PGP 5.5 For Macintosh User’s Guide PGP55mac

User Manual: PGP - 5.5 for Macintosh - User’s Guide User Guide for PGP Software, Free Instruction Manual

Open the PDF directly: View PDF .
Page Count: 138

Download
Open PDF In Browser	View PDF

PGP®
for Personal Privacy
For
Macintosh

User’s Guide

Pretty Good Privacy, Inc.

Copyright © 1997 Pretty Good Privacy, Inc. All rights reserved.
9-97. Printed in the United States of America.
PGP for Personal Privacy, Version 5.5.1 for Macintosh
Record the serial number from your License Agreement in the space provided
below:

Copyright © 1990-1997 by Pretty Good Privacy, Inc. All Rights Reserved.
PGP, Pretty Good, and Pretty Good Privacy are registered trademarks of Pretty Good
Privacy, Inc. All other trademarks and registered trademarks are the property of their
respective owners.
Portions of this software may use public key algorithms described in U.S. Patent numbers
4,200,770, 4,218,582, 4,405,829, and 4,424,414, licensed exclusively by Public Key Partners;
the IDEA™ cryptographic cipher described in U.S. patent number 5,214,703, licensed from
Ascom Tech AG; and the Northern Telecom Ltd., CAST Encryption Algorithm, licensed
from Northern Telecom, Ltd. IDEA is a trademark of Ascom Tech AG. The compression
code in PGP is by Mark Adler and Jean-loup Gailly, used with permission from the free
Info-ZIP implementation. LDAP software provided courtesy University of Michigan at Ann
Arbor, Copyright © 1992-1996 Regents of the University of Michigan. All rights reserved.
Pretty Good Privacy, Inc. may have patents and/or pending patent applications covering
subject matter in this software or its documentation; the furnishing of this software or
documentation does not give you any license to these patents.
The software provided with this documentation is licensed to you for your individual use
under the terms of the End User License Agreement and Limited Warranty provided with
the software. The information in this document is subject to change without notice. Pretty
Good Privacy, Inc. does not warrant that the information meets you requirements or that
the information is free of errors. The information may include technical inaccuracies or
typographical errors. Changes may be made to the information and incorporated in new
editions of this document, if and when made available by Pretty Good Privacy, Inc.
Export of this software and documentation may be subject to compliance with the rules and
regulations promulgated from time to time by the Bureau of Export Administration, United
States Department of Commerce, which restrict the export and re-export of certain products
and technical data.
PRETTY GOOD PRIVACY, INC.
2121 South El Camino Real, Suite 902
San Mateo, CA 94403
(650) 631-1747 voice
(650) 572-1932fax
info@pgp.com
http://www.pgp.com

LIMITED WARRANTY
Limited Warranty. Pretty Good Privacy, Inc. ("PGP") warrants that the Software
Product will perform substantially in accordance with the accompanying written materials
for a period of sixty (60) days from the date of original purchase. To the extent allowed by
applicable law, implied warranties on the Software Product, if any, are limited to such sixty
(60) day period. Some jurisdictions do not allow limitations on duration of an implied
warranty, so the above limitation may not apply to you.
Customer Remedies. PGP's and its suppliers' entire liability and your exclusive
remedy shall be, at PGP's option, either (a) return of the purchase price paid for the license,
if any or (b) repair or replacement of the Software Product that does not meet PGP's limited
warranty and which is returned at your expense to PGP with a copy of your receipt. This
limited warranty is void if failure of the Software Product has resulted from accident, abuse,
or misapplication. Any repaired or replacement Software Product will be warranted for the
remainder of the original warranty period or thirty (30) days, whichever is longer. Outside
the United States, neither these remedies nor any product support services offered by PGP
are available without proof of purchase from an authorized international source and may
not be available from PGP to the extent they subject to restrictions under U.S. export control
laws and regulations.
NO OTHER WARRANTIES. TO THE MAXIMUM EXTENT PERMITTED BY
APPLICABLE LAW, AND EXCEPT FOR THE LIMITED WARRANTIES SET FORTH
HEREIN, THE SOFTWARE AND DOCUMENTATION ARE PROVIDED "AS IS" AND PGP
AND ITS SUPPLIERS DISCLAIM ALL OTHER WARRANTIES AND CONDITIONS,
EITHER EXPRESS OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, IMPLIED
WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE,
CONFORMANCE WITH DESCRIPTION, TITLE AND NON-INFRINGEMENT OF THIRD
PARTY RIGHTS, AND THE PROVISION OF OR FAILURE TO PROVIDE SUPPORT
SERVICES. THIS LIMITED WARRANTY GIVES YOU SPECIFIC LEGAL RIGHTS. YOU
MAY HAVE OTHERS, WHICH VARY FROM JURISDICTION TO JURISDICTION.
LIMITATION OF LIABILITY. TO THE MAXIMUM EXTENT PERMITTED BY
APPLICABLE LAW, IN NO EVENT SHALL PGP OR ITS SUPPLIERS BE LIABLE FOR
ANY INDIRECT, INCIDENTAL, CONSEQUENTIAL, SPECIAL OR EXEMPLARY
DAMAGES OR LOST PROFITS WHATSOEVER (INCLUDING, WITHOUT LIMITATION,
DAMAGES FOR LOSS OF BUSINESS PROFITS, BUSINESS INTERRUPTION, LOSS OF
BUSINESS INFORMATION, OR ANY OTHER PECUNIARY LOSS) ARISING OUT OF
THE USE OR INABILITY TO USE THE SOFTWARE PRODUCT OR THE FAILURE TO
PROVIDE SUPPORT SERVICES, EVEN IF PGP HAS BEEN ADVISED OF THE
POSSIBILITY OF SUCH DAMAGES. IN ANY CASE, PGP'S CUMULATIVE AND ENTIRE
LIABILITY TO YOU OR ANY OTHER PARTY FOR ANY LOSS OR DAMAGES
RESULTING FROM ANY CLAIMS, DEMANDS OR ACTIONS ARISING OUT OF OR
RELATING TO THIS AGREEMENT SHALL NOT EXCEED THE PURCHASE PRICE PAID
FOR THIS LICENSE. BECAUSE SOME JURISDICTIONS DO NOT ALLOW THE
EXCLUSION OR LIMITATION OF LIABILITY, THE ABOVE LIMITATIONS MAY NOT
APPLY TO YOU.

Preface
This book describes how to use PGP for Personal Privacy, Version
5.5 for Macintosh. PGP Version 5.5 has some new features, which
are described in Chapter 1.

Conventions used in this document
Types of notes:
NOTE:

TIP:

ALERT

Notes provide additional information on using PGP—for example, if
you are not using PGP/MIME, you must encrypt any files you want to
send as attachments from the Finder before sending your message

Tips provide guidelines for using PGP effectively—for example, how to
create a useful passphrase.

Alerts provide information to prevent loss of data—for example, if
you are sending your key to colleagues who are using PCs, enter a
name of up to eight characters and three additional characters for
the file type extension (for example, e-mail.txt).

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

v

For more information
There are several ways to find out more about PGP and its products.

PGP Web site
PGP provides information about our products, the PGP organization,
product updates, and related topics, such as Privacy Matters, at the PGP
Web site. Visit us at www.pgp.com.

Registration information
Users are asked to register online. This information is for our internal
use only and will enable us to serve you better in the future. This data
will remain confidential -- we respect your privacy (that's why we do
what we do). See our registration page at the following URL:
http://www.pgp.com/products/online-register.cgi

Support
To get Technical Support for your PGP product, see our Technical
Support web site :http://www.pgp.com/service/ or e-mail us at
PGPSupport@pgp.com.
Service is available for PGP products by sending e-mail to
PGPservice@pgp.com or by contacting the PGP Web site at
http://www.pgp.com/service/
Service is available for the following issues:
• Product Returns
• Download Problems
• Export Control Issues
• Marketing Questions
• Product Literature Fulfillment
• Order Tracking and Information

vi

Preface

Recommended Introductory Readings
Bacard, Andre Computer Privacy Handbook, Peachpit Press, 1995.
Garfinkel, Simson Pretty Good Privacy, O’Reilly & Associates, 1995.
Schneier, Bruce Applied Cryptography: Protocols, Algorithms, and Source
Code in C, Second Edition, John Wiley & Sons, 1996.
Schneier, Bruce E-mail Security, John Wiley & Sons, 1995.
Stallings, William Protect Your Privacy, Prentice-Hall, 1994.

Other Readings:
Lai, Xuejia, “On the Design and Security of Block Ciphers,” Institute for
Signal and Information Processing, ETH-Zentrum, Zurich, Switzerland,
1992.
Lai, Xuejia, Massey, James L., and Murphy, Sean “ Markov Ciphers and
Differential Cryptanalysis,” Advances in Cryptology—EUROCRYPT’91.
Rivest, Ronald “The MD5 Message Digest Algorithm” MIT Laboratory for
Computer Science, 1991.
Wallich, Paul “Electronic Envelopes” Scientific American, Feb. 1993, page 30.
Zimmermann, Philip “A Proposed Standard Format for RSA
Cryptosystems” Advances in Computer Security, Vol. III, edited by Rein
Turn Artech House, 1988.

Your feedback is welcome
We continually enhance PGP and welcome customer feedback as we
design new versions. We appreciate your interest in PGP and your
thoughts on product content and functionality, especially as we plan
features to enhance the products for corporate settings. Feedback like
yours help us to develop richer and easier-to-use software and services.
While we cannot incorporate all suggestions, but we will give your
input serious consideration as we develop future products.
If you would like to provide input, please send e-mail to
mac-doc@pgp.com.

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

vii

viii

Preface

Contents
Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v
Conventions used in this document . . . . . . . . . . . . . . . . . . .v
Types of notes: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .v

For more information . . . . . . . . . . . . . . . . . . . . . . . . . . . vi
PGP Web site . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vi
Registration information . . . . . . . . . . . . . . . . . . . . . . . . . . . vi
Support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vi

Recommended Introductory Readings . . . . . . . . . . . . . . . .vii
Other Readings: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .vii
Your feedback is welcome . . . . . . . . . . . . . . . . . . . . . . . . . .vii

Chapter 1: Introducing PGP for Personal Privacy . . . . . . 1
What’s new in PGP Version 5.5 . . . . . . . . . . . . . . . . . . . . 1
What’s new in PGP Version 5.5 documentation . . . . . . . . . . 2
Using PGP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

A quick overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Create a private and public key pair . . . . . . . . . . . . . . . . . . 3
Exchange public keys with others . . . . . . . . . . . . . . . . . . . . 4
Validate your keys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Encrypt and sign your e-mail and files . . . . . . . . . . . . . . . . . 4
Decrypt and verify your e-mail and files . . . . . . . . . . . . . . . . 5
Wipe files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

ix

Chapter 2: Getting Started . . . . . . . . . . . . . . . . . . . . . .7
System requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Compatibility with other versions . . . . . . . . . . . . . . . . . . . 7
About PGP for Personal Privacy . . . . . . . . . . . . . . . . . . . . 8
Using PGP/MIME . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Upgrading from a previous version . . . . . . . . . . . . . . . . . 9
To upgrade from PGP Version 2.7.1 or 2.6.2 . . . . . . . 10

Installing PGP Version 5.5 . . . . . . . . . . . . . . . . . . . . . . . 11
To install PGP from a CD ROM . . . . . . . . . . . . . . . . . . 11
To install PGP from PGP’s Web site . . . . . . . . . . . . . . 11

Running PGP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Using PGP from the PGPmenu . . . . . . . . . . . . . . . . . . . . 12
Opening the PGPkeys application . . . . . . . . . . . . . . . 12
Setting PGP preferences . . . . . . . . . . . . . . . . . . . . . . . 13
Getting help . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Using PGP from supported e-mail applications . . . . . . . . 13
Using PGP from the PGPtools application . . . . . . . . . . . . . . 14
Selecting recipients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Taking shortcuts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
To access PGP functions using PGPcontextmenu . . . . . . 15
PGPkeys icon definitions . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Chapter 3: Making and Exchanging Keys . . . . . . . . . .19
Key concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Making a key pair . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
To create a new key pair . . . . . . . . . . . . . . . . . . . . . . 20
Protecting your keys . . . . . . . . . . . . . . . . . . . . . . . . . 24

Distributing your public key . . . . . . . . . . . . . . . . . . . . . . 25
Making your public key available through a key server . . . . 25
To send your public key to a key server . . . . . . . . . . . . 26
x

Contents

Including your public key in an e-mail message . . . . . . . . . 26
To include your public key in an e-mail message . . . . . 27
Exporting your public key to a file . . . . . . . . . . . . . . . . . . . 27

Obtaining the public keys of others . . . . . . . . . . . . . . . . 28
Getting public keys from a key server . . . . . . . . . . . . . . . . 28
To get someone’s public key from a key server . . . . . . 29
Adding public keys from e-mail messages . . . . . . . . . . . . . 29
Importing a public key from a file . . . . . . . . . . . . . . . . . . . 30

Verifying the authenticity of a key . . . . . . . . . . . . . . . . . . 30
Getting keys via trusted introducers . . . . . . . . . . . . . . . . . . 31

Chapter 4: Sending and Receiving Secure E-mail . . . . . 33
Encrypting and signing e-mail . . . . . . . . . . . . . . . . . . . . 33
Encrypting and signing with supported e-mail applications . 34
To encrypt and sign with supported e-mail applications 34
Encrypting e-mail to groups of recipients . . . . . . . . . . . . . . 35
To create a group . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
To add members to a group . . . . . . . . . . . . . . . . . . . . 36
To delete group members . . . . . . . . . . . . . . . . . . . . . . 36
To delete a group . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
To add a group to another group . . . . . . . . . . . . . . . . 36
To send encrypted e-mail to groups . . . . . . . . . . . . . . 36

Decrypting and verifying e-mail . . . . . . . . . . . . . . . . . . . 37
Decrypting and verifying from supported e-mail applications 37
To decrypt and verify from supported e-mail applications 38

Chapter 5: Using PGP for Secure File Storage . . . . . . . 39
Using PGP to encrypt and decrypt files . . . . . . . . . . . . . . 39
Encrypting and signing using PGPmenu . . . . . . . . . . . . . . . 39
To encrypt and sign using PGPmenu . . . . . . . . . . . . . . 40
To encrypt and sign using the PGP tools . . . . . . . . . . . . . . . 41

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

xi

Decrypting and verifying using PGPmenu . . . . . . . . . . . . . 41
To decrypt and verify using PGPmenu . . . . . . . . . . . . . 41
To decrypt and verify using PGPtools . . . . . . . . . . . . . . . . . 42

Using PGP Secure Wipe to delete files . . . . . . . . . . . . . . 42
To permanently delete a file . . . . . . . . . . . . . . . . . . . . . . . 43

Chapter 6: Managing Keys and Setting Preferences . . .45
Managing your keys . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
The PGPkeys window . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
PGPkeys attribute definitions . . . . . . . . . . . . . . . . . . . 46

Examining a key’s properties . . . . . . . . . . . . . . . . . . . . . 49
Specifying a default key pair . . . . . . . . . . . . . . . . . . . . . . 50
To specify your default key pair . . . . . . . . . . . . . . . . . 50
Adding a new user name or address . . . . . . . . . . . . . . . . . 51
To add a new user name or address to an existing key

51

Checking a key’s fingerprint . . . . . . . . . . . . . . . . . . . . . . . 51
To check a key’s fingerprint . . . . . . . . . . . . . . . . . . . . 52
Signing someone’s public key . . . . . . . . . . . . . . . . . . . . . . 52
To sign someone’s public key . . . . . . . . . . . . . . . . . . . 52
Granting trust for key validations . . . . . . . . . . . . . . . . . . . 53
To grant trust for a key . . . . . . . . . . . . . . . . . . . . . . . 53
Disabling and enabling keys . . . . . . . . . . . . . . . . . . . . . . . 54
To disable a key . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
To enable a key . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
Deleting a key or signature . . . . . . . . . . . . . . . . . . . . . . . . 54
To delete a key, signature, or user ID . . . . . . . . . . . . . 54
Changing your Passphrase . . . . . . . . . . . . . . . . . . . . . . . . 55
To change your passphrase . . . . . . . . . . . . . . . . . . . . 55

xii

Contents

Importing and Exporting Keys . . . . . . . . . . . . . . . . . . . . . . 55
To import a key from a file . . . . . . . . . . . . . . . . . . . . . 55
To add a key from an e-mail message . . . . . . . . . . . . 56
To export a key to a file . . . . . . . . . . . . . . . . . . . . . . . 56
Revoking a key . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
To revoke a key . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

Setting your preferences . . . . . . . . . . . . . . . . . . . . . . . . 57
General preferences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
Files preferences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
E-mail preferences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
PGPmenu preferences . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
Server preferences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

Advanced preferences . . . . . . . . . . . . . . . . . . . . . . . . . . 62
Searching for a key . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
To search for a user’s key . . . . . . . . . . . . . . . . . . . . . 64

Chapter 7: Troubleshooting PGP . . . . . . . . . . . . . . . . . 67
Chapter 8: Security Features and Vulnerabilities . . . . . 71
Why I wrote PGP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
Encryption basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
How public key cryptography works . . . . . . . . . . . . . . . . . 76
How your files and messages are encrypted . . . . . . . . . . . . 77
The PGP symmetric algorithms . . . . . . . . . . . . . . . . . . . . . . 78
Data compression . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
About the random numbers used as session keys . . . . . . . . 81
How decryption works . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
How digital signatures work . . . . . . . . . . . . . . . . . . . . . . . 82
About the message digest . . . . . . . . . . . . . . . . . . . . . 84
How to protect public keys from tampering . . . . . . . . . . . . . 85
How does PGP keep track of which keys are valid? . . . . . . . 89

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

xiii

How to protect private keys from disclosure . . . . . . . . . . . . 91
What if you lose your private key? . . . . . . . . . . . . . . . 93

Beware of snake oil . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
Vulnerabilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
Compromised passphrase and private key . . . . . . . . . . . . . 98
Public key tampering . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
Not Quite Deleted Files . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
Viruses and Trojan horses . . . . . . . . . . . . . . . . . . . . 100
Swap files or virtual memory . . . . . . . . . . . . . . . . . . 101
Physical security breach . . . . . . . . . . . . . . . . . . . . . . . . . 102
Tempest attacks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
Protecting against bogus timestamps . . . . . . . . . . . . . . . . 103
Exposure on multi-user systems . . . . . . . . . . . . . . . . . . . . 104
Traffic analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104

Cryptanalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105

Appendix A: Transferring Files Between the MacOS
and Windows using PGP . . . . . . . . . . . . . . . . . . .107
Sending from the MacOS to Windows . . . . . . . . . . . . . 108
MacBinary: Yes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
MacBinary: No . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
MacBinary: Smart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109

Receiving Windows files on the MacOS . . . . . . . . . . . . 110
Supported Applications . . . . . . . . . . . . . . . . . . . . . . . . . 111

A Glossary of Terms . . . . . . . . . . . . . . . . . . . . . . . . .113
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .117

xiv

Contents

Chapter 1

Introducing
PGP for Personal Privacy
Welcome to PGP! With PGP® for Personal Privacy, you can easily
and securely protect the privacy of your e-mail messages and file
attachments by encrypting them so that only the intended
recipients can read them. You can also digitally sign messages and
files, which ensures their authenticity. A signed messages verifies
that the information in it has not been tampered with in any way.

What’s new in PGP Version 5.5
PGP Version 5.5 includes these new features:
• You can create recipient groups, in which you select a group of
people’s keys and encrypt mail to all of them simultaneously.
• New key server integration capabilities that you can use to
automatically store, search, and synchronize keys.
• A new Key Search Window that you can use to locate keys on
remote servers with the same user interface that you use to search
your keyring.
• The PGP Version 1.0 Policy Management Agent for SMTP, which
enforces your organization’s e-mail encryption policy. The PGP
SMTP Policy Agent works in conjunction with a standard SMTP
mail server to ensure that incoming and outgoing e-mail adheres to
the policies enforced by a given site.

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

1

• A PGP Wipe feature that overwrites files so that they cannot be
recovered with software tools
• A configurable View menu that provides information about the keys
on your keyring.
• New key-signing capabilities that allow you to choose whether the
owner of the key that you are signing is a trusted introducer or a
meta-introducer. You can also choose whether your signature is
exportable, which means it is exported with your key, or
nonexportable.

What’s new in PGP Version 5.5 documentation
This guide contains information about PGP Version 5.5. The PGP
documentation has the following new features:
• The manuals for user products contain conceptual information and
procedures for using PGP. Online help for PGP user products is
written specifically for each plug-in, and contains more detailed
information on how to use PGP with each plug-in.
• PGP Version 5.5 for Macintosh includes online help, and an
electronic copy of the documentation in .pdf format is available on
the PGP Version 5.5 CD ROM.

Using PGP
One of the most convenient ways to use PGP is through one of the
popular e-mail applications supported by the PGP plug-ins. With these
applications, you can encrypt and sign as well as decrypt and verify
your messages while you are composing and reading your mail with a
simple click of a button.
If you are using an e-mail application that is not supported by the plugins, you can easily encrypt the text of the message using PGPmenu. In
addition, if you need to encrypt or decrypt file attachments, you can do
so directly from the Finder by choosing the appropriate menu option.
You can also use PGP to encrypt and sign files on the hard disk of your
computer for secure storage, and to securely wipe files from your hard
disk so that sensitive data can’t be retrieved with disc recovery software.

2

Chapter 1: Introducing PGP for Personal Privacy

A quick overview
PGP is based on a widely accepted encryption technology known as
public key cryptography, in which two complementary keys, called a key
pair, are used to maintain secure communications. One of the keys is
designated as a private key to which only you have access and the other is
a public key that you freely exchange with other PGP users. Both your
private and your public keys are stored in keyring files, which are
accessible from the PGPkeys window. It is from this window that you
perform all your key management functions.
To send someone a private e-mail message, you use a copy of that
person’s public key to encrypt the information, which only they can
decipher by using their private key. Conversely, when someone wants
to send you encrypted mail, they use a copy of your public key to
encrypt the data, which only you can decipher by using a copy of your
private key. You can also use PGP to encrypt or sign files that are stored
on your computer, to authenticate that they have not been altered.
You also use your private key to sign the e-mail you send to others or to
sign files to authenticate them. The recipients can then use their copy of
your public key to determine if you really sent the e-mail and whether it
has been altered while in transit. When someone sends you e-mail with
their digital signature, you use a copy of their public key to check the
digital signature and to make sure that no one has tampered with the
contents.
With the PGP program you can easily create and manage your keys and
access all of the functions for encrypting and signing as well as
decrypting and verifying your e-mail messages, files, and file
attachments.
The rest of this section takes a quick look at the procedures you normally
follow in the course of using PGP. For details concerning any of these
procedures, refer to the appropriate chapters in this book.

Create a private and public key pair
Before you can begin using PGP, you need to generate a key pair. A PGP
key pair is composed of a private key to which only you have access and
a public key that you can copy and make freely available to everyone
with whom you exchange information.

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

3

You have the option of creating a new key pair immediately after you
have finished the PGP installation procedure, or you can do so at any
time by opening the PGPkeys application.

Exchange public keys with others
After you have created a key pair, you can begin corresponding with
other PGP users. You will need a copy of their public key and they will
need yours. Your public key is just a block of text, so it’s quite easy to
trade keys with someone. You can include your public key in an e-mail
message, copy it to a file, or post it on a public or corporate key server
where anyone can get a copy when they need it.

Validate your keys
Once you have a copy of someone’s public key, you can add it to your
public keyring. You should then check to make sure that the key has not
been tampered with and that it really belongs to the purported owner.
You do this by comparing the unique fingerprint on your copy of
someone’s public key to the fingerprint on that person’s original key.
When you are sure that you have a valid public key, you sign it to
indicate that you feel the key is safe to use. In addition, you can grant the
owner of the key a level of trust indicating how much confidence you
have in that person to vouch for the authenticity of someone else’s
public key.

Encrypt and sign your e-mail and files
After you have generated your key pair and have exchanged public
keys, you can begin encrypting and signing e-mail messages and files.
• If you are using an e-mail application supported by the plug-ins, you
can encrypt and sign your messages by selecting the appropriate
options from your application’s tool bar.
• If your e-mail application is not supported by the plug-ins, you can
copy the message to the clipboard and perform the appropriate
functions from there. You can also encrypt and sign files from the
Finder before attaching them to your e-mail; encrypt files to store
them securely on your computer; and sign files to verify that they
have not been tampered with.
4

Chapter 1: Introducing PGP for Personal Privacy

Decrypt and verify your e-mail and files
When someone sends you encrypted e-mail, you can unscramble the
contents and verify any appended signature to make sure that the data
originated with the alleged sender and that it has not been altered.
• If you are using an e-mail application that is supported by the plugins, you can decrypt and verify your messages by selecting the
appropriate options from your application’s tool bar.
• If your e-mail application is not supported by the plug-ins, you can
copy the message to the clipboard and perform the appropriate
functions from there. If you want to decrypt and verify file
attachments, you can do so from the Finder. You can also decrypt
encrypted files stored on your computer, and verify signed files to
ensure that they have not been tampered with.

Wipe files
When you need to permanently delete a file, you can use the Secure
Wipe feature to ensure that the file is unrecoverable. The file is
immediately overwritten so that it cannot be retrieved using disk
recovery software.

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

5

6

Chapter 1: Introducing PGP for Personal Privacy

Chapter 2

Getting Started
This chapter explains how to run PGP and provides a quick
overview of the procedures you will normally follow in using the
product. It also contains a table of the icons used with PGPkeys.

System requirements
These are the system requirement for installing PGP Version 5.5:
• Macintosh IIci or later model with 68030 or higher
• System software 7.5.3 or later
• 8 MB RAM
• 10 MB hard disk space
• 68K Macs must be running Apple’s CFM 68K 4.0 or higher. The PGP
installer installs this if necessary.

Compatibility with other versions
PGP has gone through many revisions since it was released by Phil
Zimmermann as a freeware product in 1991, and it is estimated that
there are now over 4 million copies in circulation. Although this version
of PGP represents a significant rewrite of the original program and
incorporates a completely new user interface, it has been designed to be
compatible with earlier versions of PGP. This means that you can
exchange secure e-mail with people who are still using these older
versions of the product:
• PGP 2.6 (Released by MIT)
PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

7

• PGP 2.7.1 for the Macintosh (Released by ViaCrypt)
• PGP 4.0 (Released by ViaCrypt)
• PGP 4.5 (Released by PGP, Inc.)
• PGP for Personal Privacy, Version 5.0
• PGP for Business Security Version 5.5

About PGP for Personal Privacy
PGP for Personal Privacy Version 5.5 supports the use of two types of
keys: RSA and Diffie-Hellman. Using PGP Version 5.5 for Personal
Privacy, you can only create new Diffie-Hellman keys. You can use
existing RSA keys but you cannot generate new RSA keys.
Before PGP Version 5.0, RSA keys were the only option. PGP now offers
keys based on Diffie-Hellman encryption and DSS digital signature
technologies. The DSS portion of the key is used for signing and the
Diffie-Hellman part is used for encryption.
However, if you need to exchange e-mail or files with others who are
using keys created using RSA-encryption algorithms, you will use an
existing set of RSA keys. If you are exchanging e-mail or files with
someone who is using an older version of PGP, they should upgrade to
one of newer PGP versions to take advantage of the better user interface
and features.
If you are encrypting e-mail or files to multiple recipients, some of
whom have RSA keys and others of whom have DSS/Diffie-Hellman
keys, the e-mail is encrypted using the appropriate key for each
individual. However, in order for users of older versions of PGP to be
able to decipher or verify this e-mail, they will first need to upgrade to
one of the patched versions that remove this limitation. These previous
versions are available as free upgrades from PGP.
PGP for Business Secrity Version 5.5 contains some features of which
you may become aware if you correspond with people who are using it.
PGP for Business Security offers a Corporate Signing key and Incoming
and Outgoing Additional Decryption Keys.
A Corporate Signing Key is a public key that is designated as the
organization-wide key that all users can trust to sign other keys.

8

Chapter 2: Getting Started

Additional Decryption Keys are keys that allows security officers in
organizations, under certain circumstances, to decrypt messages that
have been sent to or from people within their organization. The
Incoming Additional Decryption Key causes encrypted mail sent to
people in an organization to also be encrypted to the Incoming
Additional Decryption Key. The Outgoing Additional Decryption Key
causes encrypted mail sent from people in an organization to also be
encrypted to the Outgoing Additional Decryption Key. You can use Key
Properties to determine if a key has an Additional Decryption Key
associated with it and you are alerted if you are encrypting to a key with
an Additional Decryption Key.
Using PGP/MIME
PGP/MIME is a standard for some of the plug-ins that integrate PGP
functions directly into popular e-mail applications. If you are using an email application that is supported by one of the plug-ins offering PGP/
MIME, you will be able to encrypt and sign as well as decrypt and verify
your e-mail messages and file attachments automatically when you send
or receive e-mail.
However, check before sending PGP/MIME e-mail to be sure that your
recipients are using an e-mail application that supports this standard, or
they may have difficulty decrypting and verifying your messages.
You can also encrypt messages and files without using PGP/MIME.

Upgrading from a previous version
If you are upgrading from a previous version of PGP (from either PGP,
Inc. or ViaCrypt), you may want to remove the old program files before
installing PGP to free up some disk space. However, you should be
careful not to delete the private and public keyring files used to store
any keys you have created or collected while using the previous version.
When you install PGP, you are given the option of retaining your
existing private and public keyrings, so you don’t have to go to the
trouble of importing all of your old keys. To upgrade from a previous
version, follow the appropriate steps listed next.

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

9

To upgrade from PGP Version 2.7.1 or 2.6.2
1. Make sure that you have exited all programs currently running on
your computer.
2. Make backups of your old PGP keyrings on another volume. Your
public keys are stored in “pubring.pgp” and your private keys are
stored in “secring.pgp”.

NOTE:

You may want to make two separate backups of your keyrings onto
two different floppy disks just to be safe. Be especially careful not to
lose your private keyring; otherwise you will never be able to decrypt
any e-mail messages or file attachments encrypted with the lost keys.
Store the keyrings in a secure place where only you have access to
them.

3. When you have successfully backed up your old keyrings, remove or
archive the (old) PGP software. You have two options here:
• Manually delete the entire old PGP folder and all of its contents; or
• Manually delete the old PGP program and archive the remaining
files, especially the configuration and keyring files.

NOTE:

If you obtain a copy of the newly patched PGP264 version from
MIT, your old software will be able to read the RSA keys on the
new 5.5 keyrings and will not fail when it encounters the new
Diffie-Hellman/DSS format keys.

4. Install PGP 5.5 using the provided Installer.
5. When the Installer asks if you have existing keyrings, click Yes,
locate your old keyrings, and follow the instructions to copy those
keys to your new PGP 5.5 keyrings.
6. Restart your computer.

10

Chapter 2: Getting Started

Installing PGP Version 5.5
Here are the ways to install PGP 5.5:
• From a CD ROM
• From PGP’s web site
• From your company’s file server
To install PGP from a CD ROM
1. Start your Macintosh.
2. Insert the CD ROM.
3. Run the Installer.
4. Follow the on-screen prompts.
To install PGP from PGP’s Web site
1. Download the PGP program onto your computer’s hard drive.
2. Double-click the PGP installation program icon.
3. Follow the on-screen prompts.

Running PGP
PGP works on the data generated by other applications. Therefore the
appropriate PGP functions are designed to be immediately available to
you based on the task you are performing at any given moment. There
are four primary ways to use PGP:
• From PGPmenu
• From within supported e-mail applications
• From the PGPtools window
• From PGPcontextmenu (for Mac OS8 users)

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

11

Using PGP from the PGPmenu
You can perform most PGP functions from the Finder or from within
most applications by choosing options from PGPmenu, which appears
as an icon in the menu bar of the Finder and any other applications you
have selected. This feature provides immediate access to the PGP
functions regardless of which application you are using and is especially
useful if you are using an e-mail application that is not supported by the
PGP plug-ins.
While using e-mail or other text-based applications to which you have
added PGPmenu, you can encrypt and sign and decrypt and verify text
by choosing options from PGPmenu. While in the Finder, you can
encrypt, sign, decrypt, verify, and wipe files.
(If you cannot find PGPmenu in one of your applications, you need to
add the application from the PGPmenu pane of the Preferences dialog
box in the PGPkeys application.)
Opening the PGPkeys application
When you choose PGPkeys from PGPmenu or from the PGP 5.5 folder,
the PGPkeys application opens, showing the private and public key
pairs that you have created for yourself as well as any public keys of
other users that you have added to your public keyring. (If you have not
already created a new key pair, the PGP Key Generation Wizard leads
you through the necessary steps. However, before going through the
process of creating a new key pair, you may wish to read Chapter 3 for
complete details about the various options.)
From the PGPkeys window you can create new key pairs and manage all
of your other keys. For instance, this is where you examine the attributes
associated with a particular key, specify how confident you are that the
key actually belongs to the alleged owner, and indicate how well you
trust the owner of the key to vouch for the authenticity of other users’
keys. For a complete explanation of the key management functions you
perform from the PGPkeys window, see Chapter 6.

12

Chapter 2: Getting Started

Setting PGP preferences
When you choose Preferences from the Edit menu in the PGPkeys
application, the Preferences dialog box appears, in which you specify
settings that affect how the PGP program functions based on your
computing environment.
By clicking the appropriate tab, you advance to the preference settings
you want to modify. For a complete explanation of these settings, see
Chapter 6.
Getting help
PGPkeys supports Apple Guide help, which can be accessed from the
Help menu in Mac OS 8 and the iconic Question M ark menu in Mac OS 7.

Using PGP from supported e-mail applications
If you have one of the popular e-mail applications supported by the PGP
plug-ins, you can access the necessary PGP functions by clicking the
appropriate buttons in your application’s toolbar. For example, you click
the lock icon to indicate that you want to encrypt your message and the
quill icon to indicate that you want to sign. Some applications have an
icon of both a lock and quill, which lets you do both at once.
When you receive e-mail from another PGP user, you decrypt the
message and verify the person’s digital signature by clicking the opened
envelope, or by selecting “Decrypt/Verify” from the menu.
Some plug-ins support a key and envelope button, which adds any keys
included in the message to your keyring. You can also access the
PGPkeys window at any time while composing or retrieving your mail
by clicking the double keys button in some plug-ins.
If you are using an e-mail application with one of the plug-ins that
supports the PGP/MIME standard, and you are communicating with
another user whose e-mail application also supports this standard, both of
you can automatically encrypt and decrypt your e-mail messages and any
attached files when you send or retrieve your e-mail. All you have to do is
turn on the PGP/MIME encryption and signatory functions from the PGP
Preferences dialog box.

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

13

When you receive e-mail from someone who uses the PGP/MIME
feature, the mail arrives with an attached icon in the message window
indicating that it is PGP/MIME encoded.
To decrypt the text and file attachments in PGP/MIME encapsulated email and to verify any digital signatures, you simply double-click the
opened envelope icon. Attachments are still encrypted if PGP/MIME is
not used, but the decryption process is usually more manual for the
receiver.

Using PGP from the PGPtools application
If you are using an e-mail application that is not supported by the plugins, or if you want to perform PGP functions from within other
applications, you can encrypt and sign, decprypt and verify, or securely
wipe messages and files directly from the PGPtools window. You can
open the PGPtools window in the following ways:
• Open the PGP folder and double-click the PGPtools icon.
• Store an alias of PGPtools in the Apple menu, and choose PGPtools
from that menu. You can also store an alias on your desktop.
When the PGPtools window opens, you can begin your encryption
work.
If you are working with text, you perform your encryption/decryption,
signature/verification, and wiping functions by selecting the text and
then dragging it onto the appropriate button in the PGPtools window.
If you are working with files, you can simply drag them to the
appropriate button.

Selecting recipients
When you send e-mail to someone whose e-mail application is
supported by the PGP plug-ins, the recipient’s e-mail address
determines which keys to use when encrypting the contents. However, if
you enter a user name or e-mail address that does not correspond to any
of the keys on your public keyring, or if you are encrypting from
PGPmenu or from PGPtools, you must manually select the recipient’s
public key from the PGP Key Selection dialog box.

14

Chapter 2: Getting Started

To select a recipient’s public key, drag the icon representing the key into
the Recipients list box and then click OK.
For complete instructions on how to encrypt and sign and decrypt and
verify e-mail, see Chapter 4. If you want to encrypt files to store on your
hard disk or to send as attachments, see Chapter 5.

Taking shortcuts
Although you will find that PGP is quite easy to use, a number of
shortcuts are available to help you accomplish your encryption tasks
even quicker. For example, you can perform most PGP functions on files
or volumes on your disk using PGPcontextmenu or PGPmenu (for Mac
OS 7 users), or by dragging the file or volume and dropping it onto one
of the PGPtools icons.
To access PGP functions using PGPcontextmenu
1. Point t-o the file or volume, either in a window or on the desktop.
2. Click once while holding down the Control key.
The contextual menu appears. PGP appears among the menu
options.
3. Choose an option from the PGP menu.
To close the menu without choosing a command, click outside the
menu.
This feature is available on machines running MacOS8 or later and is
equivalent to launching and using the features within PGPtools.
PGPmenu works similarly for MacOS7 users.

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

15

PGPkeys icon definitions
The following table shows all of the mini-icons used in the PGPkeys
window, along with a description of what they represent.
ICON

WHAT IT REPRESENTS
A pair of gold modern keys represents your Diffie-Hellman/DSS key
pair, which consists of your private key and your public key.
A single gold modern key represents a Diffie-Hellman/DSS public
key.
A pair of blue skeleton keys represents your RSA key pair, which
consists of your private key and your public key.
A single blue skeleton key represents an RSA public key.

When a key or key pair is dimmed, the keys are temporarily
unavailable for encrypting and signing. You can disable a key from
the PGPkeys window, which prevents seldom-used keys from
cluttering up the Key Selection dialog box.
A key with a red line through it indicates that the key has been
revoked. Users revoke their keys when they are no longer valid or
have been compromised in some way. A key with a red X through it
represents a corrupted or damaged key.
A key with a clock indicates that the key has expired. A key’s
expiration date is established when the key is created.

An envelope represents the owner of the key and lists the user names
and e-mail addresses associated with the key.
An empty circle indicates that the key is invalid.

16

Chapter 2: Getting Started

ICON

WHAT IT REPRESENTS
A filled circle indicates that they key is valid (green) or has a CMRK
(red).

A diamond indicates that they key is valid (green) or has a CMRK
(red).

A pencil or fountain pen indicates the signatures of the PGP users
who have vouched for the authenticity of the key. A signature with a
red line through it indicates a revoked signature. A signature with a
red X through it indicates a bad or invalid signature. A signature
with a blue arrow next to it indicates that it is exportable.
An empty bar indicates an invalid key or an untrusted user.

A half-filled bar indicates a marginally valid key or marginally
trusted user.
A full bar indicates a completely valid key or a completely trusted
user.

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

17

18

Chapter 2: Getting Started

Chapter 3

Making and Exchanging
Keys
This chapter describes how to generate the public and private key
pairs that you need to correspond with other PGP users. It also
explains how to distribute your public key and obtain the public
keys of others so that you can begin exchanging private and
authenticated e-mail.

Key concepts
PGP is based on a widely accepted and highly trusted public key
encryption system by which you and other PGP users generate a key pair
consisting of a private key and a public key. As its name implies, only
you have access to your private key, but in order to correspond with
other PGP users you need a copy of their public key and they need a
copy of yours. You use your private key to sign the e-mail messages and
file attachments you send to others and to decrypt the messages and files
they send to you. Conversely, you use the public keys of others to send
them encrypted e-mail and to verify their digital signatures.

NOTE:

Without going into too much technical detail, you might be interested to
know that it is not actually the content of the e-mail that is encrypted
using the public key encryption scheme. Instead, the data is encrypted
using a much faster single-key algorithm, and it is this single key that is
actually encrypted using the recipient’s public key. The recipient then
uses his or her private key to decrypt this key, which allows the recipient
to decipher the encrypted data.

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

19

Making a key pair
Unless you have already done so while using another version of PGP,
the first thing you need to do before sending or receiving encrypted and
certified e-mail is create a new key pair. A key pair consists of two keys:
a private key that only you possess and a public key that you freely
distribute to those with whom you correspond. You generate a new key
pair from the PGPkeys window using the PGP Key Generation Wizard,
which guides you through the process.

ALERT:

If you are upgrading from an earlier version of PGP, you have
probably already generated a private key and have distributed its
matching public key to those with whom you correspond. In this case
you don’t have to make a new key pair (as described in the next
section). Instead, you specify the location of your keys when you run
the PGPkeys application. You can go to the Key Files pane of the
Preferences dialog box and enter the correct path to your existing
keys at any time.

To create a new key pair
1.

Choose PGPkeys from PGPmenu.
The PGPkeys applucation opens.

2. Choose New from the Keys menu.
The PGP Key Generation Wizard provides some introductory
information on the first screen.
3. When you are through reading this information, click Next to
advance to the next dialog box.
The PGP Key Generation Wizard asks you to enter your name and email address.
4. Enter your name on the first line and your e-mail address on the
second line.
It’s not absolutely necessary to enter your real name or even your email address. However, using your real name makes it easier for
others to identify you as the owner of your public key. Also, by
using your correct e-mail address, you and others can take

20

Chapter 3: Making and Exchanging Keys

advantage of the plug-in feature that automatically looks up the
appropriate key on your current keyring when you address mail to a
particular recipient.
5. Click Next to advance to the next dialog box.
The PGP Key Generation Wizard asks you to specify a size for your
new keys.
6. Select a key size from 768 to 3072 bits, or enter a custom key size
from 512 to 4096 bits.

NOTE:

A custom key size may take a long time to generate, depending on
the speed of the computer you are using. Very old 68020
Macintoshes have been known to take over a day to generate a
4096 bit key. The time required to generate a key is nondeterministic, but should not take more than a few minutes using
standard key sizes.

The key size corresponds to the number of bits used to construct
your digital key. The larger the key, the less chance that someone
will be able to crack it, but the longer it takes to perform the
decryption and encryption process. You need to strike a balance
between the convenience of performing PGP functions quickly with
a smaller key and the increased level of security provided by a larger
key. Unless you are exchanging extremely sensitive information that
is of enough interest that someone would be willing to mount an
expensive and time-consuming cryptographic attack in order to read
it, you are safe using a key composed of 1024 bits.

NOTE:

When creating a Diffie-Hellman/DSS key, the size of the DSS portion
of the key is increased in fixed increments, is less than or equal to the
size of the Diffie-Hellman portion of the key, and is limited to a
maximum size of 1024 bits. The strength of a DSS 1024 bit signing
key is roughly equivalent to a 2048 bit RSA key.

7. Click Next to advance to the next dialog box.
The PGP Key Generation Wizard asks you to indicate when the key
pair should expire.

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

21

8. Indicate when you want your keys to expire. You can either use the
default selection, which is Never, or you can enter a specific date
after which the keys will expire.
Once you create a key pair and have distributed your public key to
the world, you will probably continue to use the same keys from that
point on. However, under certain conditions you may want to create
a special pair of keys that you plan to use for only a limited period of
time. In this case, when the public key expires, it can no longer be
used by someone to encrypt mail for you but it can still be used to
verify your digital signature. Similarly, when your private key
expires, it can still be used to decrypt mail that was sent to you
before your public key expired but can no longer be used to sign
mail for others.
9. Click Next to advance to the next dialog box.
The PGP Key Generation Wizard asks you enter a passphrase.
10. In the Passphrase dialog box, enter the string of characters or words
you want to use to maintain exclusive access to your private key. To
confirm your entry, press the Tab key to advance to the next line,
then enter the same passphrase again.
11. Normally, as an added level of security, the characters you enter for
the passphrase do not appear on the screen. However, if you are sure
that no one is watching, and you would like to see the characters of
your passphrase as you type, clear the Hide Typing check box.

TIP:

Your passphrase should contain multiple words and may include
spaces, numbers, and punctuation. Choose something that you can
remember easily but that others won’t be able to guess. The passphrase
is case sensitive, meaning that it distinguishes between upper and
lowercase letters. Strong passphrases include upper and lowercase
letters, numbers, punctuation, and spaces. The longer your passphrase,
and the greater the variety of characters it contains, the more secure it
is. Try to include equal quantities of upper and lowercase alphabetic
characters, numbers, punctuation marks, and so on. The quality bar
attempts to rate the strength of your passphrase compared to the
strength of the key being generated. A full bar means that they are
roughly equivalent.

12. Click Next to begin the key generation process.

22

Chapter 3: Making and Exchanging Keys

The PGP Key Generation Wizard indicates that it is busy generating
your key.
If you have entered an inadequate passphrase, a warning message
appears before the keys are generated and you have the choice of
accepting the bad passphrase or entering a more secure one before
continuing.
If there is not enough random information upon which to build the
key, the PGP Random Data dialog box appears. As instructed in the
dialog box, move your mouse around and enter a series of random
keystrokes until the progress bar is completely filled in. Your mouse
movements and keystrokes generate random information that is
needed to create a unique key pair.

NOTE:

PGP Versions 5.0 and later are contantly gathering random data
from many sources on the system, including mouse positions, timings,
and keystrokes. If the Random Data dialog box does not appear, it
indicates that PGP has already collected all the random data that it
needs to create the key pair.

After the key generation process begins, it may take a while to
generate the keys. In fact, if you specify a size other than the default
values for a Diffie-Hellman/DSS key, the fast key generation option
is not used and it may take hours to generate your key at larger sizes.
Eventually the PGP Key Generation Wizard indicates that the key
generation process is complete.
13. Click Next to advance to the next dialog box.
The PGP Key Generation Wizard indicates that you have
successfully generated a new key pair and asks if you want to send
your public key to a key server.
14. Specify whether you want your new public key to be sent to the
appropriate server for the e-mail domain you entered and then click
Next.
When you send your public key to the key server, anyone who has
access to that key server can get a copy of your key when they need
it. For complete details, see “Distributing your public key,” later in
this chapter.

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

23

When the key generation process is complete, the final dialog box
appears.
15. Click Finish.
A pair of keys representing your newly created keys appears in the
PGPkeys window. You will notice that the older RSA keys are blue and
the newer Diffie-Hellman/DSS keys are yellow. At this point you can
examine your keys by checking their properties and the attributes
associated with the keys; you may also want to add other e-mail
addresses that belong to you. See Chapter 6 for details on how to add a
new user name.

Protecting your keys
Once you have generated a key pair, it is wise to create a spare pair and
put them in a safe place in case something happens to the originals. PGP
prompts you to save a backup copy when you close the PGPkeys
application after creating a new key pair.
Your private keys and your public keys are stored in separate keyring
files, which you can copy just like any other files to another location on
your hard drive or to a floppy disk. By default, the private keyring (PGP
Private Keys) and the public keyring (PGP Public Keys) are stored along
with the other program files in the “PGP Keyrings” folder in the “PGP
5.5” folder, but you can save your backups in any location you like.
When you specify that you want to save a backup copy of your keys, the
Save As dialog box appears asking you to specify the location of the
backup private and public keyring files that are to be created.
Besides making backup copies of your keys, you should be especially
careful about where you store your private key. Even though your
private key is protected by a passphrase that only you should know, it is
possible that someone could discover your passphrase and then use
your private key to decipher your e-mail or forge your digital signature.
For instance, somebody could look over your shoulder and watch the
keystrokes you enter or intercept them on the network or even over the
airwaves.
To prevent anyone who might happen to get hold of your passphrase
from being able to use your private key, you should only store it on your
own computer. If your computer is attached to a network, you should
24

Chapter 3: Making and Exchanging Keys

also make sure that your files are not automatically included in a
system-wide backup where others might gain access to your private key.
Given the ease with which computers are accessible over networks, if
you are working with extremely sensitive information, you may want to
keep your private key on a floppy disk, which you can insert like an oldfashioned key whenever you want to read or sign your private mail.
As another security precaution, consider assigning a different name to
your private keyring file and then storing it somewhere other than in the
default PGP folder where it will not be so easy to locate. You use the
Keys pane of the PGPkeys Preferences dialog box to specify a name and
location for your private and public keyring files.

Distributing your public key
After you create your keys, you need to make them available to others so
that they can send you encrypted e-mail and verify your digital
signature. You have three alternatives for distributing your public key:
• Make your public key available through a public key server.
• Include your public key in an e-mail message.
• Export your public key or copy it to a text file.
Your public key is basically composed of a block of text, so it is quite
easy to make it available through a public key server, include it in an email message, or export or copy it to a file. The recipient can then use
whatever method is most convenient to add your public key to their
public keyring.

Making your public key available through a key
server
The best method for making your public key available is to place it on a
public key server where anyone can access it. That way, people can send
you e-mail without having to explicitly request a copy of your key. It
also relieves you and others from having to maintain a large number of
public keys that you rarely use. There are a number of key servers
worldwide, including those offered by PGP, Inc., where you can make
your key available for anyone to access.

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

25

If you are using PGP for Personal Privacy, your Security Officer will
usually pre-configure your keyserver settings so that everything works
correctly for your site.
To send your public key to a key server
1. Connect to the Internet.
2. Open the PGPkeys window.
3. Select the icon that represents the public key you want to post on the
key server.
4. Choose For Domain from the Send to Server submenu of the Keys
menu.
After you place a copy of your public key on a key server, you can tell
people who want to send you encrypted mail or to verify your digital
signature to get a copy of your key from the server. Even if you don’t
explicitly point them to your public key, they can get a copy by
searching the key server for your name or e-mail address. Many people
include the Web address for their public key at the end of their e-mail
messages; in most cases the recipient can just double-click the address to
access a copy of your key on the server.
If you ever need to change your e-mail address, or if you acquire new
signatures, all you have to do to replace your old key is send a new copy
to the server; the information is automatically updated. However, you
should keep in mind that public key servers are only capable of
updating new information and will not allow removal of user names or
signatures from your key. If your key is ever compromised, you can
revoke it, which tells the world to no longer trust that version of your
key. See Chapter 6 for more details on how to revoke a key.

Including your public key in an e-mail message
Another convenient method of delivering your public key to someone is
to include it along with an e-mail message.

26

Chapter 3: Making and Exchanging Keys

To include your public key in an e-mail message
1. Open the PGPkeys window.
2. Select your key pair and then choose Copy from the Edit menu.
3. Open the editor you use to compose your e-mail messages, place the
cursor in the desired area, and then choose Paste from the Edit
menu. In newer e-mail applications, you can simply drag your key
from the PGPkeys window into the text of your e-mail message to
transfer the key information.
When you send someone your public key, be sure to sign the e-mail.
That way, the recipient can verify your signature and be sure that no one
has tampered with the information along the way. Of course, if your
key has not yet been signed by any trusted introducers, recipients of
your signature can only truly be sure the signature is from you by
verifying the fingerprint on your key.

Exporting your public key to a file
Another method of distributing your public key is to copy it to a file and
then make this file available to the person with whom you want to
communicate. There are three ways to copy your public key to a file:
• Select the icon representing your key pair from the PGPkeys
window, then choose Export from the Keys menu and enter the
name of the file where you want the key to be saved.
• Drag the icon representing your key pair from the PGPkeys window
and drop it where you want the key to be saved.
• Select the icon representing your key pair in the PGPkeys window,
choose Copy from the Edit menu, and then choose Paste to insert the
key information into a text document.

ALERT:

If you are sending your key to colleagues who are using PCs, enter a
name of up to eight initial characters and three additional
characters for the file type extension (for example, e-mail.txt).

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

27

Obtaining the public keys of others
Just as you need to distribute your public key to those who want to send
you encrypted mail or to verify your digital signature, you need to
obtain the public keys of others so you can send them encrypted mail or
verify their digital signatures. You have three alternatives for obtaining
someone’s public key:
• Get the key from a public key server.
• Add the public key directly from an e-mail message.
• Import the public key from a file.
Public keys are really just blocks of text, so they are quite easy to add to
your keyring by importing them from a file or by copying them from an
e-mail message and then pasting them into your public keyring.

Getting public keys from a key server
If the person to whom you want to send encrypted mail is an
experienced PGP user, chances are that they have placed a copy of their
public key on a key server. This makes it very convenient for you to get a
copy of their most up-to-date key whenever you want to send them mail
and also relieves you from having to store a lot of keys on your public
keyring.
With PGP Version 5.5, you can search for keys on a key server using
these methods:
• User ID
• Key ID
• Key Type (Diffie-Hellman or RSA)
• Creation date
• Expiration date
• Revoked keys
• Disabled keys
• Key size
• Keys signed by a particular key

28

Chapter 3: Making and Exchanging Keys

The inverse of most of these operations is also available. For example,
you may search using “User ID is not Bob” as your criterion.
There are a number of public key servers, such as the one maintained by
PGP, Inc., where you can locate the keys of most PGP users. If the
recipient has not pointed you to the Web address where their public key
is stored, you can access any key server and do a search for the user’s
name or e-mail address, because all key servers are regularly updated to
include the keys stored on all the other servers. With PGP Version 5.5,
you can quickly locate a specific user’s key when you are sending e-mail
or managing your keys from the PGPkeys window.
To get someone’s public key from a key server
1. Open the PGPkeys application from the PGP menu or by doubleclicking the PGPkeys icon.
2. Choose Search Server from the Keys menu.
The Search dialog box appears.
3. Select the location or server you want to search from the Search For
Keys On menu.
4. Enter search criteria to use to locate the user’s public key. To narrow
your search, click More Choices to specify additional criteria.
If a public key for the specified user is found, you are asked whether
you want to add it to your public keyring. When you add a public
key to your keyring, the key shows up in the PGPkeys window,
where you can examine it to make sure that it is valid.

Adding public keys from e-mail messages
One convenient way to get a copy of someone’s public key is to have
that person include it in an e-mail message. If you have an e-mail
application that is supported by the PGP plug-in, then you can add the
sender’s public key to your public keyring by simply clicking a button.
For example, when an e-mail message arrives with a block of text
containing someone’s public key, click the key and envelope button to
have the key stored on your public keyring.

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

29

If you are using an e-mail application that is not supported by the plugins, you can add the public key to the keyring by copying the block of
text that represents the public key and pasting it into the PGPkeys
window.

Importing a public key from a file
Another method of obtaining someone’s public key is to have that
person save it to a file from which you can import it or copy and paste it
into your public keyring. There are three methods of extracting
someone’s public key and adding it to your public keyring.
• Choose Import from the Keys menu and then enter the name of the
file where the public key is stored.
• Drag the file containing the public key onto the PGPkeys window.
• Open the text document where the public key is stored, select the
block of text representing the key, and then choose Copy from the
Edit menu. Go to the PGPkeys window and choose Paste from the
Edit menu to copy the key. The key then shows up as an icon in the
PGPkeys window.

Verifying the authenticity of a key
When you exchange keys with someone, it is sometimes hard to tell if the
key really belongs to that person. PGP provides a number of safeguards
that allow you to check a key’s authenticity and to certify that the key
belongs to a particular owner. The PGP program also warns you if you
attempt to use a key that is not valid and also defaults to warn you when
you are about to use a marginally valid key.
One of the major vulnerabilities of public key encryption systems is the
ability of some eavesdroppers to mount a “man-in-the-middle” attack
by replacing someone’s public key with one of their own. In this way
they can intercept any encrypted e-mail intended for that person,
decrypt it using their own key, then encrypt it again with the person’s
real key and send it on to them as if nothing had ever happened. In fact,
this could all be done automatically through a sophisticated computer
program that stands in the middle and deciphers all of your
correspondence.

30

Chapter 3: Making and Exchanging Keys

Based on this scenario, you and those with whom you exchange e-mail
need a way to determine whether you do indeed have legitimate copies
of each others’ keys. The best way to be completely sure that a public
key actually belongs to a particular person is to have the owner copy it
to a floppy disk and then physically hand it to you. However, you are
seldom close enough to personally hand a disk to someone; you
generally exchange public keys via e-mail or get them from a public key
server.
Even though these are somewhat less secure methods of exchanging
tamper-proof keys, you can still determine if a key really belongs to a
particular person by checking its digital fingerprint, a unique series of
numbers generated when the key is created. By comparing the
fingerprint on your copy of someone’s public key to the fingerprint on
their original key, you can be absolutely sure that you do in fact have a
valid copy of their key.
The most definitive way to check a key’s fingerprint is to call the person
and have them read their fingerprint to you over the phone.
Once you are absolutely convinced that you have a legitimate copy of
someone’s public key, you can then sign their key. By signing someone’s
public key with your private key, you are certifying that you are sure the
key belongs to the alleged user. For instance, when you create a new
key, it is automatically certified with your own digital signature, since it
is a reasonably safe assumption that the person creating the key is in fact
the true owner. The reason for signing your own key is to prevent
anyone from modifying it, which would immediately invalidate your
signature. By default, signatures you make on other keys are not
exportable, which means they apply only to the key when it is on your
local keyring.

Getting keys via trusted introducers
PGP users often have other trusted users sign their public keys to further
attest to their authenticity. For instance, you might send a trusted
colleague a copy of your public key with a request that they certify and
return it so you can include their signature when you post your key on a
public key server. Using PGP, when someone gets a copy of your public
key, they don’t have to check the key’s authenticity themselves, but can
instead rely on how well they trust the person(s) who signed your key.
PGP provides the means for establishing this level of validity for each of
PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

31

the public keys you add to your public keyring and shows the level of
trust and validity associated with each key in the PGPkeys window.
This means that when you get a key from someone whose key is signed
by a trusted introducer, you can be fairly sure that the key belongs to the
purported user. For details on how to sign keys and validate users, see
“Signing someone’s public key” in Chapter 6.
Your Security Officer can act as a trusted introducer, and you may then
trust any keys signed by the corporate key to be valid keys. If you work
for a large company with several locations, you may have regional
introducers, and your Security Officer may be a meta-introducer, or a
trusted introducer of trusted introducers.

32

Chapter 3: Making and Exchanging Keys

Chapter 4

Sending and Receiving
Secure E-mail
This chapter explains how to encrypt and sign the e-mail you send
to others and decrypt and verify the e-mail others send to you.

Encrypting and signing e-mail
The quickest and easiest way to encrypt and sign e-mail is with an
application supported by the PGP plug-ins. Although the procedure
varies slightly between different e-mail applications, you perform the
encryption and signing process by clicking the appropriate buttons in
the application’s toolbar.
If you are using an e-mail application that is not supported by the PGP
plug-ins, you can encrypt and sign your e-mail messages via PGPmenu,
which is compatible with most popular text-based applications. When
accessing this menu from the Finder, you can encrypt and sign or
decrypt and verify files and even entire folders.
As an alternative to the other interfaces, you can also use the PGPtools
window to encrypt and sign text and files. When using this interface to
encrypt and sign text, you copy the text to the clipboard, perform the
desired operation by choosing the appropriate button and then paste the

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

33

contents back to your application. You can also encrypt and/or sign a
selected portion of text or even files by dragging them to the appropriate
button.

NOTE:

If you don’t send your e-mail immediately but instead store it in your
outbox, you should be aware that when using some e-mail
applications, the information is not encrypted until the e-mail is
actually transmitted. Before queuing encrypted messages you should
check to see if your application does in fact encrypt the messages in
your outbox. If it does not, you might want to consider encrypting
your messages via PGPmenu before queuing it in the outbox.

If you do not have one of the e-mail applications that is supported by
PGP, see Chapter 5, for information on how to encrypt and sign files.

Encrypting and signing with supported e-mail
applications
When you encrypt and sign with an e-mail application that is supported
by the PGP plug-ins, you have two choices, depending on what type of
e-mail application the recipient is using. If you are communicating with
other PGP users who have an e-mail application that supports the PGP/
MIME standard, you can take advantage of a PGP/MIME feature to
encrypt and sign your e-mail messages and any file attachments
automatically when you send them. If you are communicating with
someone who does not have a PGP/MIME-compliant e-mail
application, you should encrypt your e-mail with PGP/MIME turned off
to avoid any compatibility problems.
To encrypt and sign with supported e-mail applications
1. Use your e-mail application to compose your e-mail message just as
you normally would.
2. When you have finished composing the text of your e-mail message,
specify whether you want to encrypt and sign the text of your
message by clicking the lock and quill buttons.

NOTE:

34

If you know that you are going to use PGP/MIME regularly, you
can leave this turned on by selecting the appropriate settings
from the e-mail pane of the Preferences dialog box.

Chapter 4: Sending and Receiving Secure E-mail

3. Send your message as you normally do.
If you have elected to sign the encrypted data, the Passphrase dialog
box appears requesting your passphrase before the mail is sent. (This
dialog box appears earlier in some applications.)
4.

Enter your passphrase and then click OK.
If you have a copy of the public keys for every one of the recipients,
the appropriate keys are used. However, if you specify a recipient
for whom there is no corresponding public key, the PGP Key
Selection dialog box appears so that you can specify the correct key.

5. Drag the public keys for those who are to receive a copy of the
encrypted e-mail message into the Recipients list box. You can also
double-click any of the keys to move them from one area of the
screen to the other.
The Validity icon indicates the minimum level of confidence that the
public keys in the Recipient list are valid. This validity is based on
the signatures associated with the key, and the trust indicates how
well you can rely on the owner of the key to vouch for the
authenticity of another users key. See Chapter 6 for details.
6. Click OK to send your mail.

Encrypting e-mail to groups of recipients
You can use PGP to create recipient groups. For example, if you want to
send encrypted mail to 10 people at engineering@xyz.com, you could
create a group with that name. The View menu contains a Groups item
that toggles the display of the Groups section of the PGPkeys window.
Using the groups feature, you can create a list of people to whom you
want to send encrypted e-mail.
To create a group
1. Choose New Group from the Group menu.
2. Enter a name for the Group. Optionally, enter a group description.
3. For example, you can name a group “everyone@company.com” with
a description of “All employees.”
PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

35

4. Click OK to create the group.
To add members to a group
1. In the PGPkeys window, select the users or groups you want to add
to your group.
2.

Drag the users from the PGPkeys window to the Group pane.

NOTE:

Groups can be members of other groups.

To delete group members
1. Select the group member to be deleted.
2. Press the Delete key.
PGP asks you to confirm your choice.
To delete a group
1. Select the group from the Groups pane of the PGPkeys windows.
2. Press the Delete key or choose Delete Groups from the pop-up
menu.
To add a group to another group
1. Select the group that you want to place within another group.
2. Drag the group to the group into which you want to include it.
3. Select the Paste into Group from the pop-up menu.
To send encrypted e-mail to groups
1. Address the mail to your mail group
The name of your encryption group must correspond to the name of
the e-mail group.
2. Encrypt the message.
3. Send the message.

36

Chapter 4: Sending and Receiving Secure E-mail

Decrypting and verifying e-mail
The quickest and easiest way to decrypt and verify the e-mail sent to you
is with an application supported by the PGP plug-ins. Although the
procedure varies slightly between different e-mail applications, when
you are using an e-mail application supported by the plug-ins, you can
perform the decryption and verification process by clicking the envelope
icon in the message or your application’s toolbar. In some cases you may
need to select Decrypt/Verify from the menu in your e-mail application.
In addition, if you are using an application that supports the PGP/
MIME standard, you can decrypt and verify your e-mail messages as
well as any file attachments by clicking an icon attached to your
message.
If you are using an e-mail application that is not supported by the PGP
plug-ins, you will decrypt and verify your e-mail messages via
PGPmenu. In addition, if your e-mail includes encrypted file
attachments, you must decrypt them separately via PGPtools, PGPmenu
in the Finder, or PGPcontextmenu for MacOS8 users.

Decrypting and verifying from supported e-mail
applications
If you are communicating with other PGP users, and they have
encrypted and signed their mail using the PGP/MIME standard, a
locked envelope icon appears when you open your e-mail.

In that case, you can decrypt and verify the message and any attached
files by simply double-clicking this icon.
PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

37

If you are receiving e-mail from someone who is not using a PGP/
MIME-compliant e-mail application, you will decrypt the e-mail
messages by clicking the opened envelope icon in your application’s
toolbar or by selecting Decrypt/Verify from the Plugins menu. Also, if
there are any encrypted file attachments, you will decrypt them from
PGPtools, or PGPmenu in the Finder, or PGPcontextmenu for MacOS8
users.
To decrypt and verify from supported e-mail applications
1. Open your e-mail message just as you normally do.
You will see a block of unintelligible ciphertext in the body of your email message.
2. To decrypt and verify the contents of the e-mail message, click the
opened envelope button in your application’s toolbar.

NOTE:

If you are using Eudora, you will have to decrypt the text by
choosing the PGP Decrypt option from the Message Plug-In
menu. A convenient shortcut is to hold down the command key
and type the number 5

The PGP Enter Passphrase dialog box appears asking you to enter
your passphrase.
3. Enter your passphrase and then click OK.
The message is decrypted. If it has been signed, a message appears
indicating whether the signature is valid.
4. At this point you can save the message in its decrypted state, or you
can save the original encrypted version so that it remains secure.

38

Chapter 4: Sending and Receiving Secure E-mail

Chapter 5

Using PGP for Secure File
Storage
This chapter describes how to use PGP functions without using email plug-ins. It describes how to use PGP to encrypt and decrypt,
and how to sign and verify files from the Finder using PGPtools or
PGPmenu so that you can encrypt and sign files either for e-mail
or for secure storage on your computer. It also describes the PGP
Secure Wipe function, which deletes files by erasing their contents
completely from your computer.

Using PGP to encrypt and decrypt files
You can use PGP to encrypt and sign files without using an e-mail plugin so that you can send encrypted or signed files as e-mail attachments.
You can also use the techniques described in this chapter to encrypt and
sign files that you store on your computer.

Encrypting and signing using PGPmenu
If you plan to send an encrypted file as an attachment with your e-mail
message, or if you want to encrypt a file to protect it on your computer,
you do so using PGPmenu.

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

39

To encrypt and sign using PGPmenu
1. In the Finder, select the file or files that you want to encrypt.
2. Choose the desired option from PGPmenu or from
PGPcontextmenu, which MacOS8 users can access by holding down
the Control key while you select a file.
The PGP Key Selection dialog box appears. You can select the
recipient’s public keys for the file you are encrypting or signing.
You have the following encryption options:
• Text Output When sending files as attachments with some e-mail
applications, you may need to select the Text Output check box to
save the file as ASCII text. This is sometimes necessary in order to
send a binary file using older e-mail applications. Selecting this
option increases the size of the file by about 30 percent.
• Conventional Encrypt Select this check box to rely on a common
passphrase rather than on public key cryptography. The file is
encrypted using a session key, which encrypts using a passphrase
that you will be asked to choose.
• Wipe Original Select this check box to overwrite the original
document that you are encrypting or signing, so that your sensitive
information is not readable by anyone who can access your hard
disk.
If you have signed the files, you are asked to supply your
passphrase.
3. Select the public keys by dragging them to the Recipients list and
then click OK.
If you look in the folder where the original file was located, you will
find a file with the specified name represented by one of two icons:

.pgp
encrypted with standard output

40

.asc
encrypted with text output

Chapter 5: Using PGP for Secure File Storage

To encrypt and sign using the PGP tools
1. In the Finder, select the file or files that you want to encrypt.
You can select multiple files, but you must encrypt and sign each of
them individually.
2. Drag the file(s) onto the Encrypt, Sign, or Encrypt and Sign button in
the PGPtools window.
The PGP Key Selection dialog box appears. You can select the
recipient’s public keys for the file you are encrypting or signing.
The options available are described in “To decrypt and verify using
PGPmenu”.
3. Select the public keys by dragging them to the Recipients list and
then click OK.
The file is encrypted and/or signed and the resulting file appears in
the folder with the original.

Decrypting and verifying using PGPmenu
If the e-mail you receive has file attachments, and you are not using a
PGP/MIME-compliant e-mail application, you must decrypt them from
the Finder.
First, make sure that PGPmenu is available.
To decrypt and verify using PGPmenu
1. In the Finder, select the file or files that you want to decrypt and
verify.
You can select multiple files, but you must decrypt and verify each
file individually.
2. Choose Decrypt/Verify from PGPmenu, or hold down the Control
key and select the file to open PGPcontextmenu, then choose
Decrypt/Verify from the menu.
The passphrase dialog box appears.
3. Enter your passphrase and then click OK.

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

41

If the file has been signed, a message appears indicating whether the
signature is valid.
4. Click OK.
The decrypted file is saved in the same location as the original file.

To decrypt and verify using PGPtools
1. In the Finder, select the file or files that you want to decrypt.
You can select multiple files, but you must decrypt and verify each
of them individually.
2. Drag the file onto the Decrypt/Verify button in the PGPtools
window.
The PGP Enter Passphrase dialog box appears asking you to enter
your passphrase.
3. Enter your passphrase and then click OK.
If the message has been signed, a message appears indicating
whether the signature is valid.
4. Click OK.
The decrypted file is saved in the same location as the original file.

Using PGP Secure Wipe to delete files
The Secure Wipe item on PGPmenu deletes files and their contents. The
Wipe feature is a secure way of permanently removing a file and its
contents from the hard drive of your computer. When you delete a file
normally by placing it in the Trash, the name of the file has been
removed from the file directory, but the data in the file is still there.
Secure Wipe removes all traces of a file’s data so that no one can use a
software tool to recover the file.

42

Chapter 5: Using PGP for Secure File Storage

To permanently delete a file
1. Select the file you want to delete.
2. Select Wipe from PGPmenu.
A confirmation dialog box appears.
3. Click OK to permanently erase the file

ALERT

Even on systems with virtual memory, PGP correctly writes over all
the contents of the file. It is worth noting that application programs
that may have saved the file prior to encrypting the file, may have
left fragments of the file on your disk in locations which are no
longer considered part of the file. See “Swap files or virtual
memory” in Chapter 8. Also, be aware that many programs
automatically save files in progress, so there may be backed up
copies of the file that you want to delete.
You may wish to use a third-party utility to wipe all free space on
your disk to solve this problem. PGP does not currently have this
feature.

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

43

44

Chapter 5: Using PGP for Secure File Storage

Chapter 6

Managing Keys and
Setting Preferences
This chapter explains how to examine and manage the keys stored
on your digital keyrings. It also describes how to set your
preferences to suit your particular computing environment.

Managing your keys
The keys you create, as well as those you collect from others, are stored
on digital keyrings, which are essentially files stored on your hard drive
or on a floppy disk. Normally your private keys are stored in a file
named PGP Private Keys and your public keys are stored in another file
named PGP Public Keys. These files are usually located in the PGP
Keyrings folder. .

NOTE:

If you are not comfortable storing your keys in the usual place, you
can choose a different filename or location. For details, see “Setting
your preferences,” later in this chapter.

Occasionally, you may want to examine or change the attributes
associated with your keys. For instance, when you obtain someone’s
public key, you might want to identify its type (either RSA or DiffieHellman/DSS), check its fingerprint, or determine its validity based on
any digital signatures included with the key. You may also want to sign
someone’s public key to indicate that you believe it is valid, assign a
level of trust to the key’s owner, or change a passphrase for your private

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

45

key. You may want to search a key server for someone’s key. You
perform all of these key-management functions from the PGPkeys
window.

The PGPkeys window
To open the PGPkeys application, choose PGPkeys from PGPmenu or
double-click the application icon in the program folder.
The PGPkeys window displays the keys you have created for yourself,
as well as any public keys you have added to your public keyring.
Double keys represent the private and public key pairs you have created
for yourself, and single keys represent the public keys you have
collected from others. If you have more than one type of key, you will
notice that RSA-type keys are blue skeleton keys and Diffie-Hellman/
DSS keys are yellow modern keys.
By clicking on the Disclosure Triangle at the left side of the key icon, you
can expand the entries to reveal the user ID and e-mail address for the
owner of the key as represented by the figure icons. By clicking an
envelope icon, you can see the signatures of any users who have
certified the key, as represented by one of four icons). If you don’t want
to expand each key individually, simply select the keys of interest and
then choose Expand All from the Edit menu.
PGPkeys attribute definitions
Along the top of the window are labels that correspond to the attributes
associated with each key.

46

Name

Shows an iconic representation of the key along with
the user name and e-mail address of the owner, and the
names of the key’s signers.

Validity

Indicates the level of confidence that the key actually
belongs to the alleged owner. The validity is based on
who has signed the key and how well you trust the
signer(s) to vouch for the authenticity of a key. The
public keys you sign yourself have the highest level of
validity, based on the assumption that you only sign
someone’s key if you are totally convinced that it is
valid. The validity of any other keys, which you have

Chapter 6: Managing Keys and Setting Preferences

not personally signed, depends on the level of trust you
have granted to any other users who have signed the
key. If there are no signatures associated with the key,
then it is not considered valid, and a message indicating
this fact appears whenever you use the key.
Validity is indicated by either diamonds or striped bars,
indicating Implicit trust, and circles or half-filled bars
for Marginal trust. When you click a diamond or a
circle, the Key Signing dialog box appears, in which
you can sign other user’s keys. See “Signing someone’s
public key,” later in this chapter.
Validity is represented by either dot and diamond
shapes or bar shapes. The diamonds represent implicit
validity, filled circles represent complete validity (or
marginal also if the Treat Marginally Valid Keys as
Invalid check box is deselected in the Advanced tab of
the Preferences dialog box). Empty circles represent no
validity or marginal validity if the Treat Marginally
Valid Keys as Invalid check box is selected.
Trust

Indicates the level of trust you have granted to the
owner of the key to serve as an introducer for the public
keys of others. This trust comes into play when you are
unable to verify the validity of someone’s public key for
yourself and instead rely on the judgment of other
users who have signed the key. When you create a pair
of keys, they are considered implicitly trustworthy, as
shown by the striping in the trust and validity bars, or
by a diamond validity indicator.
When you receive a public key from someone that has
been signed by another of the user’s keys on your
public keyring, the level of authenticity is based on the
trust you have granted to the signer of that key. You
assign a level of trust, either Complete, Marginal, or
Untrusted, in the Key Properties dialog box.
Trust is not displayed initially in the PGP keys display.
You can display the Trust column by choosing
Columns from the View menu. Click the Validity

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

47

button in the main window to open the Sign Key dialog
box. The Validity button changes to a diamond when
the key is axiomatic.
If Implicit Trust is set to on, trust is displayed with a
diamond. You can change trust through the Key
Properties menu. Trust options are None, Marginal,
and Complete.
Creation

Shows the date when the key was originally created.
You can sometimes make an assumption about the
validity of a key based on how long it has been in
circulation. If the key has been in use for a while, it is
less likely that someone will try to replace it because
there are many other copies in circulation. Never rely
on creation dates as the sole indicator of validity.

Expiration

Shows the date when the key will expire. Most keys are
set to Never; however, there may be instances when
you want to use a key for a fixed period of time.

Corporate Message Recovery Key
Shows whether the key has an associated Corporate
Message Recovery Key. (See Chapter 2 for a definition.)
Size

48

Shows the number of bits used to construct the key.
Generally, the larger the key, the less chance that it will
ever be compromised. However, larger keys require
slightly more time to encrypt and decrypt data than do
smaller keys. When you create a Diffie-Hellman/DSS
key, there is one number for the Diffie-Hellman portion
and another number for the DSS portion. The DSS
portion is used for signing, and the Diffie-Hellman
portion for encryption.

Chapter 6: Managing Keys and Setting Preferences

Examining a key’s properties
In addition to the general attributes shown in the PGPkeys window, you
can also examine and change other key properties. To access the
properties for a particular key, select the desired key and then choose
Info from the Keys menu.
Key ID

A unique identifying number associated with each key.
This identification number is useful for distinguishing
between two keys that share the same user name and email address.

Created

The date when the key was created.

Key Type

The key type, either RSA or Diffie-Hellman/DSS.

Expires

The date when the key expires. Owners specify this
date when they create their keys, and the value is
usually set to Never. However, some keys are set to
expire on a particular date if the owner wants them to
be used for a limited period of time.

Key Size

The size of the key.

Cipher

CAST, Triple DES, or IDEA. This is the “preferred”
cipher by which the owner of the key requests that you
encrypt to his orher key. If this algorithm is allowed in
your Advanced preferences, it will be used whenever
encrypting to this key.

Trust Model

Indicates the validity of the key based on its
certification and the level of trust you have in the owner
to vouch for the authenticity of someone else’s public
key. You set the trust level by sliding the bar to the
appropriate level (Complete, Marginal, or Untrusted).
The bar is disabled for revoked, expired, and implicitly
trusted keys.

Enabled

Indicates whether the key is currently enabled. When a
key is disabled, it is dimmed in the PGPkeys window
and is not available for performing any PGP functions
except Decrypt and Verify. However, the key remains
on your keyring and you can enable it again at any
time. To enable or disable a key, select or clear the
Enabled check box. (The check box is not visible for

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

49

implicitly trusted keys.) This feature is useful for
preventing seldom-used keys from cluttering up the
Key Selection dialog box when you are sending
encrypted e-mail.
Fingerprint

A unique identification number that is generated when
the key is created. This is the primary means by which
you can check the authenticity of a key. One good way
to check a fingerprint is to have the owner read their
fingerprint to you over the phone so that you can
compare it with the fingerprint shown for your copy of
their public key. You can also check the authenticity of
someone’s key by comparing the fingerprint on your
copy of their public key to the one listed on a key
server, since it is assumed that the owner periodically
checks to make sure that it remains valid.

Change Passphrase
Changes the passphrase for a private key. If you ever
think that your passphrase is no longer a secret
(perhaps you caught someone looking over your
shoulder), click this button to enter a new passphrase.
It is a good idea to change your passphrase every 6
months or so. For instructions on changing your
passphrase, see “Changing your Passphrase,” later in
this chapter.

Specifying a default key pair
When you sign a message or someone’s public key, your default key
pair is used. If you have more than one pair of keys, you may want to
specifically designate one pair as your default pair. The current default
key pair is displayed in bold type to distinguish these keys from your
other keys.
To specify your default key pair
1. Select the key pair you want designated as your default pair.
2. Choose Set Default from the Keys menu.

50

Chapter 6: Managing Keys and Setting Preferences

The selected key pair is displayed in bold type, indicating that it is
now designated as your default key pair.

Adding a new user name or address
You may have more than one user name or e-mail address for which you
want to use the same set of keys. After creating a new set of keys, you
can add alternate names and addresses to the key. You can only add a
new user name or e-mail address if you have both the private and public
keys.
To add a new user name or address to an existing key
1. Select the key pair for which you want to add another user name or
address.
2. Choose Add Name from the Keys menu.
The PGP New User Name dialog box appears.
3. Enter the new name and e-mail address in the appropriate fields,
and then Click OK.
The PGP Enter Passphrase dialog box appears.
4.

Enter your passphrase and then click OK.
The new name is added to the end of the user name list associated
with the key. If you want to set the new user name and address as
the primary identifier for your key, select the name and address and
then choose Set Primary Name from the Keys menu.

Checking a key’s fingerprint
It is often difficult to know for sure that a key belongs to a particular
individual unless that person physically hands the key to you on a
floppy disk. Exchanging keys in this manner is not usually practical,
especially for users who are located many miles apart, but you can rely
on the unique fingerprint associated with each key to verify that a key
does indeed belong to the alleged owner. There are several ways to
check a key’s fingerprint, but the safest is to call the person and have
them read the fingerprint to you over the phone. It is highly unlikely
that someone would be able to intercept this random call and imitate the
PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

51

person you expect to hear on the other end. You can also compare the
fingerprint on your copy of someone’s public key to the fingerprint
listed for their original key on a public server.
To check a key’s fingerprint
1. Select the key for the fingerprint you want to check.
2. Choose Info from the Keys menu.
3. Note the fingerprint and compare it to the original.

Signing someone’s public key
When you create a set of keys, they are automatically signed using your
public key. Similarly, once you are sure that a key belongs to the proper
individual, you can sign that person’s public key, indicating that you are
sure it is a valid key.
To sign someone’s public key
1.

Select the key you want to sign.

2. Choose Sign from the Keys menu.
The PGPkeys alert box appears.
3. Click OK to indicate your certainty that the key does indeed belong
to the purported owner.
The Enter Passphrase dialog box appears.
4. Enter your passphrase and then click OK.
An icon associated with your user name is now included with the
public key that you just signed.
If you would like to send the key with your signature to a key server,
select the Send Signature to Keyserver check box. The public key on
the server is updated to reflect the inclusion of your signature. Most
users prefer to use their discretion when allowing others to sign their
keys, so it’s always a good idea to check with the owner before you
add your signature to their key on the server.
You are then asked to enter the passphrase for your default key pair.
52

Chapter 6: Managing Keys and Setting Preferences

5. Enter your passphrase and then click OK. If you have another key
pair that you want to sign with, click the down arrow and select the
desired key.
A check box is displayed that says “Allow signature to be exported.
Others may rely on your signature.”
6. Select this check box if you want your signature to be exported.
An exportable signature is one that is allowed to be sent to servers
and travels with the key whenever it is exported, such as by
dragging it to an e-mail message.
When you sign someone’s public key, an icon associated with your
user name is shown for that key.

Granting trust for key validations
Besides certifying that a key belongs to someone, you can assign a level
of trust to the user of the keys indicating how well you trust them to act
as an introducer to others whose keys you may get in the future. This
means that if you ever get a key from someone that has been signed by
an individual whom you have designated as trustworthy, the key is
considered valid even though you have not done the check yourself.
To grant trust for a key
1. Select the key for which you want to change the trust level.
2. Choose Info from the Keys menu.
The Information dialog box appears.
3. Use the Trust Level sliding bar to choose the appropriate level of
trust for the key: Never, Marginal, or Complete.
4. Close the dialog box to accept the new setting.

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

53

Disabling and enabling keys
Sometimes you may want to temporarily disable a key. The ability to
disable keys is useful when you want to retain a public key for future
use, but you don’t want it cluttering up your recipient list every time
you send mail.
To disable a key
1. Select the key you want to disable.
2. Choose Info from the Keys menu.
The Information dialog box appears.
3. Clear the Enabled check box. (The Enabled check box does not
appear for implictly trusted keys.)
4. Close the dialog box to accept the new setting.
The key is dimmed and is temporarily unavailable for use.
To enable a key
1. Select the key you want to enable.
2. Choose Info from the Keys menu.
The Information dialog box appears.
3. Select the Enabled check box.
4. Close the dialog box to accept the new setting.
The key becomes visible and can be used as before.

Deleting a key or signature
At some point you may want to remove a key, a signature, or a user ID
associated with a particular key.
To delete a key, signature, or user ID
1. Select the key, signature, or user ID you want to delete.
2.
54

Choose Clear from the Edit menu or press the Delete key.
Chapter 6: Managing Keys and Setting Preferences

Changing your Passphrase
It’s a good idea to change your passphrase periodically.
To change your passphrase
1. Select the key pair for which you want to change the passphrase.
2. Choose Info from the Keys menu.
The Information dialog box appears.
3. Click Change Passphrase.
The Change Passphrase dialog box appears.
4. Enter your old passphrase in the top field and then press the Tab key
to advance to the next field.
5. Enter your new passphrase in the center dialog box and then press
the Tab key to advance to the bottom field
6. Confirm your entry by entering your new passphrase again.
7. Click OK.

Importing and Exporting Keys
Although you often distribute your public key and obtain the public
keys of others by cutting and pasting the raw text from a public or
corporate key server, you can also exchange keys by importing and
exporting them as separate text files. For instance, someone could hand
you a disk containing their public key, or you might want to make your
public key available over an FTP server.
To import a key from a file
1. Select the key you want to import.
2. Choose Import from the Keys menu.
The Import dialog box appears.
3. Select the file that contains the key you want to import and then click
Open.
PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

55

The imported key appears in the PGPkeys window, where you can
use it to encrypt data or to verify someone’s digital signature.
To add a key from an e-mail message
If a colleague sends you an e-mail message with their key enclosed (as a
block of text) you can add it to your keyring.
1. With the e-mail message window open, open the PGPkeys window.
2. Tile the two windows so that you can see part of the PGPkeys
window behind the message window.
3. Select the key text, including the START BLOCK and END BLOCK
texts. Drag the text onto the PGPkeys window.
4. The new key or keys appear in the PGPkeys window.
To export a key to a file
1. Select the key you want to export to a file.
2. Choose Export from the Keys menu.
The Export dialog box appears.
3. Enter the name of the file to which you want the key to be exported
and then click Save.
The exported key is saved to the named file in the specified folder
location.

Revoking a key
If the situation ever arises that you no longer trust your personal key
pair, you can issue a revocation to the world telling everyone to stop
using your public key. The best way to circulate a revoked key is to place
it on a public key server.
To revoke a key
1. Select the key pair to revoke.
2. Choose Revoke from the Keys menu.

56

Chapter 6: Managing Keys and Setting Preferences

The Revocation Confirmation dialog box appears.
3. Click OK to confirm your intent to revoke the selected key.
The PGP Enter Passphrase dialog box appears.
4. Enter your passphrase and then click OK.
When you revoke a key, it is crossed out with a red line to indicate
that it is no longer valid.
5. Send the revoked key to the server so everyone will know not to use
your old key.
It is possible that you might forget your passphrase someday. In that
case, you would never be able to use your key again, and you would
have no way of revoking your old key when you create a new one. To
safeguard against this possibility, you can create a revocation key by
making a copy of your private key, revoking the copy, and saving the
revoked copy in a safe place. You can then send the revoked copy to a
public key server if you ever forget your password. However, you
should be very careful about where you store the revoked version of
your key. If someone were to get hold of the revoked key, they could
revoke your key without your approval.

Setting your preferences
PGP is configured to accommodate the needs of most users, but you
have the option of adjusting some of the settings to suit your particular
computing environment. You specify these settings through the
Preferences dialog box, which you can access by choosing Preferences
from the PGPkeys Edit menu.

General preferences
You specify general encryption settings from the General pane.
Always Encrypt to Default Key
When this setting is selected, all the e-mail messages
and file attachments you encrypt with a recipient’s
public key are also encrypted to you using your default

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

57

public key. It is useful to leave this setting turned on so
that you have the option of decrypting the contents of
any e-mail or files you have previously encrypted.
Cache Decryption Passphrase
This setting specifies the amount of time (in hours:
minutes: seconds) that your encryption passphrase is
stored in your computer’s memory. If you regularly
compose or read several e-mail messages in succession,
you may want to increase the amount of time your
passphrase is cached so you don’t have to enter it over
and over again to get through all of your mail.
However, remember that the longer your passphrase is
stored in your computer’s memory, the more time a
sophisticated snooper has to get hold of this highly
compromising bit of information. The default setting is
2 minutes, which is probably sufficient to perform most
of your PGP chores without having to enter your
passphrase too many times, but not long enough for
you to walk away and an attacker to find it in your
computer’s memory.
Cache Signing Passphrase
This setting specifies the amount of time (in hours:
minutes: seconds) that your signing passphrase is
stored in your computer’s memory. If you regularly
compose or read several e-mail messages in succession,
you may want to increase the amount of time your
passphrase is cached so you don’t have to enter it over
and over again to get through all of your mail.

NOTE:

58

Using your passphrase for signing is considered a greater threat in
some cases because the attacker can impersonate you. This is the
reason why the two caching options are handled separately. The
cache times begin and reset every time you sign a message and
erases the passphrase from memory immediately when the timer
expires. This cache defaults to off due to the potentially complex
security implications.

Chapter 6: Managing Keys and Setting Preferences

Faster Key Generation
When this setting is selected, less time is required to
generate a new Diffie-Hellman/DSS key pair. This
process is speeded up by using a previously calculated
set of prime numbers rather than going through the
time-consuming process of creating them from scratch
each time a new key is generated. However, remember
that fast key generation is only implemented for the
fixed key sizes above 1024 and below 4096 provided as
options when you create a key, and is not used if you
enter some other value. Although it would be just about
impossible for anyone to crack your key based on their
knowledge of these canned prime numbers, some may
want to spend the extra time to create a key pair with
the maximum level of security.
The general belief in the cryptographic community is
that using canned primes provides no decrease in
security for the Diffie-Hellman/DSS algorithms. If this
feature makes you uncomfortable, you may turn it off.
For more information, read the FAQ located on the PGP
website.
Warn Before Wiping Files
When this setting is selected, a dialog box appears
before you wipe a file to give you one last chance to
change your mind before PGP securely overwrites the
contents of the file and deletes it from your computer.

Files preferences
Click the Files tab to advance to the pane in which you specify the
location of the keyrings used to store your private and public keys.
Set Public Keyring Location
Shows the current location and name of the file where
the PGP program expects to find your public keyring
file. If you plan to store your public keys in a file with a
different name or in some other location, you specify
this information here. You can use the Browse button to

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

59

search through your files rather than having to type the
path. The location you specify will be used also to store
all automatic backups of the public keyring.
Set Private Key Ring Location
Shows the current location and name of the file where
the PGP program expects to find your private keyring
file. If you plan to store your private keys in a file with a
different name or in some other location, you specify
this information here. Some users like to keep their
private keyring on a floppy disk, which they insert like
a key whenever they need to sign or decrypt mail. The
location you specify will be used also to store all
automatic backups of the public keyring.
Set Random Seed Location
Shows the location of the Random Seed file. Some
users may wish to keep their Random Seed file in a
secure location to prevent tampering. This attack is
very difficult and PGP has many different protections
against it.

E-mail preferences
Click the E-mail tab to advance to the pane in which you specify
preferences that affect the way PGP functions are implemented for your
particular e-mail application. Remember that not all of the selections
may apply to your particular e-mail application.
Use PGP/MIME Encryption
If you are using Eudora and you turn this setting on, all
of your e-mail messages and file attachments are
automatically encrypted to the intended recipient. This
setting has no effect on other encryptions you perform
from PGPmenu and should not be used if you plan to
send e-mail to recipients who use e-mail applications
that are not supported by the PGP/MIME standard.
Using Eudora, attachments will always be encrypted
regardless of this setting, but if the recipient does not
have PGP/MIME, the decryption provess will be more
manual.
60

Chapter 6: Managing Keys and Setting Preferences

Use PGP/MIME Signing
If you are using Eudora and you turn this setting on, all
of your e-mail messages and file attachments
automatically include your digital signatures. This
setting has no effect on other signatures you add from
PGPmenu and should not be used if you plan to send email to recipients who are using applications that do
not support the PGP/MIME standard.
Word wrap clear-signed messages at column [ ]
This setting specifies the column number where a hard
carriage return is used to wrap the text in your digital
signature to the next line. This feature is necessary
because not all applications handle word wrapping in
the same way, which could cause the lines in your
digitally signed messages to be broken up in a way that
cannot be easily read. The default setting is 70, which
prevents problems with most applications.

ALERT

If you change the word-wrap setting in PGP, make sure that it is less
than the word-wrap settings in your e-mail application. If you set it
to be the same or a greater length, carriage returns may be added
that invalidate your PGP signature.

PGPmenu preferences
Click the PGPmenu tab to advance to the pane where you add and
remove PGPmenu for various applications.
Add

Use this option to add the PGP icon to the menu bar of
the applications you select. For example, click the Add
button and add SimpleText to the application list. The
PGP icon is added to the SimpleText menu bar, so that
you can sign, encrypt, decrypt, and verify the selected
text in the document.
The PGP icon is automatically available on the Finder
menu bar, so that you can encrypt entire folders while
using the Finder. Simply select the folder you want to
encrypt and then choose Encrypt from PGPmenu.

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

61

Double-clicking an application name in the PGPmenu
preferences brings up an Advanced PGPmenu
Preferences dialog for that particular application, which
contains settings that may help if you experience any
compatibility problems using PGPmenu with a
particular application
Remove

Use this option to remove the PGP icon from the menu
bar of applications you have previously selected.

Server preferences
Click the Server tab to advance to the pane where you specify settings
for the key server you are using.
Domain

Specifies the Internet domain (such as “company.com”)
for the public key server that is used by PGP to send
and retrieve public keys. When sending keys to a
server, this domain is used to look up the appropriate
server based on the key’s domain.

Port

The port address for the public key server. You must
enter the address in full URL format, such as “http://
pgpkeys.mit.edu:11371”. The LDAP protocol is also
supported.

Set Default

Specifies which server to use if no server can be found
for the domain of the key.

Advanced preferences
Click the Advanced tab to advance to the pane in which you make the
following choices.
Encryption algorithm
You can select the encryption algorithm for your PGP
keys: CAST (the default), IDEA, or Triple-DES. If you
want to use IDEA or Triple-DES, you must make the
selection before you generate your keys.

62

Chapter 6: Managing Keys and Setting Preferences

CAST is a new algorithm that PGP and other
cryptographers have very high confidence in, and
Triple-DES is a U.S. Government algorithm that has
withstood the test of time. IDEA is the algorithm
traditionally used in PGP. For more information about
these algorithms, see “The PGP symmetric algorithms”
in Chapter 8.
There are two reasons why PGP gives you the option to change
encryption algorithms:
• When using conventional encryption, the cipher selected here is
used to encrypt.
• When creating a key, the preferred cipher is recorded as part of the
key so that other people use that algorithm when encrypting to you.

ALERT

Use the CAST, IDEA, and Triple-DES check boxes only if you have
suddenly decided that a particular algorithm is insecure. For
example, if you become aware that Triple-DES has been broken,
you can deselect that check box and all new keys you generate will
have a record that Triple-DES may not be used when encrypting to
you.

Display Marginal Validity Level
Use this check box to specify whether to display
marginally valid keys or simply to show validity as on
or off. Green circles indicate that a key is valid; gray
indicates that the key has not been validated, has not
been signed by either a trusted introducer or by you.
Treat marginally valid keys as invalid
Use this check box to specify whether to treat all
marginally valid keys as invalid. Selecting this option
causes the Key Selection dialog box to appear whenever
you encrypt to marginally valid keys.

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

63

Searching for a key
You can search for keys on local keyrings and remote key servers.
To search for a user’s key
1. Open the PGPkeys window.
2. Choose Search from the Keys menu.
The PGPkeys Search window appears.
3. Choose the location you wish to search from the Search for Keys On
menu.
4. Specify your search criteria:
The default is User ID, but you can click the drop-down menu to
select Key ID, Key Type, Creation Date, Expiration Date, Key Status,
or Key Size. For example, you might search for all keys with the User
ID of Fred.
5. Specify the condition for which you are searching.
You can use any of the following conditions:
• Is
• Is Not
• Contains
• Does Not Contain
• Is Signed By
• Is Not Signed By
• Is On (for Creation or Expiration Dates)
• Is On or Before (for Creation or Expiration Dates)
• Is On or After (for Creation or Expiration Dates)
6. Enter the value you want to search for.
7. Click More Choices to add additional criteria to your search; for
example, Key IDs with the name Fred created on or before October 6,
1997.
64

Chapter 6: Managing Keys and Setting Preferences

8. To begin the search, click Search.
A progress bar shows that the search is being conducted.

NOTE:

To cancel a search in progress, click Stop Search

The results of the search appear in the window.
9. Click Clear Search to empty the search window and clear your
search criteria.

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

65

66

Chapter 6: Managing Keys and Setting Preferences

Chapter 7

Troubleshooting PGP
This chapter presents information about potential problems and
suggests solutions.
Error

Cause

Solution

No secret keys could
be found on your
keyring.

There are no secret
keys on your keyring.

Generate your own pair of keys in
PGPkeys.

The evaluation time
for PGP encrypting
and signing has
passed. Operation
aborted.

The product evaluation time has
expired.

Download the freeware version or
buy the commercial version of the
product.

The PGP library has
run out of memory.

The operating system has run out of
memory.

Close other running programs. If that
doesn't work, you may need more
memory in your machine.

There was an error
opening or writing the
keyring or the output
file.

A file that was
needed couldn't be
opened.

Make sure the settings in your PGP
Preferences is correct. If you've
recently deleted files in the directory
that you installed PGP, you may need
to re-install the product.

Unable to perform
operation because
this file is read-only or
otherwise protected. If
you store your
keyring files on
removable media the
media may not be
inserted.

A file that was
needed is set to
read-only or is
being used by
another program.

Close other programs that may be
accessing the same files as the program you are running. If you keep
your keyring files on a floppy disk,
check and make sure that the floppy
disk is in the floppy drive.

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

67

68

Error

Cause

Solution

There was an error
during the writing of
the keyring or the
exported file.

The program failed
to write data to a
certain file.

Your hard drive may be full, or if the
file is on a floppy, the floppy is not
present in the floppy drive.

The action could not
be completed due to
an invalid file
operation.

The program failed
to read or write data
in a certain file.

The file is probably corrupt. Try altering your PGP Preferences to use a
different file, if possible.

The passphrase you
entered does not
match the passphrase
on the key.

The passphrase you
entered is incorrect.

You may have the CAPS LOCK on, or
you simply may have mis-typed the
passphrase. Try again.

The keyring contains
a bad (corrupted) PGP
packet.

The PGP message
that you are working
with has been corrupted, or your keyring has been
corrupted.

Ask the sender to re-send the message if it's a message that you're
working with. If it's your keyring, try
restoring from your backup keyring.

The specified user ID
was not added
because it already
exists on the selected
key.

You can't add a
User ID to a key if
there is one just like
it already on the
key.

Try adding a different user ID, or
delete the matching one first.

The keyring contains
a bad (corrupted) PGP
packet.

The PGP message
that you are working
with has been corrupted, or your keyring has been
corrupted.

Ask the sender to re-send the message if it's a message that you're
working with. If it's your keyring, try
restoring from your backup keyring.

The specified user ID
was not added
because it already
exists on the selected
key.

You can't add a
User ID to a key if
there is one just like
it already on the
key.

Try adding a different user ID, or
delete the matching one first.

This key is already
signed by the
specified signing key.

You can't sign a key
that you have
already signed.

You may have accidentally picked
the wrong key. If so, choose a different key to sign.

Chapter 7: Troubleshooting PGP

Error

Cause

Solution

The specified key
could not be found
on your keyring.

The key needed to
decrypt the current
message is not on
your keyring.

Ask the sender of the message to
resend the message and make sure
they encrypt the message to your
public key.

The message/data
contains a detached
signature.

The signature for the
message/file is
located in a separate file.

Double-click on the detached signature file first.

Could not encrypt to
specified key because
it is a sign-only key.

The selected key can
only be used for
signing.

Choose a different key, or generate a
new key that can encrypt data.

Could not sign with
specified key because
it is an encrypt-only
key.

The selected key can
only be used for
encrypting.

Choose a different key, or generate a
new key that can sign data.

There is not enough
random data
currently available.

The random number generator
needs more input in
order to generate
good random numbers.

When prompted, move the mouse
around, or press random keys, in
order to generate input.

The specified input file
does not exist.

The file name typed
in does not exist.

Use the Explorer to find the exact
name and path of the file you want.

Cannot perform the
requested operation
because the output
buffer is too small.

The output is larger
than the internal
buffers can handle.

If you are encrypting or signing, you
may have to break up the message
and encrypt/sign smaller pieces at a
time. If you are decrypting or verifying, ask the sender to encrypt/sign
smaller pieces and re-send them to
you.

The keyring file is
corrupt.

The program failed
to read or write data
in a certain file.

There is a file that is probably corrupt
or missing. It may or may not be the
keyring file. Try using a different file
name or path, if possible.

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

69

70

Chapter 7: Troubleshooting PGP

Chapter 8

Security Features and
Vulnerabilities
This chapter contains introductory and background information about
cryptography written by Phil Zimmermann.
"Whatever you do will be insignificant, but it is very important that you do it."
—Mahatma Gandhi.

Why I wrote PGP
It’s personal. It’s private. And it’s no one’s business but yours. You may
be planning a political campaign, discussing your taxes, or having a
secret romance. Or you may be communicating with a political dissident
in a repressive country. Whatever it is, you don’t want your private
electronic mail (e-mail) or confidential documents read by anyone else.
There’s nothing wrong with asserting your privacy. Privacy is as applepie as the Constitution.
The right to privacy is spread implicitly throughout the Bill of Rights.
But when the United States Constitution was framed, the Founding
Fathers saw no need to explicitly spell out the right to a private
conversation. That would have been silly. Two hundred years ago, all
conversations were private. If someone else was within earshot, you
could just go out behind the barn and have your conversation there. No
one could listen in without your knowledge. The right to a private
conversation was a natural right, not just in a philosophical sense, but in
a law-of-physics sense, given the technology of the time.

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

71

But with the coming of the information age, starting with the invention
of the telephone, all that has changed. Now most of our conversations
are conducted electronically. This allows our most intimate
conversations to be exposed without our knowledge. Cellular phone
calls may be monitored by anyone with a radio. Electronic mail, sent
across the Internet, is no more secure than cellular phone calls. E-mail is
rapidly replacing postal mail, becoming the norm for everyone, not the
novelty it was in the past. And e-mail can be routinely and automatically
scanned for interesting keywords, on a large scale, without detection.
This is like driftnet fishing.
Perhaps you think your e-mail is legitimate enough that encryption is
unwarranted. If you really are a law-abiding citizen with nothing to
hide, then why don’t you always send your paper mail on postcards?
Why not submit to drug testing on demand? Why require a warrant for
police searches of your house? Are you trying to hide something? If you
hide your mail inside envelopes, does that mean you must be a
subversive or a drug dealer, or maybe a paranoid nut? Do law-abiding
citizens have any need to encrypt their e-mail?
What if everyone believed that law-abiding citizens should use
postcards for their mail? If a nonconformist tried to assert his privacy by
using an envelope for his mail, it would draw suspicion. Perhaps the
authorities would open his mail to see what he’s hiding. Fortunately, we
don’t live in that kind of world, because everyone protects most of their
mail with envelopes. So no one draws suspicion by asserting their
privacy with an envelope. There’s safety in numbers. Analogously, it
would be nice if everyone routinely used encryption for all their e-mail,
innocent or not, so that no one drew suspicion by asserting their e-mail
privacy with encryption. Think of it as a form of solidarity.
Until now, if the government wanted to violate the privacy of ordinary
citizens, they had to expend a certain amount of expense and labor to
intercept and steam open and read paper mail. Or they had to listen to
and possibly transcribe spoken telephone conversation, at least before
automatic voice recognition technology became available. This kind of
labor-intensive monitoring was not practical on a large scale. It was only
done in important cases when it seemed worthwhile.
Senate Bill 266, a 1991 omnibus anticrime bill, had an unsettling measure
buried in it. If this non-binding resolution had become real law, it would
have forced manufacturers of secure communications equipment to
insert special “trap doors” in their products, so that the government
72

Chapter 8: Security Features and Vulnerabilities

could read anyone’s encrypted messages. It reads, “It is the sense of
Congress that providers of electronic communications services and
manufacturers of electronic communications service equipment shall
ensure that communications systems permit the government to obtain
the plain text contents of voice, data, and other communications when
appropriately authorized by law.” It was this bill that led me to publish
PGP electronically for free that year, shortly before the measure was
defeated after vigorous protest by civil libertarians and industry groups.
The 1994 Digital Telephony bill mandated that phone companies install
remote wiretapping ports into their central office digital switches,
creating a new technology infrastructure for “point-and-click”
wiretapping, so that federal agents no longer have to go out and attach
alligator clips to phone lines. Now they’ll be able to sit in their
headquarters in Washington and listen in on your phone calls. Of
course, the law still requires a court order for a wiretap. But while
technology infrastructures can persist for generations, laws and policies
can change overnight. Once a communications infrastructure optimized
for surveillance becomes entrenched, a shift in political conditions may
lead to abuse of this new-found power. Political conditions may shift
with the election of a new government, or perhaps more abruptly from
the bombing of a federal building.
A year after the 1994 Digital Telephony bill passed, the FBI disclosed
plans to require the phone companies to build into their infrastructure
the capacity to simultaneously wiretap 1 percent of all phone calls in all
major U.S. cities. This would represent more than a thousandfold
increase over previous levels in the number of phones that could be
wiretapped. In previous years, there were only about a thousand courtordered wiretaps in the United States per year, at the federal, state, and
local levels combined. It’s hard to see how the government could even
employ enough judges to sign enough wiretap orders to wiretap 1
percent of all our phone calls, much less hire enough federal agents to sit
and listen to all that traffic in real time. The only plausible way of
processing that amount of traffic is a massive Orwellian application of
automated voice recognition technology to sift through it all, searching
for interesting keywords or searching for a particular speaker’s voice. If
the government doesn’t find the target in the first 1 percent sample, the
wiretaps can be shifted over to a different 1 percent until the target is
found, or until everyone’s phone line has been checked for subversive
traffic. The FBI says they need this capacity to plan for the future. This
plan sparked such outrage that it was defeated in Congress, at least this
PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

73

time around, in 1995. But the mere fact that the FBI even asked for these
broad powers is revealing of their agenda. And the defeat of this plan
isn’t so reassuring when you consider that the 1994 Digital Telephony
bill was also defeated the first time it was introduced, in 1993.
Advances in technology will not permit the maintenance of the status
quo, as far as privacy is concerned. The status quo is unstable. If we do
nothing, new technologies will give the government new automatic
surveillance capabilities that Stalin could never have dreamed of. The
only way to hold the line on privacy in the information age is strong
cryptography.
You don’t have to distrust the government to want to use cryptography.
Your business can be wiretapped by business rivals, organized crime, or
foreign governments. The French government, for example, is notorious
for using its signals intelligence apparatus against U.S. companies to
help French corporations get a competitive edge. Ironically, the United
States government’s restrictions on cryptography have weakened U.S.
corporate defenses against foreign intelligence and organized crime.
The government knows what a pivotal role cryptography is destined to
play in the power relationship with its people. In April 1993, the Clinton
administration unveiled a bold new encryption policy initiative, which
had been under development at the National Security Agency (NSA)
since the start of the Bush administration. The centerpiece of this
initiative is a government-built encryption device, called the Clipper
chip, containing a new classified NSA encryption algorithm. The
government has been trying to encourage private industry to design it
into all their secure communication products, such as secure phones,
secure faxes, and so on. AT&T has put Clipper into its secure voice
products. The catch: At the time of manufacture, each Clipper chip will
be loaded with its own unique key, and the government gets to keep a
copy, placed in escrow. Not to worry, though—the government
promises that they will use these keys to read your traffic only “when
duly authorized by law.” Of course, to make Clipper completely
effective, the next logical step would be to outlaw other forms of
cryptography.
The government initially claimed that using Clipper would be
voluntary, that no one would be forced to use it instead of other types of
cryptography. But the public reaction against the Clipper chip has been
strong, stronger than the government anticipated. The computer
industry has monolithically proclaimed its opposition to using Clipper.
74

Chapter 8: Security Features and Vulnerabilities

FBI director Louis Freeh responded to a question in a press conference in
1994 by saying that if Clipper failed to gain public support, and FBI
wiretaps were shut out by non-government-controlled cryptography,
his office would have no choice but to seek legislative relief. Later, in the
aftermath of the Oklahoma City tragedy, Mr. Freeh testified before the
Senate Judiciary Committee that public availability of strong
cryptography must be curtailed by the government (although no one
had suggested that cryptography was used by the bombers).
The Electronic Privacy Information Center (EPIC) obtained some
revealing documents under the Freedom of Information Act. In a
briefing document titled “Encryption: The Threat, Applications and
Potential Solutions,” and sent to the National Security Council in
February 1993, the FBI, NSA, and Department of Justice (DOJ)
concluded that “Technical solutions, such as they are, will only work if
they are incorporated into all encryption products. To ensure that this
occurs, legislation mandating the use of Government-approved
encryption products or adherence to Government encryption criteria is
required.”
The government has a track record that does not inspire confidence that
they will never abuse our civil liberties. The FBI’s COINTELPRO
program targeted groups that opposed government policies. They spied
on the antiwar movement and the civil rights movement. They
wiretapped the phone of Martin Luther King Jr. Nixon had his enemies
list. And then there was the Watergate mess. Congress now seems intent
on passing laws curtailing our civil liberties on the Internet. At no time
in the past century has public distrust of the government been so
broadly distributed across the political spectrum, as it is today.
If we want to resist this unsettling trend in the government to outlaw
cryptography, one measure we can apply is to use cryptography as
much as we can now while it’s still legal. When use of strong
cryptography becomes popular, it’s harder for the government to
criminalize it. Therefore, using PGP is good for preserving democracy.
If privacy is outlawed, only outlaws will have privacy. Intelligence
agencies have access to good cryptographic technology. So do the big
arms and drug traffickers. But ordinary people and grassroots political
organizations mostly have not had access to affordable “military grade”
public-key cryptographic technology. Until now.

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

75

PGP empowers people to take their privacy into their own hands.
There’s a growing social need for it. That’s why I created it.

Encryption basics
First, some elementary terminology. Suppose you want to send a
message to a colleague, whom we’ll call Alice, and you don’t want
anyone but Alice to be able to read it. As shown in Figure 1, you can
encrypt, or encipher, the message, which means scrambling it up in a
hopelessly complicated way, rendering it unreadable to anyone except
you and Alice. You supply a cryptographic key to encrypt the message,
and Alice must use the same key to decipher or decrypt it. At least that’s
how it works in conventional “secret key” encryption.
A single key is used for both encryption and decryption. This means that
this key must be initially transmitted via secure channels so that both
parties can know it before encrypted messages are sent over insecure
channels. This may be inconvenient. If you have a secure channel for
exchanging keys, then why do you need cryptography in the first place?

How public key cryptography works
In public key cryptography, as shown in Figure 2, everyone has two
related complementary keys, a public key and a private key. Each key
unlocks the code that the other key makes. Knowing the public key does
76

Chapter 8: Security Features and Vulnerabilities

not help you deduce the corresponding private key. The public key can
be published and widely disseminated across a communications
network.
This protocol provides privacy without the need for the same kind of
secure channels that conventional secret key encryption requires.
Anyone can use a recipient’s public key to encrypt a message to that
person, and that recipient uses her own corresponding private key to
decrypt that message. No one but the recipient can decrypt it, because
no one else has access to that private key. Not even the person who
encrypted the message with the recipient’s public key can decrypt it.

How your files and messages are encrypted
Because the public key encryption algorithm is much slower than
conventional single-key encryption, encryption is better accomplished
by using the process shown in Figure 3.

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

77

A high-quality fast conventional secret key encryption algorithm is used
to encipher the message. This original unenciphered message is called
“plaintext.” In a process invisible to the user, a temporary random key,
created just for this one “session,” is used to conventionally encipher the
plaintext file. Then the recipient’s public key is used to encipher this
temporary random conventional key. This public-key-enciphered
conventional “session” key is sent along with the enciphered text (called
“ciphertext”) to the recipient.

The PGP symmetric algorithms
PGP offers a selection of different secret key algorithms to encrypt the
actual message. By secret key algorithm, we mean a conventional, or
symmetric, block cipher that uses the same key to both encrypt and
decrypt. The three symmetric block ciphers offered by PGP are CAST,
Triple-DES, and IDEA. They are not “home-grown” algorithms. They
were all developed by teams of cryptographers with distinguished
reputations.

78

Chapter 8: Security Features and Vulnerabilities

For the cryptographically curious, all three ciphers operate on 64-bit
blocks of plaintext and ciphertext. CAST and IDEA have key sizes of 128
bits, while Triple-DES uses a 168-bit key. Like Data Encryption
Standard (DES), any of these ciphers can be used in cipher feedback
(CFB) and cipher block chaining (CBC) modes. PGP uses them in 64-bit
CFB mode.
I included the CAST encryption algorithm in PGP because it shows
promise as a good block cipher with a 128-bit key size, it’s very fast, and
it’s free. Its name is derived from the initials of its designers, Carlisle
Adams and Stafford Tavares of Northern Telecom (Nortel). Nortel has
applied for a patent for CAST, but they have made a commitment in
writing to make CAST available to anyone on a royalty-free basis. CAST
appears to be exceptionally well designed, by people with good
reputations in the field. The design is based on a very formal approach,
with a number of formally provable assertions that give good reasons to
believe that it probably requires key exhaustion to break its 128-bit key.
CAST has no weak or semiweak keys. There are strong arguments that
CAST is completely immune to both linear and differential
cryptanalysis, the two most powerful forms of cryptanalysis in the
published literature, both of which have been effective in cracking DES.
CAST is too new to have developed a long track record, but its formal
design and the good reputations of its designers will undoubtedly
attract the attentions and attempted cryptanalytic attacks of the rest of
the academic cryptographic community. I’m getting nearly the same
preliminary gut feeling of confidence from CAST that I got years ago
from IDEA, the cipher I selected for use in earlier versions of PGP. At
that time, IDEA was also too new to have a track record, but it has held
up well.
The IDEA (International Data Encryption Algorithm) block cipher is
based on the design concept of “mixing operations from different
algebraic groups.” It was developed at ETH in Zurich by James L.
Massey and Xuejia Lai, and published in 1990. Early published papers
on the algorithm called it IPES (Improved Proposed Encryption
Standard), but they later changed the name to IDEA. So far, IDEA has
resisted attack much better than other ciphers such as FEAL, REDOC-II,
LOKI, Snefru and Khafre. And IDEA is more resistant than DES to
Biham and Shamir’s highly successful differential cryptanalysis attack,
as well as attacks from linear cryptanalysis. As this cipher continues to
attract attack efforts from the most formidable quarters of the
cryptanalytic world, confidence in IDEA is growing with the passage of
PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

79

time. Sadly, the biggest obstacle to IDEA’s acceptance as a standard has
been the fact that Ascom Systec holds a patent on its design, and unlike
DES and CAST, IDEA has not been made available to everyone on a
royalty-free basis.
As a hedge, PGP includes three-key Triple-DES in its repertoire of
available block ciphers. The DES was developed by IBM in the mid1970s. While it has a good design, its 56-bit key size is too small by
today’s standards. Triple-DES is very strong, and has been well studied
for many years, so it might be a safer bet than the newer ciphers such as
CAST and IDEA. Triple-DES is the DES applied three times to the same
block of data, using three different keys, except that the second DES
operation is run backwards, in decrypt mode. While Triple-DES is much
slower than either CAST or IDEA, speed is usually not critical for e-mail
applications. Although Triple-DES uses a key size of 168 bits, it appears
to have an effective key strength of at least 112 bits against an attacker
with impossibly immense data storage capacity to use in the attack.
According to a paper presented by Michael Weiner at Crypto96, any
remotely plausible amount of data storage available to the attacker
would enable an attack that would require about as much work as
breaking a 129-bit key. Triple-DES is not encumbered by any patents.
PGP public keys that were generated by PGP Version 5.0 or later have
information embedded in them that tells a sender what block ciphers are
understood by the recipient’s software, so that the sender’s software
knows which ciphers can be used to encrypt. DSS/Diffie-Hellman
public keys accept CAST, IDEA, or Triple-DES as the block cipher, with
CAST as the default selection. At present, for compatibility reasons, RSA
keys do not provide this feature. Only the IDEA cipher is used by PGP to
send messages to RSA keys, because older versions of PGP only
supported RSA and IDEA.

Data compression
PGP normally compresses the plaintext before encrypting it, because it’s
too late to compress the plaintext after it has been encrypted; encrypted
data is not compressible. Data compression saves modem transmission
time and disk space and, more importantly, strengthens cryptographic
security. Most cryptanalysis techniques exploit redundancies found in
the plaintext to crack the cipher. Data compression reduces this

80

Chapter 8: Security Features and Vulnerabilities

redundancy in the plaintext, thereby greatly enhancing resistance to
cryptanalysis. It takes extra time to compress the plaintext, but from a
security point of view it’s worth it.
Files that are too short to compress, or that just don’t compress well, are
not compressed by PGP. In addition, the program recognizes files
produced by most popular compression programs, such as PKZIP, and
does not try to compress a file that has already been compressed.
For the technically curious, the program uses the freeware ZIP
compression routines written by Jean-Loup Gailly, Mark Adler, and
Richard B. Wales. This ZIP software uses compression algorithms that
are functionally equivalent to those used by PKWare’s PKZIP 2.x. This
ZIP compression software was selected for PGP mainly because it has a
really good compression ratio and because it’s fast.

About the random numbers used as session keys
PGP uses a cryptographically strong pseudo-random-number generator
for creating temporary session keys.If this random seed file does not
exist, it is automatically created and seeded with truly random numbers
derived from your random events gathered by the PGP program from
the timing of your keystroke and mouse movements.
This generator reseeds the seed file each time it is used, by mixing in
new material partially derived from the time of day and other truly
random sources. It uses the conventional encryption algorithm as an
engine for the random number generator. The seed file contains both
random seed material and random key material used to key the
conventional encryption engine for the random generator.
This random seed file should be protected from disclosure, to reduce the
risk of an attacker deriving your next or previous session keys. The
attacker would have a very hard time getting anything useful from
capturing this random seed file, because the file is cryptographically
laundered before and after each use. Nonetheless, it seems prudent to
try to keep it from falling into the wrong hands. If possible, make the file
readable only by you. If this is not possible, don’t let other people
indiscriminately copy disks from your computer.

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

81

How decryption works
As shown in Figure 4, the decryption process is just the reverse of
encryption. The recipient’s private key is used to recover the temporary
session key, and then that session key is used to run the fast
conventional secret key algorithm to decipher the large ciphertext
message.

How digital signatures work
PGP uses digital signatures to provide message authentication. The
sender’s own private key can be used to encrypt a message digest,
thereby “signing” the message. A message digest is a 160-bit or a 128-bit
cryptographically strong one-way hash function. It is somewhat
analogous to a “checksum” or CRC error checking code, in that it
compactly represents the message and is used to detect changes in the
message. Unlike a CRC, however, it is believed to be computationally
infeasible for an attacker to devise a substitute message that would
produce an identical message digest. The message digest gets encrypted
by the sender’s private key, creating a digital signature of the message.

82

Chapter 8: Security Features and Vulnerabilities

Figure 5 shows how a digital signature is generated.

The recipient (or anyone else) can verify the digital signature by using
the sender’s public key to decrypt it, as shown in Figure 6. This proves
that the sender was the true originator of the message, and that the
message has not been subsequently altered by anyone else, because the
sender alone possesses the private key that made that signature. Forgery
of a signed message is not feasible, and the sender cannot later disavow
his signature.

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

83

About the message digest
The message digest is a compact (160-bit or 128-bit) “distillate” of your
message or file checksum. You can also think of it as a “fingerprint” of
the message or file. The message digest “represents” your message, in
such a way that if the message were altered in any way, a different
message digest would be computed from it. This makes it possible to
detect any changes made to the message by a forger. A message digest is
computed using a cryptographically strong one-way hash function of
the message. It should be computationally infeasible for an attacker to
devise a substitute message that would produce an identical message
digest. In that respect a message digest is much better than a checksum,
because it is easy to devise a different message that would produce the
same checksum. But like a checksum, you can’t derive the original
message from its message digest.
The message digest algorithm now used in PGP (Version 5.0 and later) is
called SHA, which stands for Secure Hash Algorithm, designed by the
NSA for the National Institute of Standards and Technology (NIST).
84

Chapter 8: Security Features and Vulnerabilities

SHA is a 160-bit hash algorithm. Some people might regard anything
from the NSA with suspicion, because the NSA is in charge of
intercepting communications and breaking codes. But keep in mind that
the NSA has no interest in forging signatures, and the government
would benefit from a good unforgeable digital signature standard that
would preclude anyone from repudiating their signatures. That has
distinct benefits for law enforcement and intelligence gathering. Also,
SHA has been published in the open literature and has been extensively
peer reviewed by most of the best cryptographers in the world who
specialize in hash functions, and the unanimous opinion is that SHA is
extremely well designed. It has some design innovations that overcome
all the observed weaknesses in message digest algorithms previously
published by academic cryptographers. All new versions of PGP use
SHA as the message digest algorithm for creating signatures with the
new DSS keys that comply with the NIST Digital Signature Standard.
For compatibility reasons, new versions of PGP still use MD5 for RSA
signatures, because older versions of PGP used MD5 for RSA signatures.
The message digest algorithm used by older versions of PGP is the MD5
Message Digest Algorithm, placed in the public domain by RSA Data
Security, Inc. MD5 is a 128-bit hash algorithm. In 1996, MD5 was all but
broken by a German cryptographer, Hans Dobbertin. Although MD5
was not completely broken at that time, it was discovered to have such
serious weaknesses that no one should keep using it to generate
signatures. Further work in this area might completely break it,
allowing signatures to be forged. If you don’t want to someday find
your PGP digital signature on a forged confession, you might be well
advised to migrate to the new PGP DSS keys as your preferred method
for making digital signatures, because DSS uses SHA as its secure hash
algorithm.

How to protect public keys from tampering
In a public key cryptosystem, you don’t have to protect public keys from
exposure. In fact, it’s better if they are widely disseminated. But it’s
important to protect public keys from tampering, to make sure that a
public key really belongs to the person to whom it appears to belong.
This may be the most important vulnerability of a public key
cryptosystem. See “Protecting your keys” in Chapter 3 for procedures.
Let’s first look at a potential disaster, then describe how to safely avoid it
with PGP.
PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

85

Suppose you want to send a private message to Alice. You download
Alice’s public key certificate from an electronic bulletin board system
(BBS). You encrypt your letter to Alice with this public key and send it to
her through the BBS’s e-mail facility.
Unfortunately, unbeknownst to you or Alice, another user named
Charlie has infiltrated the BBS and generated a public key of his own
with Alice’s user ID attached to it. He covertly substitutes his bogus key
in place of Alice’s real public key. You unwittingly use this bogus key
belonging to Charlie instead of Alice’s public key. All looks normal
because this bogus key has Alice’s user ID. Now Charlie can decipher
the message intended for Alice because he has the matching private key.
He may even reencrypt the deciphered message with Alice’s real public
key and send it on to her so that no one suspects any wrongdoing.
Furthermore, he can even make apparently good signatures from Alice
with this private key because everyone will use the bogus public key to
check Alice’s signatures.
The only way to prevent this disaster is to prevent anyone from
tampering with public keys. If you got Alice’s public key directly from
Alice, this is no problem. But that may be difficult if Alice is a thousand
miles away or is currently unreachable.
Perhaps you could get Alice’s public key from a mutually trusted friend,
David, who knows he has a good copy of Alice’s public key. David
could sign Alice’s public key, vouching for the integrity of Alice’s public
key. David would create this signature with his own private key.
This would create a signed public key certificate, and would show that
Alice’s key had not been tampered with. This requires that you have a
known good copy of David’s public key to check his signature. Perhaps
David could provide Alice with a signed copy of your public key also.
David is thus serving as an “Introducer” between you and Alice.
This signed public key certificate for Alice could be uploaded by David
or Alice to the BBS, and you could download it later. You could then
check the signature via David’s public key and thus be assured that this
is really Alice’s public key. No impostor can fool you into accepting his
own bogus key as Alice’s because no one else can forge signatures made
by David.
A widely trusted person could even specialize in providing this service
of “introducing” users to each other by providing signatures for their
public key certificates. This trusted person could be regarded as a
86

Chapter 8: Security Features and Vulnerabilities

“Certifying Authority.” Any public key certificates bearing the
Certifying Authority’s signature could be trusted as truly belonging to
the person to whom they appear to belong to. All users who wanted to
participate would need a known good copy of just the Certifying
Authority’s public key, so that the Certifying Authority’s signatures
could be verified. In some cases, the Certifying Authority may also act as
a key server, allowing users on a network to look up public keys by
asking the key server, but there is no reason why a key server must also
certify keys.
A trusted centralized Certifying Authority is especially appropriate for
large impersonal centrally controlled corporate or government
institutions. Some institutional environments use hierarchies of
Certifying Authorities.
For more decentralized environments, allowing all users to act as
trusted introducers for their friends would probably work better than a
centralized key certification authority.
One of the attractive features of PGP is that it can operate equally well in
a centralized environment with a Certifying Authority or in a more
decentralized environment where individuals exchange personal keys.
This whole business of protecting public keys from tampering is the
single most difficult problem in practical public key applications. It is
the “Achilles heel” of public key cryptography, and a lot of software
complexity is tied up in solving this one problem.
You should use a public key only after you are sure that it is a good
public key that has not been tampered with, and that it actually belongs
to the person with whom it purports to be associated. You can be sure of
this if you got this public key certificate directly from its owner, or if it
bears the signature of someone else that you trust, from whom you
already have a good public key. Also, the user ID should have the full
name of the key’s owner, not just her first name.
No matter how tempted you are, you should never give in to expediency
and trust a public key you downloaded from a bulletin board, unless it is
signed by someone you trust. That uncertified public key could have
been tampered with by anyone, maybe even by the system
administrator of the bulletin board.

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

87

If you are asked to sign someone else’s public key certificate, make
certain that it really belongs to the person named in the user ID of that
public key certificate. This is because your signature on her public key
certificate is a promise by you that this public key really belongs to her.
Other people who trust you will accept her public key because it bears
your signature. It can be ill-advised to rely on hearsay—don’t sign her
public key unless you have independent first-hand knowledge that it
really belongs to her. Preferably you should sign it only if you got it
directly from her.
In order to sign a public key, you must be far more certain of that key’s
ownership than if you merely want to use that key to encrypt a message.
To be convinced of a key’s validity enough to use it, certifying
signatures from trusted introducers should suffice. But to sign a key
yourself, you should require your own independent first-hand
knowledge of who owns that key. Perhaps you could call the key’s
owner on the phone and read the key fingerprint to her, to confirm that
the key you have is really her key—and make sure you really are talking
to the right person.
Bear in mind that your signature on a public key certificate does not
vouch for the integrity of that person, but only vouches for the integrity
(the ownership) of that person’s public key. You aren’t risking your
credibility by signing the public key of a sociopath, if you are completely
confident that the key really belongs to him. Other people would accept
that key as belonging to him because you signed it (assuming they trust
you), but they wouldn’t trust that key’s owner. Trusting a key is not the
same as trusting the key’s owner.
It would be a good idea to keep your own public key on hand with a
collection of certifying signatures attached from a variety of
“introducers,” in the hope that most people will trust at least one of the
introducers who vouch for the validity of your public key. You could
post your key with its attached collection of certifying signatures on
various electronic bulletin boards. If you sign someone else’s public key,
return it to them with your signature so that they can add it to their own
collection of credentials for their own public key.
Make sure that no one else can tamper with your own public keyring.
Checking a newly signed public key certificate must ultimately depend
on the integrity of the trusted public keys that are already on your own
public keyring. Maintain physical control of your public keyring,
preferably on your own personal computer rather than on a remote
88

Chapter 8: Security Features and Vulnerabilities

time-sharing system, just as you would do for your private key. This is
to protect it from tampering, not from disclosure. Keep a trusted backup
copy of your public keyring and your private key on write-protected
media.
Since your own trusted public key is used as a final authority to directly
or indirectly certify all the other keys on your keyring, it is the most
important key to protect from tampering. You may want to keep a
backup copy on a write-protected floppy disk.
PGP generally assumes that you will maintain physical security over
your system and your keyrings, as well as your copy of PGP itself. If an
intruder can tamper with your disk, then in theory he can tamper with
the program itself, rendering moot the safeguards the program may
have to detect tampering with keys.
One somewhat complicated way to protect your own whole public
keyring from tampering is to sign the whole ring with your own private
key. You could do this by making a detached signature certificate of the
public keyring.

How does PGP keep track of which keys are valid?
Before you read this section, you should read the previous section,
“How to protect public keys from tampering.”
PGP keeps track of which keys on your public keyring are properly
certified with signatures from introducers that you trust. All you have to
do is tell PGP which people you trust as introducers, and certify their
keys yourself with your own ultimately trusted key. PGP can take it
from there, automatically validating any other keys that have been
signed by your designated introducers. And of course you can directly
sign more keys yourself.
There are two entirely separate criteria that PGP uses to judge a public
key’s usefulness—don’t get them confused:

1. Does the key actually belong to the person to whom it appears to
belong? In other words, has it been certified with a trusted signature?
2. Does it belong to someone you can trust to certify other keys?

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

89

PGP can calculate the answer to the first question. To answer the second
question, you must tell PGP explicitly. When you supply the answer to
question 2, PGP can then calculate the answer to question 1 for other
keys signed by the introducer you designated as trusted.
Keys that have been certified by a trusted introducer are deemed valid
by PGP. The keys belonging to trusted introducers must themselves be
certified either by you or by other trusted introducers.
PGP also allows for the possibility of your having several shades of trust
for people to act as introducers. Your trust for a key’s owner to act as an
introducer does not just reflect your estimation of their personal
integrity—it should also reflect how competent you think they are at
understanding key management and using good judgment in signing
keys. You can designate a person as untrusted, marginally trusted, or
completely trusted to certify other public keys. This trust information is
stored on your keyring with their key, but when you tell PGP to copy a
key off your keyring, PGP does not copy the trust information along
with the key, because your private opinions on trust are regarded as
confidential.
When PGP is calculating the validity of a public key, it examines the
trust level of all the attached certifying signatures. It computes a
weighted score of validity—for example, two marginally trusted
signatures are deemed to be as credible as one fully trusted signature.
The program’s skepticism is adjustable—for example, you can tune PGP
to require two fully trusted signatures or three marginally trusted
signatures to judge a key as valid.
Your own key is “axiomatically” valid to PGP, needing no introducer’s
signature to prove its validity. PGP knows which public keys are yours
by looking for the corresponding private keys on the private key. PGP
also assumes that you completely trust yourself to certify other keys.
As time goes on, you will accumulate keys from other people whom you
may want to designate as trusted introducers. Everyone else will choose
their own trusted introducers. And everyone will gradually accumulate
and distribute with their key a collection of certifying signatures from
other people, with the expectation that anyone receiving it will trust at
least one or two of the signatures. This will cause the emergence of a
decentralized fault-tolerant web of confidence for all public keys.

90

Chapter 8: Security Features and Vulnerabilities

This unique grass-roots approach contrasts sharply with standard public
key management schemes developed by government and other
monolithic institutions, such as Internet Privacy Enhanced Mail (PEM),
which are based on centralized control and mandatory centralized trust.
The standard schemes rely on a hierarchy of Certifying Authorities who
dictate who you must trust. The program’s decentralized probabilistic
method for determining public key legitimacy is the centerpiece of its
key management architecture. PGP lets you alone choose who you trust,
putting you at the top of your own private certification pyramid. PGP is
for people who prefer to pack their own parachutes.
Note that while this decentralized, grass-roots approach is emphasized
here, it does not mean that PGP does not perform equally well in the
more hierarchical, centralized public key management schemes. Large
corporate users, for example, will probably want a central figure or
person who signs all the employees’ keys. PGP handles that centralized
scenario as a special degenerate case of PGP’s more generalized trust
model.

How to protect private keys from disclosure
Protect your own private key and your passphrase very carefully. If
your private key is ever compromised, you’d better get the word out
quickly to all interested parties before someone else uses it to make
signatures in your name. For example, someone could use it to sign
bogus public key certificates, which could create problems for many
people, especially if your signature is widely trusted. And of course, a
compromise of your own private key could expose all messages sent to
you.
To protect your private key, you can start by always keeping physical
control of it. Keeping it on your personal computer at home is OK, or
keep it in your notebook computer that you can carry with you. If you
must use an office computer that you don’t always have physical control
of, then keep your public and private keyrings on a write-protected
removable floppy disk, and don’t leave it behind when you leave the
office. It wouldn’t be a good idea to allow your private key to reside on a
remote timesharing computer, such as a remote dial-in UNIX system.
Someone could eavesdrop on your modem line and capture your

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

91

passphrase and then obtain your actual private key from the remote
system. You should only use your private key on a machine that is
under your physical control. See Chapter 6 for additional information.
Don’t store your passphrase anywhere on the computer that has your
private key file. Storing both the private key and the passphrase on the
same computer is as dangerous as keeping your PIN in the same wallet
as your Automatic Teller Machine bank card. You don’t want somebody
to get their hands on your disk containing both the passphrase and the
private key file. It would be most secure if you just memorize your
passphrase and don’t store it anywhere but your brain. If you feel you
must write down your passphrase, keep it well protected, perhaps even
better protected than the private key file.
And keep backup copies of your private key—remember, you have the
only copy of your private key, and losing it will render useless all the
copies of your public key that you have spread throughout the world.
The decentralized noninstitutional approach that PGP supports for
management of public keys has its benefits, but unfortunately it also
means that you can’t rely on a single centralized list of which keys have
been compromised. This makes it a bit harder to contain the damage of a
private key compromise. You just have to spread the word and hope
that everyone hears about it.
If the worst case happens—your private key and passphrase are both
compromised (hopefully you will find this out somehow)—you will
have to issue a “key compromise” certificate. This kind of certificate is
used to warn other people to stop using your public key. You can use
PGP to create such a certificate by using the Revoke command from the
PGPkeys menu. Then you must somehow send this compromise
certificate to everyone else on the planet, or at least to all your friends
and their friends, et cetera. Their own PGP software installs this key
compromise certificate on their public keyrings and automatically
prevents them from accidentally using your public key ever again. You
can then generate a new private/public key pair and publish the new
public key. You could send out one package containing both your new
public key and the key compromise certificate for your old key.

92

Chapter 8: Security Features and Vulnerabilities

What if you lose your private key?
Normally, if you want to revoke your own private key, you can use the
Revoke command from the PGPkeys menu to issue a revocation
certificate, signed with your own private key.
But what can you do if you lose your private key, or if your private key
is destroyed? You can’t revoke it yourself, because you must use your
own private key to revoke it, and you don’t have it anymore. You ask
each person who signed your key to retire his or her certification. Then
anyone attempting to use your key based on the trust of one of your
introducers will know not to trust your public key.

Beware of snake oil
When examining a cryptographic software package, the question always
remains, why should you trust this product? Even if you examined the
source code yourself, not everyone has the cryptographic experience to
judge the security. Even if you are an experienced cryptographer, subtle
weaknesses in the algorithms could still elude you.
When I was in college in the early seventies, I devised what I believed
was a brilliant encryption scheme. A simple pseudorandom number
stream was added to the plaintext stream to create ciphertext. This
would seemingly thwart any frequency analysis of the ciphertext, and
would be uncrackable even to the most resourceful government
intelligence agencies. I felt so smug about my achievement.
Years later, I discovered this same scheme in several introductory
cryptography texts and tutorial papers. How nice. Other cryptographers
had thought of the same scheme. Unfortunately, the scheme was
presented as a simple homework assignment on how to use elementary
cryptanalytic techniques to trivially crack it. So much for my brilliant
scheme.
From this humbling experience I learned how easy it is to fall into a false
sense of security when devising an encryption algorithm. Most people
don’t realize how fiendishly difficult it is to devise an encryption
algorithm that can withstand a prolonged and determined attack by a
resourceful opponent. Many mainstream software engineers have
developed equally naive encryption schemes (often even the very same

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

93

encryption scheme), and some of them have been incorporated into
commercial encryption software packages and sold for good money to
thousands of unsuspecting users.
This is like selling automotive seat belts that look good and feel good,
but snap open in the slowest crash test. Depending on them may be
worse than not wearing seat belts at all. No one suspects they are bad
until a real crash. Depending on weak cryptographic software may
cause you to unknowingly place sensitive information at risk when you
might not otherwise have done so if you had no cryptographic software
at all. Perhaps you may never even discover that your data has been
compromised.
Sometimes commercial packages use the Federal Data Encryption
Standard (DES), a fairly good conventional algorithm recommended by
the government for commercial use (but not for classified information,
oddly enough—Hmmm). There are several “modes of operation” that
DES can use, some of them better than others. The government
specifically recommends not using the weakest simplest mode for
messages, the Electronic Codebook (ECB) mode. But they do
recommend the stronger and more complex Cipher Feedback (CFB) and
Cipher Block Chaining (CBC) modes.
Unfortunately, most of the commercial encryption packages I’ve looked
at use ECB mode. When I’ve talked to the authors of a number of these
implementations, they say they’ve never heard of CBC or CFB modes,
and don’t know anything about the weaknesses of ECB mode. The very
fact that they haven’t even learned enough cryptography to know these
elementary concepts is not reassuring. And they sometimes manage
their DES keys in inappropriate or insecure ways. Also, these same
software packages often include a second faster encryption algorithm
that can be used instead of the slower DES. The author of the package
often thinks his proprietary faster algorithm is as secure as DES, but
after questioning him I usually discover that it’s just a variation of my
own brilliant scheme from college days. Or maybe he won’t even reveal
how his proprietary encryption scheme works, but assures me it’s a
brilliant scheme and I should trust it. I’m sure he believes that his
algorithm is brilliant, but how can I know that without seeing it?
In fairness I must point out that in most cases these terribly weak
products do not come from companies that specialize in cryptographic
technology.

94

Chapter 8: Security Features and Vulnerabilities

Even the really good software packages, that use DES in the correct
modes of operation, still have problems. Standard DES uses a 56-bit key,
which is too small by today’s standards, and can now be easily broken
by exhaustive key searches on special high-speed machines. The DES
has reached the end of its useful life, and so has any software package
that relies on it.
There is a company called AccessData (87 East 600 South, Orem, Utah
84058, phone 1-800-658-5199) that sells a package for $185 that cracks the
built-in encryption schemes used by WordPerfect, Lotus 1-2-3, MS Excel,
Symphony, Quattro Pro, Paradox, MS Word, and PKZIP. It doesn’t
simply guess passwords—it does real cryptanalysis. Some people buy it
when they forget their password for their own files. Law enforcement
agencies buy it too, so they can read files they seize. I talked to Eric
Thompson, the author, and he said his program only takes a split second
to crack them, but he put in some delay loops to slow it down so it
doesn’t look so easy to the customer.
In the secure telephone arena, your choices look bleak. The leading
contender is the STU-III (Secure Telephone Unit), made by Motorola and
AT&T for $2,000 to $3,000, and used by the government for classified
applications. It has strong cryptography, but requires some sort of
special license from the government to buy this strong version. A
commercial version of the STU-III is available that is watered down for
NSA’s convenience, and an export version is available that is even more
severely weakened. Then there is the $1,200 AT&T Surity 3600, which
uses the government’s famous Clipper chip for encryption, with keys
escrowed with the government for the convenience of wiretappers.
Then, of course, there are the analog (nondigital) voice scramblers that
you can buy from the spy-wannabe catalogs, that are really useless toys
as far as cryptography is concerned, but are sold as “secure”
communications products to customers who just don’t know any better.
In some ways, cryptography is like pharmaceuticals. Its integrity may be
absolutely crucial. Bad penicillin looks the same as good penicillin. You
can tell if your spreadsheet software is wrong, but how do you tell if
your cryptography package is weak? The ciphertext produced by a
weak encryption algorithm looks as good as ciphertext produced by a
strong encryption algorithm. There’s a lot of snake oil out there. A lot of
quack cures. Unlike the patent medicine hucksters of old, these software
implementors usually don’t even know their stuff is snake oil. They may
be good software engineers, but they usually haven’t even read any of
PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

95

the academic literature in cryptography. But they think they can write
good cryptographic software. And why not? After all, it seems
intuitively easy to do so. And their software seems to work OK.
Anyone who thinks they have devised an unbreakable encryption
scheme either is an incredibly rare genius or is naive and inexperienced.
Unfortunately, I sometimes have to deal with would-be cryptographers
who want to make “improvements” to PGP by adding encryption
algorithms of their own design.
I remember a conversation with Brian Snow, a highly placed senior
cryptographer with the NSA. He said he would never trust an
encryption algorithm designed by someone who had not “earned their
bones” by first spending a lot of time cracking codes. That made a lot of
sense. I observed that practically no one in the commercial world of
cryptography qualifies under this criterion. “Yes,” he said with a selfassured smile, “And that makes our job at NSA so much easier.” A
chilling thought. I didn’t qualify either.
The government has peddled snake oil too. After World War II, the
United States sold German Enigma ciphering machines to third-world
governments. But they didn’t tell them that the Allies cracked the
Enigma code during the war, a fact that remained classified for many
years. Even today many UNIX systems worldwide use the Enigma
cipher for file encryption, in part because the government has created
legal obstacles against using better algorithms. They even tried to
prevent the initial publication of the RSA algorithm in 1977. And they
have for many years squashed essentially all commercial efforts to
develop effective secure telephones for the general public.
The principal job of the United States government’s National Security
Agency is to gather intelligence, principally by covertly tapping into
people’s private communications (see James Bamford’s book, The Puzzle
Palace). The NSA has amassed considerable skill and resources for
cracking codes. When people can’t get good cryptography to protect
themselves, it makes NSA’s job much easier. NSA also has the
responsibility of approving and recommending encryption algorithms.
Some critics charge that this is a conflict of interest, like putting the fox
in charge of guarding the hen house. In the 1980s, NSA had been
pushing a conventional encryption algorithm that they designed (the
COMSEC Endorsement Program), and they won’t tell anybody how it
works because that’s classified. They wanted others to trust it and use it.
But any cryptographer can tell you that a well-designed encryption
96

Chapter 8: Security Features and Vulnerabilities

algorithm does not have to be classified to remain secure. Only the keys
should need protection. How does anyone else really know if NSA’s
classified algorithm is secure? It’s not that hard for NSA to design an
encryption algorithm that only they can crack, if no one else can review
the algorithm. And now with the Clipper chip, the NSA is pushing
SKIPJACK, another classified cipher they designed. Are they
deliberately selling snake oil?
There are three main factors that have undermined the quality of
commercial cryptographic software in the United States.
• The first is the virtually universal lack of competence of
implementors of commercial encryption software (although this is
starting to change since the publication of PGP). Every software
engineer fancies himself a cryptographer, which has led to the
proliferation of really bad crypto software.
• The second is the NSA deliberately and systematically suppressing
all the good commercial encryption technology, by legal
intimidation and economic pressure. Part of this pressure is brought
to bear by stringent export controls on encryption software which,
by the economics of software marketing, has the net effect of
suppressing domestic encryption software.
• The third principle method of suppression comes from the granting
of all the software patents for all the public key encryption
algorithms to a single company, affording a single choke point to
suppress the spread of this technology (although this crypto patent
cartel broke up in the fall of 1995).
The net effect of all this is that before PGP was published, there was
almost no highly secure general purpose encryption software available
in the United States.
I’m not as certain about the security of PGP as I once was about my
brilliant encryption software from college. If I were, that would be a bad
sign. But I don’t think PGP contains any glaring weaknesses (although
I’m pretty sure it contains bugs). I have selected the best algorithms from
the published literature of civilian cryptologic academia. For the most
part, these algorithms have been individually subject to extensive peer
review. I know many of the world’s leading cryptographers, and have
discussed with some of them many of the cryptographic algorithms and
protocols used in PGP. It’s well researched, and has been years in the

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

97

making. And I don’t work for the NSA. But you don’t have to trust my
word on the cryptographic integrity of PGP, because source code is
available to facilitate peer review.
One more point about my commitment to cryptographic quality in PGP:
Since I first developed and released PGP for free in 1991, I spent three
years under criminal investigation by United States Customs for PGP’s
spread overseas, with risk of criminal prosecution and years of
imprisonment. By the way, you didn’t see the government getting upset
about other cryptographic software—it’s PGP that really set them off.
What does that tell you about the strength of PGP? I have earned my
reputation on the cryptographic integrity of my products. I will not
betray my commitment to our right to privacy, for which I have risked
my freedom. I’m not about to allow a product with my name on it to
have any secret back doors.

Vulnerabilities
No data security system is impenetrable. PGP can be circumvented in a
variety of ways. In any data security system, you have to ask yourself if
the information you are trying to protect is more valuable to your
attacker than the cost of the attack. This should lead you to protect
yourself from the cheapest attacks, while not worrying about the more
expensive attacks.
Some of the discussion that follows may seem unduly paranoid, but
such an attitude is appropriate for a reasonable discussion of
vulnerability issues.
“If all the personal computers in the world—260 million—were put to
work on a single PGP-encrypted message, it would still take an
estimated 12 million times the age of the universe, on average, to break a
single message.” William Crowell, Deputy Director, National Security
Agency, March 20, 1997.

Compromised passphrase and private key
Probably the simplest attack comes if you leave the passphrase for your
private key written down somewhere. If someone gets it and also gets
your private key file, they can read your messages and make signatures
in your name.
98

Chapter 8: Security Features and Vulnerabilities

Here are some recommendations for protecting your passphrase:

1. Don’t use obvious passphrases that can be easily guessed, such as the
names of your kids or spouse.
2. Use spaces and a combination of numbers and letters in your passphrase. If you make your passphrase a single word, it can be easily
guessed by having a computer try all the words in the dictionary until
it finds your password. That’s why a passphrase is so much better
than a password. A more sophisticated attacker may have his computer scan a book of famous quotations to find your passphrase.
3. Be creative. Use an easy to remember but hard to guess passphrase;
you can easily construct one by using some creatively nonsensical
sayings or obscure literary quotes.

Public key tampering
A major vulnerability exists if public keys are tampered with. This may
be the most crucially important vulnerability of a public key
cryptosystem, in part because most novices don’t immediately recognize
it. The importance of this vulnerability, and appropriate hygienic
countermeasures, are detailed in “How to protect public keys from
tampering,” earlier in this chapter.
To summarize: When you use someone’s public key, make certain it has
not been tampered with. A new public key from someone else should be
trusted only if you got it directly from its owner, or if it has been signed
by someone you trust. Make sure no one else can tamper with your own
public keyring. Maintain physical control of both your public keyring
and your private key, preferably on your own personal computer rather
than on a remote timesharing system. Keep a backup copy of both
keyrings.

Not Quite Deleted Files
Another potential security problem is caused by how most operating
systems delete files. When you encrypt a file and then delete the original
plaintext file, the operating system doesn’t actually physically erase the
data. It merely marks those disk blocks as deleted, allowing the space to
be reused later. It’s sort of like discarding sensitive paper documents in
the paper recycling bin instead of the paper shredder. The disk blocks
still contain the original sensitive data you wanted to erase, and will
PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

99

probably be overwritten by new data at some point in the future. If an
attacker reads these deleted disk blocks soon after they have been
deallocated, he could recover your plaintext.
In fact, this could even happen accidentally, if something went wrong
with the disk and some files were accidentally deleted or corrupted. A
disk recovery program may be run to recover the damaged files, but this
often means that some previously deleted files are resurrected along
with everything else. Your confidential files that you thought were gone
forever could then reappear and be inspected by whoever is attempting
to recover your damaged disk. Even while you are creating the original
message with a word processor or text editor, the editor may be creating
multiple temporary copies of your text on the disk, just because of its
internal workings. These temporary copies of your text are deleted by
the word processor when it’s done, but these sensitive fragments are still
on your disk somewhere.
The only way to prevent the plaintext from reappearing is to somehow
cause the deleted plaintext files to be overwritten. Unless you know for
sure that all the deleted disk blocks will soon be reused, you must take
positive steps to overwrite the plaintext file, and also any fragments of it
on the disk left by your word processor. You can take care of any
fragments of the plaintext left on the disk by using PGP’s Secure Wipe
feature.
Viruses and Trojan horses
Another attack could involve a specially tailored hostile computer virus
or worm that might infect PGP or your operating system. This
hypothetical virus could be designed to capture your passphrase or
private key or deciphered messages and to covertly write the captured
information to a file or send it through a network to the virus’s owner.
Or it might alter PGP’s behavior so that signatures are not properly
checked. This attack is cheaper than cryptanalytic attacks.
Defending against this kind of attack falls into the category of defending
against viral infection generally. There are some moderately capable
antiviral products commercially available, and there are hygienic
procedures to follow that can greatly reduce the chances of viral
infection. A complete treatment of antiviral and antiworm
countermeasures is beyond the scope of this document. PGP has no

100

Chapter 8: Security Features and Vulnerabilities

defenses against viruses, and assumes that your own personal computer
is a trustworthy execution environment. If such a virus or worm actually
appeared, hopefully word would soon get around warning everyone.
A similar attack involves someone creating a clever imitation of PGP
that behaves like PGP in most respects, but that doesn’t work the way
it’s supposed to. For example, it might be deliberately crippled to not
check signatures properly, allowing bogus key certificates to be
accepted. You should make an effort to get your copy of PGP directly
from Pretty Good Privacy.
There are other ways to check PGP for tampering, using digital
signatures. You could use another trusted version of PGP to check the
signature on a suspect version of PGP. But this won’t help at all if your
operating system is infected, nor will it detect if your original copy of
pgp.exe has been maliciously altered in such a way as to compromise its
own ability to check signatures. This test also assumes that you have a
good trusted copy of the public key that you use to check the signature
on the PGP executable.
Swap files or virtual memory
PGP was originally developed for MS-DOS, a primitive operating
system by today’s standards. But as it was ported to other more complex
operating systems, such as Microsoft Windows and the Macintosh OS, a
new vulnerability emerged. This vulnerability stems from the fact that
these fancier operating systems use a technique called virtual memory.
Virtual memory allows you to run huge programs on your computer
that are bigger than the space available in your computer’s
semiconductor memory chips. This is handy because software has
become more and more bloated since graphical user interfaces became
the norm and users started running several large applications at the
same time. The operating system uses the hard disk to store portions of
your software that aren’t being used at the moment. This means that the
operating system might, without your knowledge, write out to disk
some things that you thought were kept only in main memory—-things
like keys, passphrases, and decrypted plaintext. PGP does not keep that
kind of sensitive data lying around in memory for longer than
necessary, but there is some chance that the operating system could
write it out to disk anyway.

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

101

The data is written out to some scratchpad area of the disk, known as a
swap file. Data is read back in from the swap file as needed, so that only
part of your program or data is in physical memory at any one time. All
this activity is invisible to the user, who just sees the disk chattering
away. Microsoft Windows swaps chunks of memory, called pages, using
a Least Recently Used (LRU) page-replacement algorithm. This means
pages that have not been accessed for the longest period of time are the
first ones to be swapped to the disk. This approach suggests that in most
cases the risk is fairly low that sensitive data will be swapped out to
disk, because PGP doesn’t leave it in memory for very long. But we
don’t make any guarantees.
This swap file can be accessed by anyone who can get physical access to
your computer. If you are concerned about this problem, you may be
able to solve it by obtaining special software that overwrites your swap
file. Another possible cure is to turn off your operating system’s virtual
memory feature. Microsoft Windows allows this, and so does the Mac
OS. Turning off virtual memory may mean that you need to have more
physical RAM chips installed in order to fit everything in RAM.

Physical security breach
A physical security breach may allow someone to physically acquire
your plaintext files or printed messages. A determined opponent might
accomplish this through burglary, trash-picking, unreasonable search
and seizure, or bribery, blackmail, or infiltration of your staff. Some of
these attacks may be especially feasible against grass-roots political
organizations that depend on a largely volunteer staff.
Don’t be lulled into a false sense of security just because you have a
cryptographic tool. Cryptographic techniques protect data only while
it’s encrypted—direct physical security violations can still compromise
plaintext data or written or spoken information.
This kind of attack is cheaper than cryptanalytic attacks on PGP.

Tempest attacks
Another kind of attack that has been used by well-equipped opponents
involves the remote detection of the electromagnetic signals from your
computer. This expensive and somewhat labor-intensive attack is
probably still cheaper than direct cryptanalytic attacks. An
102

Chapter 8: Security Features and Vulnerabilities

appropriately instrumented van can park near your office and remotely
pick up all of your keystrokes and messages displayed on your
computer video screen. This would compromise all of your passwords,
messages, and so on. This attack can be thwarted by properly shielding
all of your computer equipment and network cabling so that it does not
emit these signals. This shielding technology, known as “Tempest,” is
used by some government agencies and defense contractors. There are
hardware vendors who supply Tempest shielding commercially.

Protecting against bogus timestamps
A somewhat obscure vulnerability of PGP involves dishonest users
creating bogus timestamps on their own public key certificates and
signatures. You can skip over this section if you are a casual user and
aren’t deeply into obscure public-key protocols.
There’s nothing to stop a dishonest user from altering the date and time
setting of his own system’s clock, and generating his own public key
certificates and signatures that appear to have been created at a different
time. He can make it appear that he signed something earlier or later
than he actually did, or that his public/private key pair was created
earlier or later. This may have some legal or financial benefit to him, for
example by creating some kind of loophole that might allow him to
repudiate a signature.
I think this problem of falsified timestamps in digital signatures is no
worse than it is already in handwritten signatures. Anyone can write
any date next to their handwritten signature on a contract, but no one
seems to be alarmed about this state of affairs. In some cases, an
“incorrect” date on a handwritten signature might not be associated
with actual fraud. The timestamp might be when the signator asserts
that he signed a document, or maybe when he wants the signature to go
into effect.
In situations where it is critical that a signature be trusted to have the
actual correct date, people can simply use notaries to witness and date a
handwritten signature. The analog to this in digital signatures is to get a
trusted third party to sign a signature certificate, applying a trusted
timestamp. No exotic or overly formal protocols are needed for this.
Witnessed signatures have long been recognized as a legitimate way of
determining when a document was signed.

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

103

A trustworthy Certifying Authority or notary could create notarized
signatures with a trustworthy timestamp. This would not necessarily
require a centralized authority. Perhaps any trusted introducer or
disinterested party could serve this function, the same way real notary
publics do now. When a notary signs other people’s signatures, it creates
a signature certificate of a signature certificate. This would serve as a
witness to the signature in the same way that real notaries now witness
handwritten signatures. The notary could enter the detached signature
certificate (without the actual whole document that was signed) into a
special log controlled by the notary. Anyone could read this log. The
notary’s signature would have a trusted timestamp, which might have
greater credibility or more legal significance than the timestamp in the
original signature.
There is a good treatment of this topic in Denning’s 1983 article in IEEE
Computer. Future enhancements to PGP might have features to easily
manage notarized signatures of signatures, with trusted timestamps.

Exposure on multi-user systems
PGP was originally designed for a single-user PC under your direct
physical control. If you run PGP at home on your own PC, your
encrypted files are generally safe, unless someone breaks into your
house, steals your PC and persuades you to give them your passphrase
(or your passphrase is simple enough to guess).
PGP is not designed to protect your data while it is in plaintext form on
a compromised system. Nor can it prevent an intruder from using
sophisticated measures to read your private key while it is being used.
You will just have to recognize these risks on multiuser systems, and
adjust your expectations and behavior accordingly. Perhaps your
situation is such that you should consider only running PGP on an
isolated single-user system under your direct physical control.

Traffic analysis
Even if the attacker cannot read the contents of your encrypted
messages, he may be able to infer at least some useful information by
observing where the messages come from and where they are going, the
size of the messages, and the time of day the messages are sent. This is
analogous to the attacker looking at your long-distance phone bill to see
104

Chapter 8: Security Features and Vulnerabilities

who you called and when and for how long, even though the actual
content of your calls is unknown to the attacker. This is called traffic
analysis. PGP alone does not protect against traffic analysis. Solving this
problem would require specialized communication protocols designed
to reduce exposure to traffic analysis in your communication
environment, possibly with some cryptographic assistance.

Cryptanalysis
An expensive and formidable cryptanalytic attack could possibly be
mounted by someone with vast supercomputer resources, such as a
government intelligence agency. They might crack your RSA key by
using some new secret factoring breakthrough. But civilian academia
has been intensively attacking it without success since 1978.
Perhaps the government has some classified methods of cracking the
IDEA conventional encryption algorithm used in PGP. This is every
cryptographer’s worst nightmare. There can be no absolute security
guarantees in practical cryptographic implementations.
Still, some optimism seems justified. The IDEA algorithm’s designers
are among the best cryptographers in Europe. It has had extensive
security analysis and peer review from some of the best cryptanalysts in
the unclassified world. It appears to have some design advantages over
DES in withstanding differential cryptanalysis.
Besides, even if this algorithm has some subtle unknown weaknesses,
PGP compresses the plaintext before encryption, which should greatly
reduce those weaknesses. The computational workload to crack it is
likely to be much more expensive than the value of the message.
If your situation justifies worrying about very formidable attacks of this
caliber, then perhaps you should contact a data security consultant for
some customized data security approaches tailored to your special
needs.
In summary, without good cryptographic protection of your data
communications, it may be practically effortless and perhaps even
routine for an opponent to intercept your messages, especially those sent
through a modem or e-mail system. If you use PGP and follow
reasonable precautions, the attacker will have to expend far more effort
and expense to violate your privacy.
PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

105

If you protect yourself against the simplest attacks, and you feel
confident that your privacy is not going to be violated by a determined
and highly resourceful attacker, then you’ll probably be safe using PGP.
PGP gives you Pretty Good Privacy.

106

Chapter 8: Security Features and Vulnerabilities

Appendix A

Transferring Files
Between the MacOS and
Windows using PGP
Transferring files to and from MacOS is a classic problem in using
almost any kind of data exchange software, such as e-mail
applications, FTP, compression utilities, and PGP. This appendix
is intended to document how this problem is finally solved by
PGP Version 5.5, and to discuss how to communicate with
previous versions of PGP.
The MacOS stores files differently from other operating systems. Even
the text file format of the MacOS is different. MacOS files are really two
files consisting of a Data segment and a Resource segment. In order to
send a file from MacOS to Windows without losing data, the two
segments must be merged into one. The standard method by which a
MacOS file is converted into a single file so that it can be transferred to
another Macintosh or PC without losing either of its halves is called
MacBinary.
The problem is that, without special software, Windows and other
platforms cannot inherently understand the MacBinary format. If a
situation occurs where the receiving software fails to convert a
MacBinary format file into a Windows file, the resulting file is unusable.
Third-party utilities exist on Windows to convert it after the fact into a
usable file, but that can be rather inconvenient.

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

107

Previous versions of PGP and most utilities available on the market
today generally try to ignore this problem as much as possible and leave
all decisions up to the user as to whether or not to encode a file with
MacBinary when sending from MacOS. This places the burden of
deciding to send with MacBinary, and not risk losing any data, or send
without MacBinary, with hope that no important data will be lost on the
user, who often has no idea what the correct decision is. The decision
should generally be based on whether the file is being sent to Windows
or MacOS. But what about if you're sending to both at the same time?
There is no good solution to that problem with older versions of PGP
and many other utilities. This has resulted in great confusion and
inconvenience for users.
The reverse, sending a file from Windows to the MacOS, has also been a
major problem. Windows uses filename extensions, such as .doc, to
identify the type of a file. This is meaningless on MacOS. These files are
sent to a Macintosh computer without any file type or creator
information. The process of making them readable after receipt
generally involves various arcane motions in the Open dialog of the
creator application, or in many cases requires the user to understand
MacOS lore of creator and type codes by setting them manually in a
third-party utility.
Fortunately, PGP Version 5.5 finally leads the way out of this confusion.
If all PGP users were to use PGP Version 5.5, no one would have to think
about how to send files from MacOS to Windows and vice versa.

Sending from the MacOS to Windows
On the MacOS, there are three options in PGP 5.5 when encrypting or
signing a file on MacOS:
• MacBinary: Yes
• MacBinary: No
• MacBinary: Smart

108

Appendix A: Transferring Files Between the MacOS

MacBinary: Yes
This is the recommended option for all encryptions when sending to
another user of PGP Version 5.5 or above on any platform. This means
that MacOS users will receive the exact file that was intended, and the
Windows version will automatically decode the MacBinary and even
append the appropriate file extension, such as .doc for Microsoft Word
or .ppt for Microsoft PowerPoint. PGP includes information on most
popular application filename extensions and Macintosh-creator codes.
In cases where the type is unknown or known to be a MacOS-only file
such as a MacOS application, the file remains in MacBinary format so
that it can later be forwarded to a Macintosh fully intact.

MacBinary: No
If you are communicating with users who have an older version of PGP,
the decision of whether to send with MacBinary generally ends up in the
sender's hands as in most other programs and in previous versions of
PGP for MacOS. When sending to a PC using an older version, if you
know that the file you are sending can be read by Windows applications
when no MacBinary is used, select this option. This includes most files
that are generally cross-platform such as those created by the Microsoft
Office applications, graphics files, compressed files, and many others.
The sender or the recipient will have to manually rename the file to have
the correct filename extension on Windows. This is required because the
Windows recipient does not have the creator information normally
encoded with MacBinary.

MacBinary: Smart
There are some very limited cases where this option can be useful when
communicating with users who are not using Version 5.5. This option
makes a decision as to whether to encode with MacBinary based on an
analysis of the actual data in the file. If the file is one of the following
types, it will not be encoded with MacBinary, thereby making it readable
on a PC with any version of PGP:
• * PKzip compressed file
• * Lempel-Ziv compressed file
• * MIDI music format file
PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

109

• * PackIt compressed file
• * GIF graphics file
• * StuffIt_ compressed file
• * Compactor compressed file
• * Arc compressed file
• * JPEG graphics file
As shown, only a limited selection of files will result in a readable file by
old versions of PGP on other platforms using the Smart option. Any
other file received on a PC with an older version of PGP will be
unreadable without stripping the MacBinary encoding with a thirdparty utility. Also, the file will not have the correct filename extension
on the PC unless that extension was manually added by the user on the
sending side. Using Smart mode, the resulting file may not be the same
as the original when sent to a Macintosh, because it may lose its creator
and type codes. This mode remains in the product mostly due to the fact
that it was in PGP Version 5.0 and some users may only have a need to
send the above file types. This option is not recommended in most cases.
In summary, if you are sending only to Versions 5.5 or above, always
select MacBinary: Yes (the default). Thus, no thought is required if your
environment is using PGP Version 5.5 exclusively. When sending to
users with older versions, you should select MacBinary: No for crossplatform file types and MacBinary: Yes for files which simply wouldn't
be readable to PC users anyway (such as a MacOS application).
PGP Version 5.0 Note: PGP Version 5.0 did not have a MacBinary: No
option. In order to send file types without MacBinary, which are not
included in the MacBinary: Smart list to a PC using 5.0, the file must be
manually set to one of the creator and type codes on the Smart list before
sending.

Receiving Windows files on the MacOS
When decrypting, PGP Version 5.5 automatically attempts to translate
filename extensions for non-MacBinary files into MacOS creator and
type information. For example, if you receive a file from Windows with
an extension of .doc, the file will be saved as a Microsoft Word
document. The same list of applications used when adding filename
110

Appendix A: Transferring Files Between the MacOS

extensions upon receipt of a MacBinary file on Windows is used to
translate filename extensions back into the MacOS equivalent when
received on a Macintosh computer. In almost all cases, this results in
files which are immediately readable and double-clickable on MacOS.
Previous versions of PGP for MacOS do not have this feature. The user
will have to manually determine that a file named “report.doc” is a
Microsoft Word file. After determining the creator application, in the
case of Microsoft Word, one can simply use the Open dialog to open the
file by selecting Show All Files from the popup menu. Many other
applications also have this feature, but some don't. If the document
cannot be opened from within the application, the user must find out
what the appropriate Macintosh creator and type codes are for the file
and manually set them with a third-party utility. There are many free
utilities to do this. Upgrading to Version 5.5 is probably the easiest
option in this case, as it eliminates this problem.

Supported Applications
The following list of major applications produce documents which are
automatically translated by PGP 5.5 when sent from Windows to MacOS
and vice versa. At this time, there is no way for a user to add or change
these conversions, however, we may add such functionality in the
future.
• PhotoShop(GIF, native Photoshop documents, TGA, JPEG)
• PageMaker(Versions 3.X, 4.X, 5.X, 6.X)
• Microsoft Project(project and template files)
• FileMaker Pro
• Adobe Acrobat
• Lotus 123
• Microsoft Word (text, RTF, templates)
• PGP
• Microsoft PowerPoint
• StuffIt
• QuickTime

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

111

• Corel WordPerfect
• Microsoft Excel(many different types of files)
• Quark XPress
The following general filename extensions are also converted:

112

cvs

.arj

.ima

.eps

.mac

.cgm

.dl

.fli

.ico

.iff

.img

.lbm

msp

.pac

pbm

.pcs

.pcx

.pgm

plt

.pm

.ppm

.rif

.rle

.shp

spc

.sr

.sun

.sup

.wmf

.flc

.gz

.vga

hal

lzh

.Z

.exe

.mpg

dvi

tex

.aif,

zip

.au

mod,

.svx

.wav

.tar

.pct

pic

.pit

.txt

mdi

pak

.tif,

.eps

Appendix A: Transferring Files Between the MacOS

A Glossary of Terms
ASCII-armored text: Binary information that has been encoded using a
standard, printable, 7-bit ASCII character set, for convenience in
transporting the information through communication systems. In the PGP
program, ASCII armored text files are given the default filename
extension, and they are encoded and decoded in the ASCII radix-64
format.
authentication: The determination of the origin of encrypted information
through the verification of someone’s digital signature or someone’s
public key by checking its unique fingerprint.
certify: To sign another person's public key.
certifying authority: One or more trusted individuals who are assigned
the responsibility of certifying the origin of keys and adding them to a
common database.
conventional encryption: Encryption that relies on a common
passphrase instead of public key cryptography. The file is encrypted
using a session key, which encrypts using a passphrase that you will be
asked to choose
decryption: A method of unscrambling encrypted information so that it
becomes legible again. The recipient's private key is used for decryption.
digital signature: See signature.
encryption: A method of scrambling information to render it unreadable
to anyone except the intended recipient, who must decrypt it to read it.
fingerprint: A uniquely identifying string of numbers and characters
used to authenticate public keys. This is the primary means for checking
the authenticity of a key.

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

113

introducer: A person or organization who is allowed to vouch for the
authenticity of someone’s public key. You designate an introducer by
signing their public key.
key: A digital code used to encrypt and sign and decrypt and verify emaile-mail messages and files. Keys come in key pairs and are stored on
keyrings.
key escrow: A practice where a user of a public key encryption system
surrenders their private key to a third party thus permitting them to
monitor encrypted communications.
key fingerprint: A uniquely identifying string of numbers and characters
used to authenticate public keys. For example, you can telephone the
owner of a public key and have him or her read the fingerprint associated
with their key so you can compare it with the fingerprint on your copy of
their public key to see if they match. If the fingerprint does not match,
then you know you have a bogus key.
key ID: A legible code that uniquely identifies a key pair. Two key pairs
may have the same user ID, but they will have different Key IDs.
key pair: A public key and its complimentary private key. In public-key
cryptosystems, like the PGP program, each user has at least one key pair.
keyring: A set of keys. Each user has two types of keyrings: a private
keyring and a public keyring.
message digest: A compact “distillate” of your message or file checksum.
It represents your message, such that if the message were altered in any
way, a different message digest would be computed from it
passphrase: A series of keystrokes that allow exclusive access to your
private key which you use to sign and decrypt e-mail messages and file
attachments.
plaintext: Normal, legible, unencrypted, unsigned text.
private key: The secret portion of a key pair-used to sign and decrypt
information. A user's private key should be kept secret, known only to the
user.
private keyring: A set of one or more private keys, all of which belong to
the owner of the private keyring.

114

public key: One of two keys in a key pair-used to encrypt information
and verify signatures. A user's public key can be widely disseminated to
colleagues or strangers. Knowing a person's public key does not help
anyone discover the corresponding private key.
public keyring: A set of public keys. Your public keyring includes your
own public key(s).
public-key cryptography: Cryptography in which a public and private
key pair is used, and no security is needed in the channel itself.
sign: To apply a signature.
signature: A digital code created with a private key. Signatures allow
authentication of information by the process of signature verification.
When you sign a message or file, the PGP program uses your private key
to create a digital code that is unique to both the contents of the message
and your private key. Anyone can use your public key to verify your
signature.
text: Standard, printable, 7-bit ASCII text.
trusted: A public key is said to be trusted by you if it has been certified by
you or by someone you have designated as an introducer.
trusted introducer: Someone who you trust to provide you with keys
that are valid. When a trusted introducer signs another person’s key,
you trust that their keys are valid, and you do not need to verify their
keys before using them.
user ID: A text phrase that identifies a key pair. For example, one
common format for a user ID is the owner's name and e-mail address.
The user ID helps users (both the owner and colleagues) identify the
owner of the key pair.
verification: The act of comparing a signature created with a private key
to its public key. Verification proves that the information was actually
sent by the signer, and that the message has not been subsequently
altered by anyone else.
web of trust:: A distributed trust model used by PGP to validate the
ownership of a public key where the level of trust is cumulative, based
on the individuals’ knowledge of the introducers.

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

115

116

Index
A
Additional Decryption Key 9
address
e-mail
adding to an existing key 51
attackers
protecting against 24, 85
attributes
changing your keyrings’ 46–50
viewing your keyrings’ 46–50

C
certifying
authority 87
public keys 4, 86
Certifying Authority, description 87
Change passphrase property 50
changing
your passphrase 55
checking
authenticity of a key 30
fingerprints 51
comparing
fingerprints 31
compatibility
PGP/MIME standard 9
versions of PGP 7

creating
key pairs 12, 20
recipient groups 35
cryptography
overview 3

D
decrypting
e-mail 3, 5, 37
files 41
using PGP menu 41, 41–42
decryption
how it works 19
defaults
specifying 50
deleting
digital signatures 54
files 42
keys 54
recipient groups 36
user IDs 54
using Secure Wipe 42
digital signatures
and authenticity 31
deleting 54
description 3, 82
verifying 3
disabling keys 54

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

117

disclosure
protecting private keys against 91
disks
system requirements 7
wiping files from 42
distributing your public keys 25

E
e-mail
adding a new user name 51
checking signatures 3
copying public keys from 29
creating recipient groups 35
decrypting 5, 37, 38
deleting recipient groups 36
encrypting 4, 33, 34–35
to groups of people 35
including your public key in 26
receiving private 3, 33
selecting recipients 14
sending private 3, 33
signing 3, 4, 33, 34–35
using PGP with 13
verifying 5, 37, 38
Enabled property 49
enabling keys 54
encrypting
e-mail 4, 33, 34–35, 38
overview 3
to groups of people 35
via the PGP menu 39–40
encryption
how it works 19
setting preferences 57
exchanging
public keys 4
obtaining others’ 28–30
expiration
setting for key pairs 21
Expire property 49
exporting
118

keys, to files 27, 56

F
files 41
decrypting
using PGP menu 41–42
deleting 42
encrypting
via the PGP menu 39–40
exporting keys to 56
exporting public keys to 27
importing keys from 55
importing public keys from 30
setting location of keyring files 59
signing
via the PGP menu 39–40
verifying
with PGP menu 41–42
wiping 42
Finder
using PGP from 12
finding
keys 64
Fingerprint property 50
fingerprints
checking 51
comparing 31
fingerprints, description 84

G
generating
key pairs 20
setting preferences 59
granting
trust for key validations 53
groups
creating 35
deleting 36

Index

H
hash function, description 84
help
getting 13

I
importing
public keys, from files 30, 55
installing
PGP 11
installing PGP 11
from the Web 11
introducer 86
trusted
description 87
introducers
and digital signatures 88, 104
description 88
trusted 90

K
key compromise certificate
issuing 92
Key ID property 49
key pairs
creating 3, 20, 20–24
creating with PGP Key Wizard 12
description of 20
examining 12
generating 20
making 20
setting expiration of 21
specifying defaults 50
viewing your 12
key server
getting someone’s public key from 28
sending your public key to 23, 25–26
setting preferences 62
using to circulate revoke keys 56
key servers

searching 64
key size
Diffie-Hellman portion 21
DSS portion 21
setting 21
trade-offs 21
Key Type property 49
keyboard shortcuts 15
keyrings
changing attributes of 46–50
description of 45
location of 45
overview of 3
searching 64
storing elsewhere 45
viewing attributes of 46–50
keys
backing up 24
checking fingerprints 51
colors of 24
deleting 54
disabling 54
distributing 25
enabling 54
examining 12
exporting to files 56
finding 64
generating 20
granting trust for validations 53
importing from files 55
locating 64
managing 45
overview of 19
protecting 24, 91
revoked 57
revoking 56
saving 24
searching for 64
setting size of 21
signing 52
verifying authenticity of 30

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

119

L

key concepts 19
keyrings 3
private keys 3
public key cryptography 3
public keys 3
signing e-mail 3
verifying digital signatures 3

legitimacy
determining a key’s 30
locating
keys 64

M
Macintosh
system requirements 7
making
key pairs 20
managing
keys 45
memory
system requirements 7
message digest
description 84
message digest, description 84
MIME standard
using to decrypt e-mail 38
using to encrypt e-mail 34–35

N
new e-mail address
adding 51

O
obtaining
others’ public keys 28–30
online help
getting 13
opening
PGPkeys window 12
overviews
checking digital signatures 3
cryptography 3
decrypting e-mail 3
digital signatures 3
encrypting e-mail 3
120

P
passphrases
Change Passphrase property 50
changing 55
compromised 98
forgotten 57
setting 22
suggestions for 22
PGP
compatibility 7
history 7
installing 11
overview of 3
running 11
setting preferences 13
upgrading from a previous version 9
upgrading from PGP, Inc. 9
upgrading from ViaCrypt 9
using from PGPtools window 14
using from the Finder 12
using with supported e-mail applications 13
ways to use 11
PGP Key Wizard
using to create key pairs 12, 20
PGP menu
decrypting files 41, 41–42
encrypting via 39–40
using Secure Wipe 42
verifying files 41–42
PGP/MIME standard
compatibility 9
using to decrypt e-mail 38
Index

using to encrypt e-mail 34–35
PGPkeys window
creating key pairs with 20–24
Creation label 48
description 46
examining keys’ properties 49
Change Passphrase 50
Enabled 49
Expire 49
Fingerprint 50
Key ID 49
Key Type 49
Trust Model 49
opening 12
Size label 48
Trust label 47
uses 46
Validity label 46
PGPmenu
setting preferences 61
PGPtools window
using PGP from 14
platforms
supported 7
preferences
encryption 57
general 57
key generation 59
key server 62
PGPmenu 61
setting 13, 57
Privacy Enhanced Mail institution 91
private and public key pairs
creating 3
creating with PGP Key Wizard 12
viewing your 12
private keys
compromised 98
creating 3
key pairs 3
creating with PGP Key Wizard 12

location of 45
overview 3
protecting 24
protecting against 91
setting location of 60
storing 24
viewing your 12
protecting
your keys 24
public key cryptography
overview 3
public keys
advantages of sending to key server 25
certifying 4, 86
consequences of sending to key server

23
copying from e-mail messages 29
creating 3
key pairs 3
creating with PGP Key Wizard 12
distributing your 25
exchanging with other users 4
exporting to files 27
getting from a key server 28
giving to other users 4
importing from files 30
including in an e-mail message 26
location of 45
obtaining others’ 28–30
overview 3
protecting 24
protecting against tampering 85
sending to key server 23, 25–26
setting location of 59
signing 52, 86
storing 24
trading with other users 4
validating 4
viewing your 12

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

121

R
random numbers
their use as session keys 81
receiving
private e-mail 33
recipient groups
creating 35
deleting 36
recipients
groups of 35
selecting 14
removing files
using Secure Wipe 42
revoking
keys 56
running
PGP 11, 12

S
saving
keys 24
searching
for keys 64
Secure Wipe
using 42
security breach
description 102
selecting
e-mail recipients 14
sending
private e-mail 33
setting
passphrase for a key 22
preferences 57
shortcuts 15
signing
deleting signatures 54
e-mail 4, 33, 34–35
checking signatures 3
overview 3
122

files
via the PGP menu 39–40
keys 52
public keys 52, 86
storing
keys 24
system requirements 7

T
tampering
protecting your keys against 24, 85
traffic analysis
as an attack 104
trust
granting for key validations 53
Trust Model property 49
trusted introducer
description 87, 90

U
upgrading
from a previous version of PGP 9
from ViaCrypt 9
user ID
checking a public key’s 87
user name
adding 51
using
PGP 11
from the Finder 12

V
validating
keys
granting trust for 53
public keys 4
validity
checking a key’s 30
verifying
authenticity of a key 30
Index

e-mail 5, 37, 38
files
using PGP menu 41–42
versions
of PGP, compatible 7
upgrading to new 9
ViaCrypt
upgrading from 9
viewing
attributes of keyrings 46–50
key attributes 12
private and public key pairs 12
virus
as attacker 100

W
wiping files 42
worm
as attacker 100

PGP for Personal Privacy, Version 5.5 Macintosh User’s Guide

123

124

Index



---

Source Exif Data:

File Type                       : PDF
File Type Extension             : pdf
MIME Type                       : application/pdf
PDF Version                     : 1.1
Linearized                      : No
Page Count                      : 138
Create Date                     : 1997:10:23 18:31:51
Producer                        : Acrobat Distiller 3.0 for Windows

EXIF Metadata provided by EXIF.tools