XOREncryption Instructions

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CS-207: Programming II
Spring 2016
Northeastern Illinois University
Research Lab: XOR Encryption
Due to D2L on Tuesday, 04/19/16 by 9:00 a.m.
Research Lab Goal
The goal of this research lab is to encrypt a file using the XOR encryption algorithm while
using exception handling techniques described in your course. In addition, students will
become more familiar with reference the Java 8 docs when working with new API classes.
Please note that his lab is due to D2L by the above date and time and there will be not be
an extended deadline to resubmit.
Getting Started
1. Create a new Java class named XOREncryption that has the main method.
2. You need one specific Java package imported in order to work with File objects. What
is it? Import this package before your class definition.
3. Download the data.txt file from the NeededFiles.zip file and save it in the same folder
as your Java file. Look at the data in this file - do you recognize it?
4. Create a .txt file named key.txt that has the following text: This is a key
5. Compile your code.
Creating and Using File Objects
1. Each file that you read from needs to be represented by a File object.
2. Create a File object for each of your files.
3. You will need to be able to read from each of your File objects using the FileInputStream
class.
4. Find the constructor for the FileInputStream class in the Java 8 docs that takes a
File object as a parameter. What type/name of the exception does it throw? Why
do you need to know this before you create a FileInputStream object?
5. Create a FileInputStream object for each of your File objects.
6. Handle the FileNotFoundException by printing out "No file."
7. Compile your code.
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Reading From a File
•The read method of the FileInputStream class takes an initialized byte array as a
parameter.
•Create and initialize two byte arrays. How do you know the length of the arrays?
(Hint: Look at the lecture!)
•The length method of the File class returns a long. However, in order to declare and
create your arrays, you need an int. Cast the result of calling the length method to
an int.
•Look at the Java 8 docs for the read method. What does this method return?
•Look at the Java 8 docs the read method again. Does it throw any exceptions, and if
so, what is the name/type of the exception?
•Read from your data.txt and key.txt files and save the results to int variables.
•Handle the IOException by printing out "Error reading file."
•Compile your code.
Implementing the Encryption Algorithm
•To implement the algorithm, you need to iterate over the byte array created by reading
from the data.txt file.
•For each byte in that array, you should do the following:
- Find the index of the key byte array by taking the for-loop counter mod the length of
the key byte array.
- Encrypt the byte: (byte at for loop counter) XOR (key_byte at index found using
mod)
- Assign the encrypted byte back to the byte array (for the data.txt file).
•Hint: Look at the lecture to find the operator for XOR.
•Compile your code.
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Writing the Encrypted Data
•Find the constructor for the FileOutputStream class in the Java 8 docs that takes a
String as a parameter. What type/name of the exception does it throw and when
does it throw it? Do you need to write another catch block for this exception?
•Create a FileOutputStream object and pass in the String "data_encrypt.txt" as a
parameter to the constructor.
•Next, we need to write the encrypted byte array to using the FileOutputStream object
and the method write.
•What type of parameter does the write method take and what does it return?
•Write the encrypted byte array.
•Close both of the FileInputStream objects and the FileOutputStream object.
•Compile your code.
Testing Your Code
•Run your code. A file named data_encrypt.txt should appear in the same location
as your Java file.
•Open the data_encrypt.txt file and verify that the contents of the file look like
nonsense.
•In your XOREncryption.java file, change the file that you are reading from to be
"data_encrypt.txt" and the file that you are writing to be "data_decrypt.txt".
Do not change your key.txt file!
•Run your code. A file named data_decrypt.txt should appear in the same location
as your Java file. This file should be exactly the same (with the same formatting) as
your data.txt file.
•Try different keys and different text files to see how your encryption works!
•Submit only your XOREncryption.java file to the Research Lab #3 Dropbox on D2L.
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