Programming Assignment 3

Due May 4, 10 points

Overview

In this programming assignment, you will implement decryption with the RSA public key system. You will be modifying two java class files, starting from skeletons that can be accessed from my public directories. After the modifications, you will be able to decrypt a message that I have encrypted.

Setting Up

To set up for this programming assignment, you should copy the directory ~gshute/public/cs5651/pa3 into your own directory hierarchy. You can do this by logging into a UNIX account, moving into the directory where you want to create a subdirectory for this programming assignment, and giving the following command:

    cp -r ~gshute/public/cs5651/pa3 .

After you have done this, you will have a new subdirectory named "pa3". If you list the contents of this directory you should see the following files:

• Decrypt.java -
  the main program for decryption
• BigIntegerReader.java -
  a class for reading a file or stream of big integers
• ciphertext -
  a file containing encrypted text
• key.pub -
  a file containing the key you should use for decryption

To complete this assignment, you will need to add code to both of the .java files and compile them. These files contain "FIXME" comments that indicate where you need to add code and what the added code should do.

When your code is working correctly, you can see the plaintext message by giving the following command:

    java Decrypt key.pub < ciphertext

    java Decrypt key.pub < ciphertext > plaintext

File Formats

The "ciphertext" and "key.pub" files contain sequences of big integers encoded in a textual format. The textual format for each big integer is the hexadecimal representation of the number, chopped into lines with no more than 72 digits per line. Each big integer is followed by a blank line.

Code Organization

The BigIntegerReader class provides a read() method for reading big integers from files, one at a time. The Decrypt class just has a main() method that reads key modulus and exponent big integers from a key file specified on the command line, then reads ciphertext big integers from System.in. All of the big integer reads are done by setting up BigIntegerReader objects and invoking their read() methods. The RSA decryption algorithm is applied to each ciphertext big integer to get a plaintext big integer. A plaintext big integers named "plainNumber" can be converted to a string with the following code:

    String plainString = new String(plainNumber.toByteArray());

The modpow() method of the java.math.BigInteger class makes easy work of the RSA decryption algorithm. You should check the Java documentation for this method to get its parametrization correct.

What to Turn in

You should turn in copies of your modified .java files for the BigIntegerReader and Decrypt classes along with a printout of the output of your program using the provided key and ciphertext. These should be turned in to your TA on or before May 4. This is the last day of lecture for the course.