CSC350 - Lab 4
Modern Ciphers - Simple DES

Assigned Sep 18th
Due Sep 25th Beginning of Class

Part 1 - GF(2ⁿ)

Add the following numbers from GF(32). Record your number in binary.
1. 01001 (+) 10011
2. 10111 (+) 10111
Multiply the following numbers from GC(32) using x⁵ + x² + 1 as the irreducible polynomial. Show your work using the multiplication algorithm on page 113. Record your number in binary.
1. 0101 (*) 1001
2. 1011 (*) 1011

Write up your answers in a file called lab4.txt

Part 2 - Simple DES

In this project, you will implement a simplified version of the DES block cipher algorithm, called SDES, and it is designed to have the features of the DES algorithm but scaled down so it is more tractable to understand. (Note however, that SDES is in no way secure and should not be used for serious cryptographic applications.)

This algorithm is discussed in detail in Appendix O of the textbook. Your input will be a String representing in binary an 8 bit integer, and String representing in binary a 10 bit key, and your output will be a String representing the encrypted bits.

You must write the following classes (described here in Java, but you are welcome to implement them in Python). You may write whatever additional classes you feel necessary to complete the project (such as a round key generator class, or a mixer class).

PBox Class

The first class you will implement is PBox.java. This can capture straight, expansion and compression boxes.

Data Members

int size stores the maximum size of input integers
int[] perm stores the permutation

Methods

public PBox(int size, int[] perm) is the constructor, and sets the size and permutation.
public int permute(int num) runs the input number num through the permutation.

You can add a main method to this class for unit testing, and should, but it is not necessary.

SBoxTable Class

You will also implement SBoxTable.java. This is an implementation of an SBox interface where the output value is found through table lookup. Your class declaration must be public class SBoxTable implements SBox {....

Data Members

int[][] table stores the table for lookup of the output numbers.
int bitmask stores a number, where in binary representation, 1's mean the bit will be used as part of the row lookup, and 0's mean the bit will be used as part of the column lookup. The ordering of the bits will not change.

Methods

public SBoxTable(int[][] table, int bitmask) is the constructor, and sets the table and bitmask.
public int substitute(int num) runs the input number num through the substitution.

You can add a main method to this class for unit testing, and should, but it is not necessary.

SDES class

This class will implement the SDES encryption cipher. It should hold the initial and final permutation PBoxes, the PBoxes in the key generator, the PBoxes and SBoxes for the mixer, and a function to perform repeated rounds and swap the bits in between rounds.

The bitwise operators

You must use bitwise operators in Java or Python in this assignment.

Operator Name Example Result Description

a & b and 3 & 5 1 1 if both bits are 1.

a | b or 3 | 5 7 1 if either bit is 1.

a ^ b xor 3 ^ 5 6 1 if both bits are different.

~a not ~3 -4 Inverts the bits.

n << p left shift 3 << 2 12 Shifts the bits of n left p positions. Zero bits are shifted into the low-order positions.

n >> p right shift 5 >> 2 1 Shifts the bits of n right p positions. If n is a 2's complement signed number, the sign bit is shifted into the high-order positions.

n >>> p right shift -4 >>> 28 15 (Only in Java) Shifts the bits of n right p positions. Zeros are shifted into the high-order positions.

Operator	Name	Example	Result	Description
`a & b`	and	3 & 5	1	1 if both bits are 1.
`a \| b`	or	3 \| 5	7	1 if either bit is 1.
`a ^ b`	xor	3 ^ 5	6	1 if both bits are different.
`~a`	not	~3	-4	Inverts the bits.
`n << p`	left shift	3 << 2	12	Shifts the bits of n left p positions. Zero bits are shifted into the low-order positions.
`n >> p`	right shift	5 >> 2	1	Shifts the bits of n right p positions. If n is a 2's complement signed number, the sign bit is shifted into the high-order positions.
`n >>> p`	right shift	-4 >>> 28	15	(Only in Java) Shifts the bits of n right p positions. Zeros are shifted into the high-order positions.

Test Cases

You can generate test cases and intermediate tests with the SDES Calculator.

You may not download the code from this site or any other, you must code this from the algorithm description in Appendix O.

To verify that your implementation of SDES is correct, try the following test cases:

 Raw Key    Plaintext Ciphertext 
0000000000  10101010   00010001 
1110001110  10101010   11001010 
1110001110  01010101   01110000 
1111111111  10101010   00000100

Use your implementation to complete the following table:

 Raw Key    Plaintext Ciphertext 
0000000000  00000000      ? 
1111111111  11111111      ? 
0000011111  00000000      ? 
0000011111  11111111      ? 
1000101110     ?       00011100 
1000101110     ?       11000010 
0010011111     ?       10011101 
0010011111     ?       10010000

Write the results of your table in lab4.txt

What to turn in

PBox.java, SBox.java and SDES.java or
SDES.py and
lab4.txt

To hand in your Lab 4 files, follow the steps listed below from the Wright Lab computers:

Open the Terminal application.
SSH with ssh username@cs.centenary.edu to connect with the cs server.
Type in your password.
Make a new directory with mkdir csc350/lab4
Log out of the cs server with logout
Change to the directory where you have your files on the local computer.
Securely copy your files to your directory with scp filenames username@cs.centenary.edu:"csc350/lab4"

CSC350 - Lab 4Modern Ciphers - Simple DES

Assigned Sep 18th Due Sep 25th Beginning of Class

Part 1 - GF(2n)

Part 2 - Simple DES

PBox Class

Data Members

Methods

SBoxTable Class

Data Members

Methods

SDES class

The bitwise operators

Test Cases

What to turn in

CSC350 - Lab 4
Modern Ciphers - Simple DES

Assigned Sep 18th
Due Sep 25th Beginning of Class

Part 1 - GF(2ⁿ)