# Computer Science

IFT202 Name: ________________________

Activity 1: page 2

# Activity I

## Symmetric Encryption

Symmetric encryption is the process of encoding and decoding messages using the same key. An important aspect of this type encryption is to ensure that the key is not compromised and that only the sender and receiver have access to the key used to modify the plain text.

Substitution is one technique used in symmetric encryption. The first part of this activity is intended to illustrate symmetric encryption using substitution. The second part of this activity illustrates the block cipher technique using a product technique that emulates the Feistel structure with substitution and transposition.

**Part One**

1. A file explaining the Special Forces symmetric substitution cipher is provided. Use this technique for this exercise. Make sure you show all your work.

2. Using the two keys (memory words): *cryptanalysis* and *network cryptology*, encrypt the following message:

a. Be at the third pillar from the left outside the lyceum theater tonight at seven. If you are distrustful bring two friends.

b. You may make reasonable assumptions about how to treat redundant letters and excess letters in the memory words and how to treat spaces and punctuation. Identify your assumptions in you summary.

3. Decrypt the ciphertext to show the original message.

4. What are the advantages and disadvantages of using this technique? Would this technique have any merit in today’s technology rich environment?

**Part Two**

1. A file explaining the Tiny Encryption Algorithm (TEA) which is a Feistel Block Cipher method is located in the Content Section of Blackboard. Use this technique for this exercise. Make sure you show all your work.

2. TEA operates on 64-bit blocks of plaintext using a 128 bit key. The plaintext is divided into two 32 bit blocks (L0, R0), and the key is divided into four 32 bit blocks (K0, K1, K2, K3). Encryption involves repeated application of a pair of rounds, defined as follows for rounds *i* and *i + 1:*

Li = Ri-1

Ri = Li-1 + F(Ri-1, K0, K1, δi)

Li+1 = Ri

Ri+1 = Li + F(Ri-1, K2, K3, δi+1)

where F (M, Kj, Kk, δi) = ((M<< 4) + Kj) + ((M>> 5) ) + Kk) + (M + δ1)

and where the logical shift of x by y bits is denoted by x<<y, the logical right shift of x by y bits is denoted by x >> y, and δi is a sequence of predetermined constants.

3. Using the TEA concept, but limiting the plaintext block to 16 bits and the key block to 32 bits, complete the following block cipher given the following plaintext message and key, respectively (in hexadecimal): A2B9H and 4C8AF6B9.

4. Illustrate the operation of TEA using a block diagram or flow chart. Complete the first four rounds of iterations. Show the output values for Li and Ri for each of the rounds (Round 1, Round 2, Round 3, Round 4).

**Deliverable Items**

**Theory or Principle Illustrated:**

The description should include any mathematical analysis, results, or truth tables, as appropriate. All mathematical analyses must use the equation editor. All simulation results, and tables must be electronically generated and merged into the document text. This section develops the expected values of the activity. There should be a one to one correlation between the Part 1 and Part 2 “Procedure Steps” and the “Theory or Principle Illustrated” sections.

**Experimental Data and Presentation:**

This section is the recording and presentation of the data. This includes tables, charts, etc. Each step in the procedure that requires a measurement or observation should have a corresponding entry in this section. It may be necessary to record observations in tables to facilitate readability. Measurement values may need to be presented in table and graph format to facilitate the evaluation of the information. It is acceptable to include any error analysis along with the measured and theoretical or simulation data in this section. The procedure step should be referenced with the measurement value or observation. All information must be electronically generated and merged into the document text.

**Discussion of Results:**

This section is a detailed discussion of the results of the experiment. Each step in the procedure that requires a measurement or observation should have a corresponding discussion in this section. Discussions should include analysis and a comparison of the results with the expected value from the “Theory or Principle Illustrated” section. Explain differences between the actual and expected results here. It is acceptable to reference other sections of the report, namely the “Theory or Principle Illustrated”, “Experimental Data and Results”, and “Procedure” sections. Details of all tests performed including % error relative to the expected values are required. In addition, a conclusion relative to the purpose based on the comparison of actual test data to expected values is required.