## 17 Jul Exercise Sheet 1

1.Decipher the following ciphertext that has been doubly enciphered with a rail fence of height 5 and a monoalphabetic substitution cipher with keyword CRYPTOGRAPHY: WHAWPAVNAMTLPTTUTHLVTWMCV

- Carry out the following computations on residue classes:

(a) [8]11 + [5]11 (b) [5]14 − [7]14

(c) [5]13 ∗ [7]13 (d) [8]12 ∗ [11]12

3.Assume a simple two round Feistel block cipher with an 8 bit key and 16 bit block size. Key derivation is defined as Ki = K +75·i( mod 256) where Ki is the ith subkey and K is the decimal representation of the key. The encryption function is f(Ki , Ri) = 127 · (Ki + Ri)(mod256) where Ri is the decimal representation of the right 8 bit of the input block.

Encrypt the message block M = (V, S) with key K = Y. Use the ASCII encoding of capital letters where A is encoded as 65. It is sufficient to give your result as a pair of decimal numbers.

- By defining a suitable strategy for the attacker, show that the one-time pad is not a secure block cipher.

**Order Description**

Q1- Monoalphabetic Substitution Ciphers

1. choosing a keyword,

2. removing all duplicate letters from the keyword

3. start the cipher-alphabet with the letters from duplicate-free keyword

4. at the end of the codeword continue with the next unused letter of the alphabet that follows the last

letter in the codeword

5. continue filling in letters in alphabetical order leaving out those letters already used; if necessary,

start from the beginning of the alphabet again similar to a shift cipher

Q3- Feistel Ciphers

Q4: one time is not correct but tow time is correct. how?