April 1999
SC223: COMPUTER SECURITY

QUESTION 2

Total Marks: 20 Marks

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SAMPLE STUDENT'S SOLUTIONS
for Question 2

 
(a) (i) What is a block cipher? [1]
(ii) Name one example of a block cipher. [1]
(iii) Name one advantage and one disadvantage of block ciphers.

 

 [2]
(b) Define the following terms.
(i) Monoalphabetic cipher. [2]
(ii) Polyalphabetic cipher.

 

 [2]
(c) A Caesar cipher was used to produce the following ciphertext message.

AOPZ PZ AOL LUK VM AOL WYVISLT

Use the most efficient cryptanalysis you can to deduce the plaintext message.

 

 [4]
(d) (i) Perform encryption and decryption using the RSA algorithm, where p = 5, q = 11, e = 3, and M = 9. Show each step in detail. [7]
(ii) In the RSA algorithm, n is usually chosen to be the product of two large prime numbers. Why is this? [1]