Tribhuvan University

Institute of Science and Technology

2076

Bachelor Level / fifth-semester / Science

Computer Science and Information Technology( CSC316 )

Cryptography

Full Marks: 60 + 20 + 20

Pass Marks: 24 + 8 + 8

Time: 3 Hours

Candidates are required to give their answers in their own words as far as practicable.

The figures in the margin indicate full marks.

**Section A**

**Attempt any TWO questions**

1

Among monoalphabetic and polyalphabetic cipher, which one is more vulnerable? Justify your statement. Which types of keys are considered weak keys in DES? Explain the round operation in IDEA.

2

State Fermat’s theorem with an example. Given the prime number p=29 and its primitive root g=8, private key sender with X=9 and random integer K=11, encrypt the message m=13 using ElGamal cryptosystem.

3

Compare the SHA parameters between SHA-1 and SHA-2 families. Decrypt the cipher text DRJI with the key \(\begin{pmatrix}7 & 8\\ 11 & 11\end{pmatrix}\) using the Hill cipher.

**Section B**

**Attempt any EIGHT questions**

4

Define discrete logarithm. Explain the procedure of sharing the secret key in Diffie Hellman.

5

Distinguish between stream cipher and block cipher. Encrypt the message WE ARE IN SAME RACE UNTILL OVER LIVE END using Rail fence cipher using 4 as a number of rails.

6

Define digital signature. Describe the approaches of DSS.

7

What is the task of a firewall? List the elements of X.509.

8

How does the nature of worms differ from viruses? Define PKI with its architecture model.

9

Explain the procedure of mix column transformation in AES with an example.

10

What is the role of the prime number in the Euler totient Function? Find the GCD of 12 and 16 using the Euclidean algorithm.

11

Write down any two limitations of MAC. What do policy and mechanism mean in cryptography? Describe with a scenario.

12

Write short notes on

- Classes of Intruder
- SSL
- DoS Attack

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