Introduction to Cryptography 2069

Attempt all questions.

1.  Answer the following questions in short (Any Five).                                         (5 × 2 = 10) 

        a. How monoalphabetic substitution differs from polyalphabetic. Briefly define with suitable example.

        b. What are the components of authentication system? Give an example of authentication system.

        c. What do you mean by avalanche effect?

        d. How chosen plaintext attack differs from chosen ciphertext attack?

        e. What do you mean by multiplicative inverse? Find multiplicative inverse of each nonzero elements in Z₁₁.

        f. Even though we have a strong algorithm like 3-DES, still AES is preferred as a reasonable candidate for long term use. Why?

        g. Give an example for a situation that compromise in confidentiality leads to compromise in integrity.

2.     a) Consider a Deffie-Hellman scheme with a common prime p = 11 and a primitive root g= 2.

i. Show that 2 is a primitive root of 11.

ii. If user A has public key Y_a = 9, what is A’s private key X_a?

iii. If user B has public key Y_b = 3, what is shared key K, shared with A.         (3x2=6) 

    b) Construct a playfair matrix with the key “KEYWORD”. Using this matrix encrypt the   message “WHY DON’T YOU”.                                                                                            (4)

3.     a) How Trojan horse differs from viruses? Discuss about possible types of Trojan horses.                                                                                                                                    (2+3)

    b) Does Kerberos protocol ensures authentication and confidentiality in secure system? Explain.                                                                                                                              (5)

4.     a) How Hash functions differ from MAC? Given a message m, discuss what arithmetic and logical functions are used by MD4 to produce message digest of 128 bits.           (2+4) 

    b) Discuss the five principle services provided by PGP protocol.                                    (4)

5.     a) What is the purpose of S-Boxes in DES? Prove that DES satisfies complementation property?                                                                                                                            (6) 

    b) Given the plaintext “ABRA KA DABRA”, compute the ciphertext for                       (4)

i. The Ceaser cipher with key = 8

ii. The Railfence cipher with rails = 3

6.     a) What do you mean by digital signature? How digital signatures can be enforced using encryptions? Illustrate with an example.                                                                       (1+5)

    b) Determine whether the integers 105 and 294 are relatively prime. Explain your answer using Euclidean algorithm.                                                                                                (4)