Question 1 (total of 6 marks) Critique the following statement: We achieve security by obscurity: by keeping our algorithms secret we obtain the best guarantee of security. Your answer should cover roughly half a page of A4 paper. (6 marks) (ps: You are strongly encouraged to answer this question using a word processing / typesetting program, e.g., LaTex, Word. Please avoid submitting it handwritten.)

IY 3660 Assessed Coursework 2 Question 1 Explain what hybrid encryption is and discuss why it is a commonly used approach in cryptog- raphy.

Question 1 Explain what hybrid encryption is and discuss why it is a commonly used approach in cryptography.

Question 2 2a. The ephemeral Diffie-Hellman Key Exchange (DHKE) protocol allows two parties to agree on keying material in the presence of an adversary. The protocol assumes the two parties have already agreed on two primes p, q such that q divides p - 1, and a value g> 1 such that gº = 1 mod p. From this starting point, describe the remainder of the protocol. [2 marks] 2b. Assuming that the adversary is passive (i.e., acts only as an eavesdropper), identify and define the mathematical problem underlying the security of this protocol. How does it relate to the Discrete Logarithm Problem (DLP) in the given setting? [4 marks] 2c. Discuss the security weaknesses of ephemeral DHKE in the situation where the adversary is an active party. 2d. Explain how you might modify the ephemeral DHKE protocol to avoid the weaknesses identified in your answer to item 2c.

Question 3 3a. Describe, using a diagram, the IND-CCA security game for public-key encryption. (Your diagram should include clearly the entities in the game, their interaction, and the winning condition.) 3b. Identify a public-key encryption scheme that satisfies IND-CCA security. For the scheme you have identified, name the mathematical assumption(s) required for the security to hold.

1. Prove that if (b,c) = 1, then (a, bc) = (a,b)(a,c). Here (n, m) denotes the greatest common divisor of n and m.

Question 4: What is the motivation for public-key encryption? Give an overview of Data Encryption Standard (DES), Advanced Encryption System (AES), and Rivest Shamir-Adelman algorithms.

Information Security Question 1: Define cryptography. What are the objectives of cryptography? Explain substitution

Question 3: Define cryptosystem, cryptanalyst, cryptographer, and cryptology.

Question 2: Define the following with examples: symmetric encryption, asymmetric encryption, stream encryption, and block encryption.