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CM30173 University of Bath DEPARTMENT OF COMPUTER SCIENCE EXAMINATION CM30173 May 2012 No calculators may be brought in and used. Full marks will be given for correct answers to THREE questions. Only the best three answers will contribute towards the assessment. Examiners will attach importance to the number of well-answered questions. CM30173 CM30173 2. 1.

CM30173: Cryptography eserved@d =[@let@token art IV CM30173: Cryptography Part IV New directions in cryptography Idea 1: A public-key cryptosystem Idea 2: A signature scheme Idea 3: Public-key distribution scheme RSA Mathematical background Part V Public-key cryptography CM30173: Cryptography Part IV Mathematical background Arithmetic modulo n The Euclidean algorithm The extended Euclidean algorithm The Chinese remainder

CM30173 University of Bath DEPARTMENT OF COMPUTER SCIENCE EXAMINATION CM30173: Cryptography Full marks will be given for correct answers to THREE questions. If you opt to answer more than the specified number of questions, you should clearly identify which of your answers you wish to have marked. In cases where you have failed to identify

1. (a) Give the definition of a Feistel cipher encryption system, including the definition of the encryption and decryption functions. [6] (b) Give an example of an encryption system based on a Feistel cipher. [2] (c) What is an S-box? Describe a property of the S-boxes used in a block cipher which is necessary for

Announcements Announcements • Exam 2 on Friday – Please let me know if you cannot take during class time (and didn’t already tell me for exam 1) • No class on Monday Last Time • Greedy Algorithms – Find decision Procedure – Repeatedly make best available choice – Repeat until done • Exchange arguments •

O. Braun 1Algorithmic Complexity Algorithmic Complexity and NP-hard problems O. Braun 2Algorithmic Complexity Try again harder! NP-hard problems Continue with the next problem. Yes No Problem Can you find an efficient algorithm? Still no? How do you convey the bad information to your boss? O. Braun 3Algorithmic Complexity Approach 1: Take the loser‘s way out

1. (a) What is meant by the following properties of an encryption system? • Confusion. • Diffusion. [4] (b) Explain how a substitution-permutation network can be used to encrypt a block of plaintext. [4] (c) Describe how differential cryptanalysis may be used to attack a SPN-based encryption system. Your answer should identify the goal of