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# MID_NS_V - cipher text blocks Show that if an error occurs...

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Birla Institute of Technology & Science, Pilani Distance Learning Programmes Division Second Semester 2007-2008 Mid-Semester Test (EC-1 Regular) Course No. : SS ZG513 Course Title : NETWORK SECURITY Nature of Exam : Closed Book Weightage : 40% Duration : 2 Hours Date of Exam : 02/02/2008 (FN) Note: 1. Please follow all the Instructions to Candidates given on the cover page of the answer book. 2. All parts of a question should be answered consecutively. Each answer should start from a fresh page. 3. Mobile phones and computers of any kind should not be brought inside the examination hall. 4. Use of any unfair means will result in severe disciplinary action. Q.1 Show how encryption and decryption are done on the plain text using playfair cipher. Plain text : Long live the king Key : Computer. [3 + 3 = 6] Q.2 The cipher block chaining (CBC) mode has the property that it recovers from errors in
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Unformatted text preview: cipher text blocks. Show that if an error occurs in C j , then this only affects two blocks for decryption. Which two blocks? [5] Q.3 Using Extended Euclid method find the smallest positive integer X such that, . 73 mod 4 31 = • X [8] Q.4 RSA problem: [4 + 2 +2 = 8] Let p = 11, q = 13 and e =7. (a) Find d (b) Encrypt the message 99. (c) Using two prime numbers p and q that are close to each other in value is a bad idea for RSA. Q.5 In the round key expansion procedure AES uses the 8-bit constants Ci, i = 1, 2, 3,…. .,30 that can be computed as Ci = 2 i-1 In polynomial arithmetic modulo m(x) = x 8 + x 4 + x 3 + x + 1, that is, in Galois field GF(2 8 ) with polynomial m(x). Compute C8, C9 and C16. [10] Q.6 Explain the following [3] a. Factoring Problem b. Timing Attacks ******** No. of Pages = 1 No. of Questions = 6...
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