Birla Institute of Technology & Science, Pilani Distance Learning Programmes Division Second Semester 2008-2009 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 : 07/02/2009 (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 used inside the examination hall. 4. Use of any unfair means will result in severe disciplinary action. Q.1 Find the smallest positive integer x such that x . 31= 1 mod 73.  Q.2 Assume that there is a block cipher ABC that always encrypts block of b=72 bits using key of k= 60 bits. We know that ABC will be broken even using simple brute-force attacking by guessing the encryption key. To enhance the security level, assume that someone proposes to use XYZ by use of 2 encryptions of ABC
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