MID_NS_IV - wouldn’t What happens if you double the key...

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Birla Institute of Technology & Science, Pilani Distance Learning Programmes Division First 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 : 02/08/2008 (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 used inside the examination hall. 4. Use of any unfair means will result in severe disciplinary action. Q.1 Assuming you can do 2 20 encryptions per second and the key size is 40 bits, how long would a brute force attack take? Give a scenario where this would be practical and another where it
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Unformatted text preview: wouldn’t. What happens if you double the key size? [10] Q.2 Use the extended Euclidean algorithm to compute the greatest common divisor d of 654 and 123 and to find integers m and n such that 654m + 123n = d. [10] Q.3 In a Feistel system, we divide the state into left and right halves LiRi and then define the new state by Li+1 = Ri and Ri+1 = Li f(Ki,Ri) where Ki is the key for the i th round and f is a function of the key and half of the state. Prove that no matter what the function f is, the round transformation is 1-to-1, i.e., we can recover the old state from the new state and the key. [10] Q.4 (a). What is the main drawback of the one time pad cryptosystem? [5] Q.4 (b). Show that the transposition cipher is vulnerable to a known plaintext attack. [5] ********** No. of Pages = 1 No. of Questions = 4...
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