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hw2_sol_Security

# hw2_sol_Security - CSC474/574 Information Systems Security...

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CSC474/574 - Information Systems Security Homework 2 Solutions Sketch October 24, 2004 1. Problem 3 on page 236 (Multiple KDC Domains) Alice KDC A : let me talk to KDC A KDC A generates key K aC KDC A KDC C : K A - C (talk to Alice; use key K aC ) KDC A Alice : K Alice (use key K aC ) Alice KDC C : let me talk to KDC B KDC C generates key K aB KDC C KDC B : K B - C (talk to Alice at A; use key K aB ) KDC C Alice : K aC (use key K aB ) Alice KDC B : let me talk to Boris KDC B generates key K ab KDC B Boris : K Boris (talk to Alice at A; use key K ab ) KDC B Alice : K aB (use key K ab ) 2. Problem 2 on page 255 (Eavesdropping) Let r be the no. of passwords in the list, q be the no. of passwords re- quested on each login, and s the no. of logins that have been eavesdropped on. Then the probability that the eavesdropper knows all q passwords for the next login is approximately [1 - (1 - q/r ) s ] q . For q = 1, the probability is roughly 1 - e - s/r . For values of s for which this probability is reasonably small, say s < r/k

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