hw4-sol_Security

# Hw4-sol_Security - CMSC 414 Hw 1 Solution and Grading Guidelines 1(text 3.3 In DES how many keys on the average encrypt a particular plaintext

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CMSC 414: Hw 1 Solution and Grading Guidelines ___________________________________________________________ 1. (text 3.3) In DES, how many keys, on the average, encrypt a particular plaintext block to a particular ciphertext block. Each key maps 2 64 plaintext blocks to 2 64 ciphertext blocks. So it has a 1/2 64 chance of mapping a plaintext block b to a ciphertext block c. There are 2 56 keys, so the total probability of mapping p to c is (1/2 64 ) 2 56 = 1/256. ___________________________________________________________ 2. (text 3.5) Suppose the DES mangler function maps every 32-bit value to zero, regardless of the value of its input. What function would DES then compute? DES does the following (see text figure 3-2): Initial permutation 16 DES rounds Swap left and right halves final permutation (inverse of initial permuation) With a mangler function that outputs 0 always, each DES round just swaps L and R. So after 16 (even number) DES rounds, the initial 64-bit word would be unchanged. So DES would do the following: Initial permutation Swap left and right halves final permutation Based on the initial permutation, the net result is a permutation that interchanges consecutive even and odd bits. [If the swap were not there, DES would have no affect at all.]

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## This note was uploaded on 12/03/2008 for the course CSC 574 taught by Professor Ning during the Spring '04 term at N.C. State.

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Hw4-sol_Security - CMSC 414 Hw 1 Solution and Grading Guidelines 1(text 3.3 In DES how many keys on the average encrypt a particular plaintext

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