hw1_sol_Security

# hw1_sol_Security - CSC474/574 Information Systems Security...

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CSC474/574 - Information Systems Security: Homework1 Solutions Sketch September 20, 2004 1. Consider slide 12 in the handout for topic 2.2. Prove that the decryp- tion process of a one-round Feistel cipher is the same as the input of the corresponding encryption process. That is, ( L 0 2 ,R 0 2 ) = ( L 0 ,R 0 ). Ans. R 0 = L 1 = R 2 = R 0 0 = L 0 1 = R 0 2 L 0 2 = R 0 1 = L 0 0 L F ( R 0 0 ,K 1 ) = L 2 L F ( R 2 ,K 1 ) = R 1 L F ( R 0 ,K 1 ) = L 0 2. Random J. Protocol-Designer has been told to design a scheme to prevent messages from being modiﬁed by an intruder. Random J. decides to append to each message a hash of that message. Why doesn’t this solve the problem? Ans. Anybody can generate and append a hash to any message. A malicious adversary can easily modify the message and append the recomputed hash value. This modiﬁcation goes undetected at the receiving end. 3. How many DES keys, on the average, encrypt a particular plaintext block to a particular ciphertext block? Ans. There are 2 56 possible keys and 2 64 possible ciphertext blocks for a particu- lar plaintext block. So only about 2 56 - 64 = 1 / 256 of the possible ciphertext blocks can be obtained with a DES key. 1

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4. Alice developed a message authentication code (MAC) based on DES. Her algorithm works as follows: For a given input message M, rep- resent M as M = ( X 1 k X 2 k ... k Xm ), where Xi is a 64-bit block and k represents concatenation. Compute Delta ( M ) = X 1 X 2 ... Xm , where represents bit-wise XOR. Then the MAC for M is computed as C K ( M ) = E K ( Delta ( M )), where E is DES encryption algorithm and K is
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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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hw1_sol_Security - CSC474/574 Information Systems Security...

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