CISC464VSicaHWSolutions.pdf

CISC464VSicaHWSolutions.pdf - CISC464/664 Homework-2...

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CISC464/664 Homework-2 – Vincent Sica DUE: March 5th 11:59PM 1. Chapter 5. Problem 2. Message digests are reasonably fast, but here is a much faster function to compute. Take your message, divide it into 128-bit chunks, and L all the chunks together to get a 128-bit result. Do the standard message digest on the result. Is this a good message digest function? Explain why. (10 points) Answer: No. It is fairly easy to generate another message with the same 128-bit XOR 2. Chapter 5. Problem 6. Why do MD5 and SHA-1 require padding of messages that are already a multiple of 512 bits? (10 points) Answer: Otherwise it would be easy to find two messages with the same hash. Let M' be any message that is not a multiple of 512 bits. Let M be M' padded as in MD4, so M is a multiple of 512 bits. If no padding is used for M (because it is a multiple of 512 bits) then MD4(M) would be the same as MD4(M'). 3. Suppose that H(m) is a collision-resistant hash function that maps a message of arbitrary bit length into an n-bit hash value. Is it true that, for all messages
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