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2003_quiz1sol

# 2003_quiz1sol - Massachusetts Institute of Technology 6.857...

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2 Handout 13: Quiz 1 Name: Problem Q1-1. Short Answer [24 points] (a) At a recent Red Sox game, you observed the catcher making a variety of signals to the pitcher with his hands before every pitch. Following the signals, you observe the type of pitch: e.g., fast ball, curve ball, slider, knuckleball, bean-ball, etc. By the end of the game, you are able to predict every pitch after seeing the catcher’s signals. What sort of cryptographic attack have you successfully executed? Solution: This is a known-plaintext attack. Half credit was given for the answer “passive attack.” (b) What is 11 1 (mod 29)? Show your work. Solution: 11 1 (mod 29) = 8, because 8 · 11 = 88 = 29 + 1. We can get this 3 · result by using Euclid’s extended algorithm. Half credit was given for the answer 11 1 = 11 28 (mod 29), based on Fermat’s Little Theorem. (c) You are watching an encrypted conversation between Alice and Bob. You notice that the prefixes of many of the ciphertexts agree for several hundred bytes. In addition, these identical prefixes are always a multiple of 16 bytes long. However, you never observe two identical chunks of ciphertext of any significant length following the identical prefixes. Conjecture what cipher is being used, what mode of operation is being used, and what Alice and Bob are doing wrong. Solution: The answer we had in mind was AES (or DES) under CBC mode, (incor- rectly) using the same IV for every message. We also gave full credit for an answer such as AES or DES in ECB mode, with some explanation (e.g., all messages have long, common headers.) (d) Next, you start spying on a different encrypted conversation between Alyssa and Ben.
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2003_quiz1sol - Massachusetts Institute of Technology 6.857...

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