2003_quiz1sol

2003_quiz1sol - 1 2 3 4 5 6 7 Massachusetts Institute of...

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Unformatted text preview: 1 2 3 4 5 6 7 Massachusetts Institute of Technology Handout 13 6.857: Network and Computer Security October 9, 2003 Professor Ronald L. Rivest Quiz 1 1. This quiz is intended to provide a fair measure of your understanding of the course material to date (Homeworks 13 and Lectures 110). 2. Do not open this quiz booklet until the quiz begins. Read all the instructions first. 3. Do not discuss any aspect of this quiz with anyone (except 6.857 staff ) until noon on Friday, October 10, 2003. 4. When the quiz begins, write your name on every page of this quiz booklet. 5. This quiz booklet contains 12 pages, including this one. An extra sheet of scratch paper is attached. 6. This quiz is open-book, open-notes. No calculators or programmable devices (including laptop computers) are permitted. 7. Write your solutions in the space provided. If you need more space, write on the back of the sheet containing the problem. Do not put part of the answer to one problem on the back of the sheet for another problem; pages may be separated for grading. 8. Partial credit will be given. You will be graded not only on the correctness of your answer, but also on the clarity with which you express it. Be neat. 9. Good luck! Problem Points Grade Initials 24 9 36 12 10 10 10 Total 111 Your Name: 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 catchers 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 Euclids extended algorithm. Half credit was given for the answer 11 1 = 11 28 (mod 29), based on Fermats 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 - 1 2 3 4 5 6 7 Massachusetts Institute of...

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