ps2sols

ps2sols - Massachusetts Institute of Technology Handout 10...

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Unformatted text preview: Massachusetts Institute of Technology Handout 10 6.857: Network and Computer Security September 30, 2003 Professor Ronald L. Rivest Problem Set 2 Solutions Todays fun-fact is about jointly-authored submissions. During your professional lives, you will frequently be asked to jointly submit a letter, a paper, or an article. When you submit a document that has multiple names on it, you should not write sentences that use the word I as their subject. Doing so is a dead give-away that the paper was not authored by the group, but was instead authored by a single person. (It also comes across as somewhat haughty to the reader.) For example, this sentence appeared in a homework solution to problem set #2 that had four names on the top: The approach I used to solve this problem is as follows. How should we take such a sentence? Please remember that these homeworks are meant to be done in groups. This means that each student is responsible for all of the work. It does not mean that you have license to split up the problems and have each student work on a different one. Submitted homework solutions that used the first-person pronoun were penalized between 1 and 2 points. Problem 2-1. Two-time pad (Courtesy of Rui L. Viana, Conor M. Murray, Pallavi Naresh and Richard Hansen.) TA Notes: Many students enjoyed the two-time pad problem, and so many people got it that we should make the problem harder next year. The vast majority of students tried XORing each of the texts with the other texts and then seeing which of the 3 results had no high-order bits set. One group of students did a statistical analysis of the distribution of the resultant XOR strings with the theoretical distribution of an XOR of standard english text against standard english text, but it turned out that this approach was not needed. Once the pairs were distinguished, most students wrote a program that allowed them to type in sample text and then to see the results of XORing the text in a 1-character sliding window along the entire length of the chosen combined text. This seems to have been a pretty manual process, although one group of students automated the process by doing a brute-force dictionary lookup off all the words in the web2a file and then doing a statistical analysis on the result. (As it turns out, it was easier to work manually because of the high predictability of the text.) One of those most interesting experiences in solving this assignment is reported below: Oddly enough, we had the movie Cube playing, and got to talking about people dying. Some how we someone suggested Johnny Cash, and we tried Ring of fire. Oddly enough our program spouted (among many other things) a bunch of primes. (This really freaked us out, because as I said ... we had just been watching Cube.) Our only dissappointment with the example was that many students, once they identified the source string, simply reported the source string, rather than what was actually in the plaintext .....
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This note was uploaded on 04/28/2009 for the course CS 6.857 taught by Professor Rivest during the Spring '03 term at MIT.

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ps2sols - Massachusetts Institute of Technology Handout 10...

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