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hw1_sol - CSC 574 Section 001 Spring 2011 Homework#1ANSWER...

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1 CSC 574 Section 001 Spring 2011 Homework #1ANSWER KEY Keywords: Secret Key Cryptography, DES, Authentication, Integrity, MAC, Multiple Encryption DES (EDE), Encrypting Large Messages (CBC, OFB, CFB, CTR). Instructions You can do this homework in groups of two (at most). The total number of points is 60. You must answer all questions for full credit. The due date is as posted on the web page (submission through Wolfware). In general, you can make any reasonable assumptions in your answers. Do NOT make unnecessary assumptions.
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2 Question 1:[Affine Cipher] [10 points] Decrypt the following ciphertext produced by the affine cipher C = (9P + 4) mod 26 XERKXABTPOJEKTTUAUOOQKMACPELONOORBOEFYRGENACTTPONEFNBOEQYGA TMOTTAFEMYWARKYFOBIMKOZXTPOZCWQYOKTIERARTPOXEWOAXTPOOEBTPYP ELONOORYRNEZZJEBQKXABKOLORTOORMOEBKERFPELOROLOBBOWOYLOFERMT PYRGNCTQYRFROKKERFORWACBEGOIORTXBAIMACXERK Solution: To decrypt this affine cipher, one needs to subtract 4 and then divide by 9 (mod 26) every ciphertext letter to get the corresponding plaintext letter. Dividing by 9 is the same as multiplying by the multiplicative inverse of 9, which is 3 (i.e., 9*3 = 27 = 1 mod 26). The result (with spaces and punctuation added for readability) is an excerpt of a famous speech by Lou Gehrig (after whom Lou Gehrig's disease is named): FANS, FOR THE PAST TWO WEEKS YOU HAVE BEEN READING ABOUT THE BAD BREAK I GOT. YET TODAY I CONSIDER MYSELF THE LUCKIEST MAN ON THE FACE OF THE EARTH. I HAVE BEEN IN BALLPARKS FOR SEVENTEEN YEARS, AND HAVE NEVER RECEIVED ANYTHING BUT KINDNESS AND ENCOURAGEMENT FROM YOU FANS. Question 2:[Monoaphabetic Cipher][10 points] Find or write a program (I will provide one written in Java) that reads an input ciphertext, and outputs the (sorted) letter frequencies, digram (consecutive pairs of letters) frequencies, and trigram (consecutive triplets of letters) frequencies. Using this program, decode the following ciphertext. The original plaintext that produced this ciphertext contains only upper case letters; all punctuation and spaces were removed before encoding. It was encrypted with a mono-alphabetic cipher. Explain how you decoded the ciphertext in your answer. WMTLXJAVQNXBTECJIZMWDMEIGXXVTSXRAWHXHTNHVMTVWKVMXBTECQGXUCX SWXNDXWYNQZQGNQZTIQGPAHRWNRQGVMXGAVASXJAVJIVMXCTKVTNHPAHRWN RJIVMXCTKVWZWKMVQYNQZZMTVVMXSXMTKJXXNWNVMXDQNHADVQGVMXJSWVW KMEWNWKVSIGQSVMXBTKVVXNIXTSKVQPAKVWGIVMQKXMQCXKZWVMZMWDMRXN VBXEXNMTLXJXXNCBXTKXHVQKQBTDXVMXEKXBLXKTNHVMXMQAKXWKWVVMTVW NKWHWQAKKEWBXZWVMZMWDMQASCXVWVWQNMTKJXXNBTVXBISXDXWLXH Solution: The best starting place was to find the word THE, which gives you 3 letters. From there you have to guess a few main letters, possibly using backtracking if your guesses were wrong, and then things start getting obvious.
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