Zhou_Zhang_HW2

# Zhou_Zhang_HW2 - Zhou HuanXian(0225418 Zhang Huan(0336630...

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Zhou, HuanXian (0225418) Zhang, Huan (0336630) CS392 HW2 HW #2: Private Key Cryptography CS 392/6813: Computer Security Fall 2010 [100pts] DUE 09/30/2010 (midnight) Problem 1 The following strings are ciphertexts that have been encrypted using one of the classical cryptosystems that were discussed in class. Each of them decrypt to an English sentence without any whitespace or punctuation. Cryptanalyze each one and provide the decrypted plaintext as well as the secret key used to produce it. Explain the process you went through and include any script you used in the cryptanalysis process. 1) [15 points] Vigenere Cipher: QFZMZZJLXGYCLMZPZQTRNGXMHHAMEIWMJGMMGCFEWKTRWOA NGCIGBOECAMEUDOYBPVLJUATJAWZLGSDGSSYRHOYBHSNMCRW YGUPQIWYYGSLGIWDYAGZRWSHCHHPPCAZQIQZSCHJMCZZLVWDJ PBOZGCZIAMYUPGLLXBOCESYBTBEAXHJSCHTJXHDADBDMAWOYI WZLLWEFCSHWDFVAXHJYCRNMCHTLJSDRDALGCHLGCOOGHHTLR HNSAHFPTWYBTDPLSSYRPFEQRSYCPBOSCWBSTOCAWWECRHFPPZ SCGWEYVS In order to break the Vigenere cipher, we have to find repeating pattern in the string. For example the string AME appears twice and each string is 30 characters apart from each other. For there we can determine the possible key lengths, which are “2, 3, 4, 5, 6, and 10.” We repeat this step until all the patterns have being tally. Repeated Sequence Spacing 2 3 4 5 6 7 8 9 10 AME 30 x x x x x OYB 24 x x x x x WZL 120 x x x x x x x SSY 176 x x x SYR 176 x x x NMC 124 x x ZSC 159 x SCH 44 x x PBO 124 x x YBT 84 x x x x x AXH 36 x x x x x XHJ 36 x x x x x CHT 40 x x x x x

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Zhou, HuanXian (0225418) Zhang, Huan (0336630) CS392 HW2 HTL 20 x x x x LGC 4 x x HFP 40 x x x x x SSYR 176 x x x AXHJ 36 x x x x x As we can see from the table, the key lengths that appear the most frequent are key length 2 & 4. From there we can use frequency analysis to determine the actual key. Secret key: POLY Decrypted plaintext: BROOKLYNISNEWYORKCITYSMOSTPOPULOUSBOROUGHWITHAP PROXIMATELYTWOANDAHALFMILLIONRESIDENTSANDSECONDL ARGESTINAREAITISALSOTHEWESTERNMOSTCOUNTYONLONGISL ANDBROOKLYNWASANINDEPENDENTCITYUNTILITSCONSOLIDAT IONWITHNEWYORKCITYANDCONTINUESTOMAINTAINADISTINC TCULTUREINDEPENDENTARTSCENEANDUNIQUEARCHITECTURAL HERITAGE 2) [15 points] Substitution Cipher: DPNMGTXVUDFXODXPVDPNMGTFOZUDFXODXUPNXTFNXUBAAVX HFBTXCDUFUTEXUTGCKPYTEXTEXPVXTFDBSYPGOCBTFPOUPYFOY PVNBTFPOBOCDPNMGTBTFPOBOCPYMVBDTFDBSTXDEOFWGXUYP VTEXFVFNMSXNXOTBTFPOBOCBMMSFDBTFPOFODPNMGTXVUKUT XNUFTFUYVXWGXOTSKCXUDVFAXCBUTEXUKUTXNBTFDUTGCKPY BSZPVFTENFDMVPDXUUXUTEBTDVXBTXCXUDVFAXBOCTVBOUYP VNFOYPVNBTFPODPNMGTXVUDFXODXEBUNBOKUGAYFXSCUUPNX
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Zhou_Zhang_HW2 - Zhou HuanXian(0225418 Zhang Huan(0336630...

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