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crypto-slides-03-intro-tutor.1x1

crypto-slides-03-intro-tutor.1x1 - Classic Cryptography...

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Classic Cryptography Tutorial c circlecopyrt Eli Biham - August 18, 2010 67 Classic Cryptography Tutorial (3)
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Ciphertext-Only Attack on Substitution Cipher Monoalphabetic substitution ciphers cannot protect against known plaintext and chosen plaintext attacks. Therefore, we restrict our discussion to ciphertext- only attacks, and try to prove that even under a ciphertext-only scenario they are insecure. We will see that there are algorithmic shortcuts that help the attacker using additional information. Monoalphabetic substitution ciphers are vulnerable to ciphertext-only attacks if the ciphertext and the distribution of the plaintext letters (i.e., in an English text) are known to the attacker. The main observation is that the distribution of the letters is invariant to the permutation, and that each letter is permuted to another letter, whose frequency (in the ciphertext) is the same as the frequency of the letter in the original text. c circlecopyrt Eli Biham - August 18, 2010 68 Classic Cryptography Tutorial (3)
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Ciphertext-Only Attack on Substitution Cipher (cont.) For example, the most frequent letter in an English text is e : Letter Frequency Letter Frequency Letter Frequency e 12.31% l 4.03% b 1.62% t 9.59% d 3.65% g 1.61% a 8.05% c 3.20% v 0.93% o 7.94% u 3.10% k 0.52% n 7.19% p 2.29% q 0.20% i 7.18% f 2.28% x 0.20% s 6.59% m 2.25% j 0.10% r 6.03% w 2.03% z 0.09% h 5.14% y 1.88% c circlecopyrt Eli Biham - August 18, 2010 69 Classic Cryptography Tutorial (3)
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Ciphertext-Only Attack on Substitution Cipher (cont.) The most frequent English word is the : Word Frequency Word Frequency Word Frequency the 6.421% a 2.092% i 0.945% of 4.028% in 1.778% it 0.930% and 3.150% that 1.244% for 0.770% to 2.367% is 1.034% as 0.764% c circlecopyrt Eli Biham - August 18, 2010 70 Classic Cryptography Tutorial (3)
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Breaking Monoalphabetic Substitutions Exercise : Solve UCZCS NYEST MVKBO RTOVK V RVKC Z O SJM UCJMO MBRJM VESZB SMOSJ OBKYE MJTRV VEMP Y JMOMJ AMVEM HKOVJ KT RVK CZ CQV EMNMV VMJOS ZHVER O VEMP BSZTM MSOKN PTJCI MZ The frequency of the letters in this ciphertext: Letter A B C D E F G H I J K L M Occurs 1 5 7 0 8 0 0 2 1 10 8 0 19 Letter N O P Q R S T U V W X Y Z Occurs
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