Unformatted text preview: of the letters of the alphabet in this sample of ciphertext: A B C D E F G H I J K L M 2 10 2 5 3 1 8 2 2 5 1 3 1 N O P Q R S T U V W X Y Z 2 1 10 8 1 8 5 2 1 3 5 1 8 Compute the Index of Coincidence IC for this sample. (c) Assuming the IC computed in Part (b) is near its expected value, what is your best guess for t ? Remember that your answer to Part (b) is just an approximation. (d) Now suppose that the Kasiski method applied to the same ciphertext of length 100 characters suggests that t is a divisor of 18. Would this information change your answer to Part (c)? If so, what is your new guess for t ? Explain your answer. 2. Use Shannon’s theorem to prove that Vernam’s cipher is perfectly secret. Your proof should be shorter and simpler than the proof of this fact given in class and in the slides. 1...
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 Spring '10
 Wagstaff
 Cryptography, Index of Coincidence IC

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