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12 June, 07:45

Consider an improved version of the Vigen ere cipher, where instead of using multiple shift ciphers, multiple mono-alphabetic substitution ciphers are used. That is, the key consists of t random permutations of the alphabet, and the plaintext characters in positions i, t + i, 2t + i, and so on are encrypted using the ith permutation. Show how to break this version of the cipher.

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  1. 12 June, 09:14
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    Kasiski's method for determining 't' works for Vigenère cipher as well. The only difference is therefore in the second stage of the attack. In the second stage, one needs to build a frequency table for each of the 't' keys, and carry out an attack like on the mono-alphabetic cipher. Given a long enough plaintext, this will work successfully.

    Explanation:

    Kasiski method is a method of attacking polyalphabetic substitution ciphers such as Vigenère cipher. It is also called Kasiski test or Kasiski examination.

    The method involve finding the length of the keyword and then dividing the message into that many simple substitution cryptograms. Frequency analysis could then be used to solve the resulting simple substitution.
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