Test of Kasiski

The test of Kasiski is a technique of Cryptanalyse invented by Friedrich Wilhelm Kasiski in 1863. It makes it possible to attack the Chiffrement of Vigenère.

Principle

The coding of Vigenère is a polyalphabetic substitution (for example, the letter M can be quantified to P or Z). The key is a sequence of letters of fixed size, which is repeated and truncated so as to have the same size as the message to be quantified. The goal of the test of Kasiski is to find the size of the key.

Kasiski realizes that sequences appear several times in the quantified text. Vis-a-vis relatively random contents, these repetitions within the coded message are not alleviating. Such a redundancy indicates that the same continuation of letters in the message in light is quantified on several occasions.

The analysis consists in examining the distance between the repetitions. If the key has a length M and that the distance between two sequences is D then to obtain this repetition it is necessary that M is a divider of D (if not the key would be shifted compared to the sequence in the plaintext and coding would be different). One thus draws up a list of the distances between similar sequences and one calculates the largest common divider between these distances. This result is the length of the key.

to see the article Cryptanalyse of the figure of Vigenère for a complete example

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