Cryptography
Computer Science & Statistics at University of Rhode Island
Vigenere Cipher Cryptanalysis The Vigenere Cipher initially seems very secure, however it can be broken fairly easily once the length of the keyword is known. If you know that the length of the keyword is n, you can break the ciphertext into n cosets and attack the cipher using frequency analysis if the ciphertext sample is long enough. This page will look at two methods to determine the length of the keyword, the Friedman and Kasiski tests.
Friedman Test
Kasisiki Text The Kasiski Test uses the occasional aligning of groups
of letters with the keyword to determine the length of the keyword. This
will produce repeated groups of letters in the ciphertext. By counting the
number of letters between the beginnings of these repeated groups of
letters and finding a number which is the multiple of those distances, we
can estimate the length of the keyword.
The second group of repeated letters IVE occurs 12 letters after the first. The third group of letters appears 6 letters after the first. Since both of these numbers are multiples of 6, this suggests that the length of the keyword is 6.
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