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Unformatted text preview: of the key is grab every
Lth letter from the ciphertext and perform frequency analysis
– Then, to get the 2nd letter of the key, we grab the 2nd, L+2, 2L+2,
… letter from the ciphertext
– And so on… The trick lies in finding the length of the key! Finding the Key Length in Vigenere
Vigenere
Write the ciphertext on two strips of paper. Put one on top of the
other, but displaced by a certain amount of places.
VVHQWVVRHMUSGJGTH
VVHQWVVRHMUSGJGTH
* Mark a * each time a letter and the one below it are the same.
Count the total number of coincidences for different
displacements
The displacement with the most coincidences is the most likely
key size. Finding the Key Length Explained, pg. 1
Finding
So why does this work?
Suppose we write down the vector of frequencies for English
letters:
A0 = (.082, .015, .028, …, .020, .001)
We may shift A0 by i spaces to the right to get Ai , which
corresponds to the probabilities we would get if we applied a shift
cipher where we shifted by i letters
Look at the dot products...
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This note was uploaded on 01/07/2013 for the course 332 519 taught by Professor Wadetrappe during the Fall '12 term at Rutgers.
 Fall '12
 WadeTrappe

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