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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.

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