This corresponds to a shift of the corresponding

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Unformatted text preview: < Ai , Aj >, which depend only on |i-j| (Why?) The maximum value is when i-j =0. – This just says that a vector is most similar to itself. Finding the Key Length Explained, pg. 2 Finding Now take a strip of ciphertext, and look at a random letter. This corresponds to a shift of the corresponding plaintext letter by an amount i, corresponding to an element of the key. Now, place a second strip below, and displace it. The letter below the first will correspond to a random letter of English shifted by j. What is the probability they are both “A”? This is Ai(0) * Aj(0) Similarly, the probability they are both “B” is Ai(1) * Aj(1), and so on… The total probability that these letters are the same is < Ai , Aj >. When i=j, we have the maximum. This happens when the letters lying one above the other have been shifted by the same amount! Binary and ASCII Binary Generally, our data is not simply letters, but may more generally be characters, or they may be binary data generated by a program Binary numbers are a way to represent numbers base 2 Example: 110101 = 25 + 24+ 22 + 1 ASCII is the America Standard Code for Inf...
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