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Unformatted text preview: < Ai , Aj >, which depend only on ij
(Why?)
The maximum value is when ij =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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 Fall '12
 WadeTrappe

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