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Unformatted text preview: (mod 26) Chosen plaintext: You get to choose the plaintext x. For
simplicity, take x = ‘a’ = 0. The ciphertext is
y = k (mod 26) Chosen Ciphertext: You get to choose the ciphertext y. Choose
y=‘A’, then plaintext is x = k (mod 26).
x =  k (mod 26) What about Known Ciphertext? This is the toughest one. We
have the least information. Ciphertext Only on Shift Cipher
Ciphertext
If all Eve gets is the ciphertext, she may try one of two
strategies:
– She could try all choices for k. There are only 26 and most likely
(if message is long enough) only one key will produce something
intelligent.
– She could do a frequency counting:
Use knowledge of the underlying language. For example, ‘e’
occurs the most often in English.
Whatever letter occurs the most in the ciphertext probably
corresponds to the plaintext ‘e’. (If you have a long enough
message).
The key is probably the value needed to shift ‘e’ to the most
frequent ciphertext letter.
If the most frequent fails, try the next most frequent… Affine Cipher
Affine
The affine...
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 Fall '12
 WadeTrappe

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