16-page3 - where the addition is taken “mod 26”(for the...

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Substitution Ciphers • Julius Caesar used a simple cryptosystem to send secret messages to his generals. Each letter was encoded as the third letter preceding, so D became A, E became B, and so on, with the last letters wrapping around. (So that in our alphabet A would become X, B would become Y, and C would become Z.) • A substitution cipher is a system where individual letters are encoded, and the ciphertext letters occur in the same order as the plaintext letters. We can generalize Caesar’s “-3” or “+23” cipher to add a chosen number to each letter,
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Unformatted text preview: where the addition is taken “mod 26” (for the key k, we map x to (x+k)%26). • This is easy to encode and decode by hand, but not very secure. Even if your enemy does not know the key, they can quickly determine which of the 26 possible keys you are using. And even an arbitrary mapping of letters to letters is subject to frequency analysis-- the most common ciphertext letter is likely to be the translation of E, the most common pair is TH, and so forth....
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