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Unformatted text preview: H(K C) = H(K) + H(M)  H(C) Recall two basic equations next next next Recall the experiment we carried out in class ? The sequence that gave the number of guesses for each letter may be viewed as an encoding of the sentence next The Unicity distance The least amount of ciphertext the opponent needs to know to uniquely recover the plaintext = 0 ? next The unicity distance equation then becomes Computation of the Unicity Distance for monoalphabetic ciphers Thus the encryption of such a message will use only 14 letters of the permuted alphabet and we get In conclusion: for a monoalphabetic cipher 23 characters of ciphertext should be sufFcient to uniquely determine the plaintext next SAMPLE PROBLEMS next Sample quiz problems 1 2 next next next...
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 Spring '10
 MATH187
 Math, Cryptography, perfect secrecy, unicity distance

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