1. Substitution Ciphers Reference –Matt Bishop, Computer Security, Addison Wesley, 2003.

2. Substitution Ciphers (p.2) Substitution Cipher –Changes characters in the plaintext to produce the ciphertext. Examples –Caesar Cipher –Vigenere Cipher –One Time Pad

3. Substitution Ciphers (p.3) Caesar Cipher –Consider the plaintext to be the letters A,B,C,...,Z. –Now shift the sequence, say, by 3 to get D,E,F,...Z,A,B,C. –Then the cipher text becomes D for A, E for B, and so on. –If each letter is represented by integers 0,1,...,25, we can describe this process as C=(M + K) mod 26, where the key is K=3.

4. Substitution Ciphers (p.4) Vigenère Cipher –The Vigenère cipher chooses a sequence of keys, represented by a string. –Key letters are applied to successive plaintext. –When the end of the key sequence is reached, the key starts over again. –The length of the key is called the period of the cipher.

5. Substitution Ciphers (p.5) One-Time Pad –A variant of the Vigenère cipher. –The key is chosen at random. –The length of the key is at least as long as that of the message, and so it does not repeat.