Vigenere Encryption: given,

m = theclassistough,

k = apple,

do the encryption and then do the decryption from the ciphertext using this.

Example 2.5. The Vigenre cipher of (key) length is a classical example of a symmetric variable-length scheme over the alphabet of letters. One sets = = and = , where = {, , , } 26. The letters A to Z can be represented by integers modulo 26 (see Example 1.21). The letter A corresponds to the residue class 0, B to 1, , Z to 25. A cyclic shift then becomes addition or subtraction of residue classes. (1) generates a uniform random key string $ of length . For encryption and decryption, the message and the ciphertext is split into blocks of length , although the last block can be shorter. Each letter in a plaintext block is transformed by a cyclic shift, where the number of positions is determined by the corresponding key letter. For encryption the shifting is in the positive direction, and for decryption it is in the opposite direction. = () = (12 ) = (1 + 2 + ) mod 26, = () = (12 ) = (1 - 2 - ) mod 26