advertisement

Mathematical Description of Affine Ciphers Given a and b in Z 26 {0, 1, 2, , 24, 25} where gcd( a, 26) 1 . We encipher a plaintext letter x to obtain a ciphertext letter y by computing y (ax b) MOD 26. Here, the key is made up of a multiplicative parameter a and an additive parameter b . The next example illustrates how a message is enciphered. Example 13: Encipher “RADFORD” using the affine cipher y (5 x 4) MOD 26. Solution: █ Note: Recall that for an affine cipher y (ax b) MOD 26 to be defined properly, gcd( a, 26) 1 . Besides allowing a recipient to decipher a message, the next example illustrates another reason why this requirement is essential.