A generalization of the Caesar cipher, known as the affine Caesar cipher, has the following form: For each plaintext letter p, substitute the ciphertext letter C : C=E([a, b], p)=(a p b) 26 A basic requirement of any encryption algorithm is that it be one-to-one. That is, if p ≠q, then E(k, p) ≠E(k, q). Otherwise, decryption is impossible, because more than one plaintext character maps into the same ciphertext character. The affine Caesar cipher is not one-to-one for all values of a. For example

Shakespeare's Cipher