A substitution cipher does not have to be a character-to-character transformation. In a Polybius cipher, each letter in the plaintext is encrypted as two integers. The key is a 5 × 5 matrix of characters. The plaintext is the character in the matrix, the ciphertext is the two integers (each between 1 and 5) representing row and column numbers. Encipher the message €œAn exercise€ using the Polybius cipher with the following key:

3 4 5 f 2 3 h 4 i/j b m 2.