The Hill cipher uses matrices and congruence arithmetic. First, the alphabetic message is converted to a string of numbers, use 0 for a, 1 for b and so on until we reach 25 for z. The message is then broken up into segments of equal length, and each of these segments treated as a column matrix and mutiplied by an enciphering matrix, C, and the result modulo 26 is used to give the ciphertext. The deciphering process uses the same procedure, but with a deciphering matrix, D. We will work with the Hill cipher with 5 12 (a) Encipher the message 'ok'. (b) Show that the deciphering matrix 19 12 3 21 recovers the original message. (c) Calculate the matrix CD. (Remember to give your answer modulo 26.)