you have it right M=loveyou K=spgbwoz C_i =letter(order(M_i)+order(K_i) mod 26) where order() and letter() backward and forward lookup in this table: 0 - a 1

you have it right M=loveyou K=spgbwoz C_i =letter(order(M_i)+order(K_i) mod 26) where order() and letter() backward and forward lookup in this table: 0 - a 1 - b 2 - c ... 24 - y 25 - z The Decryption function is similar to the encryption function but you subtract the key from the cyphertext to get back the message. Claude Shannon proved that the one-time pad is unconditionally secure using the concept of entropy H(). Very simple argument which you could try to lookup.