Which of the following schemes is a cryptosystem? What is the plaintext space, the ciphertext space, and the key space?
(a) Each letter x ε Z26 is replaced by kx (mod 26), k ε {1, 2, . . . , 26}.
(b) Each letter x ε Z26 is replaced by kx (mod 26), k ε {1, 2, . . . , 26}, gcd(k, 26) = 1.