encrypt : {a}(fin a) => [8] -> [a][8] -> [a][8]
encrypt key plaintext = [pt ^ key | pt <- plaintext ]

decrypt : {a}(fin a) => [8] -> [a][8] -> [a][8]
decrypt key ciphertext = [ct ^ key | ct <- ciphertext ]

property roundtrip k ip = decrypt k (encrypt k ip) == ip