Caesar cipher implementation in Python

caesar.py

def caesar_enc(plain, key):
    plain = plain.encode('utf-8')
    cipher = bytearray(plain)
    for i, c in enumerate(plain):
        cipher[i] = (c + key) & 0xff
    return bytes(cipher)

def caesar_dec(cipher, key):
    plain = bytearray(len(cipher))    # at most, len(plain) <= len(cipher)
    for i, c in enumerate(cipher):
        plain[i] = (c - key) & 0xff
    return plain.decode('utf-8')

message = 'السلام عليكم'
key = 1436

assert caesar_dec(caesar_enc(message, key), key) == message