A method and apparatus for a signal encryption device constructed to perform synchronous stream cipher encryption for a sequence of input words with restricted codes. The encryption device includes a keystream generator for producing a sequence of pseudorandom words from a key, and an adder that is used to sum the output of the keystream generator and the input words. A lookup table of size substantially twice the number of possible input words provides encrypted codes excluding restricted codes from the summed signal. A signal encryption and decryption system is constructed by including a second keystream generator for producing a second sequence of pseudorandom words from the key, and a second adder to produce a second summed signal from the output of the second keystream generator and the encrypted codes. A corresponding lookup table provides decrypted codes excluding restricted codes from the second summed signal.