Method and apparatus for synchronous stream cipher encryption with reserved codes

A signal decryption device constructed to receive a key and an encrypted input word with restricted codes, the device comprising:

• a keystream generator for producing a pseudorandom word from the key;
• an adder coupled to the keystream generator for receiving the pseudorandom word and coupled to the encrypted input word and producing a summed signal therefrom;
• and
• a lookup table using the summed signal to produce a decrypted code excluding restricted codes for the encrypted input word, wherein the signal decryption device does not generate or operate on the restricted codes, wherein the device performs synchronous stream cipher decryption for a sequence of input words, and wherein the keystream generator produces a sequence of pseudorandom words in response to the key with a period of repetition and the key is changed before the period of repetition repeats.

A signal encryption and decryption system comprising:

• a first keystream generator for producing a first pseudorandom word from a received key;
• a first adder coupled to the first keystream generator and receiving an input word with restricted codes, and producing a first summed signal from the first pseudorandom word and the input word;
• a first lookup table using the first summed signal to produce an encrypted word excluding the restricted codes;
• a second keystream generator for producing a second pseudorandom word from the key;
• a second adder coupled to the second keystream generator for receiving the second pseudorandom word and receiving the encrypted word to produce a second summed signal therefrom;
• and
• a second lookup table using the second summed signal to produce a decrypted code excluding the restricted codes for the encrypted word, wherein the decrypted code is substantially the same as the input word, and wherein the signal encryption and decryption systems do not generate or operate on the restricted codes, wherein the system performs synchronous stream cipher encryption for a sequence of input words, the first and second keystream generators produce a sequence of pseudorandom words in response to the key with a period of repetition, and the key is changed before the period of repetition repeats.

A method of producing an encrypted code for an input word with restricted codes from a key, the method comprising:

• performing synchronous stream cipher encryption for a sequence of input words by:
• generating a pseudorandom word from the key using a keystream generator;
• summing the input word and the pseudorandom word to produce a summed signal therefrom;
• applying a lookup table to the summed signal to produce the encrypted code for the input word excluding the restricted codes, wherein the restricted codes are not operated upon or generated while producing the encrypted code;
• and
• generating a sequence of pseudorandom words with a period of repetition in response to the key using the keystream generator, and changing the key before the period of repetition repeats.

A method of producing a decrypted code for an encrypted input word with restricted codes from a key, the method comprising:

• performing synchronous stream cipher decryption for a sequence of encrypted input words by:
• generating a pseudorandom word from the key using a keystream generator;
• summing the encrypted input word and the pseudorandom word to produce a summed signal therefrom;
• and
• applying a lookup table to the summed signal to produce a decrypted word for the encrypted input word excluding the restricted codes, wherein the restricted codes are not operated upon or generated while producing the decrypted code, and wherein the keystream generator produces a sequence of pseudorandom words in response to the key with a period of repetition and the key is changed before the period of repetition repeats.

A method of encrypting and decrypting an input word with restricted codes from a key, the method comprising:

• producing a pseudorandom word from the key using a keystream generator;
• summing the input word and the pseudorandom word to produce a first summed signal therefrom;
• applying a first lookup table to the first summed signal to produce an encrypted code for the input word excluding the restricted codes;
• producing a second pseudorandom word from the key using a second keystream generator;
• summing the encrypted code and the second pseudorandom word to produce a second summed signal therefrom;
• applying a second lookup table to the second summed signal to produce a decrypted code excluding the restricted codes for the encrypted code, wherein the restricted codes are not operated upon or generated during the encrypting or decrypting;
• performing synchronous stream cipher encryption for a sequence of input words;
• producing a sequence of pseudorandom words with a period of repetition with the keystream generator in response to the key, and changing the key before the period of repetition repeats.