Communications Satellite Corporation

Russell J Fang

Lin-nan Lee

Shyue-Ching Lu

In a secure communications system, a key number which is changed periodically, e.g. monthly, and a random number from a random number generator are combined and used as a seed to reset a PN sequence generator, with the output of the generator being used to control encryption of transmission data in a signal processor. The key is also provided to a first encipherer to encipher the random number for transmission with the encrypted data. At the receiver, the key is provided on common to a decipherer for deciphering the random number and a PN sequence generator which is periodically reset by the combination of the key and random number in the same manner as in the transmitter. The PN sequence is then used to decrypt the information. User identification codes are stored in the transmitter and are used to encipher the key, with each employing its ID code to decipher the key. The user ID codes are known only to the system operator, so that not even a particular user can know the key.

Security system for SSTV encryption

The integrated data enciphering and channel encoding system consists of a message expansion function F, a convolutional encoder G.sub.e of code rate r.sub.e greater than the computational cutoff rate R.sub.comp, and an artificial noise generator emitting a noise sequence n. The expansion function F is the key to this integrated cryptosystem and can be any linear time varying or time invariant function, with or without memory. It can also be a block code or a convolutional code itself provided that the resultant concatenated convolutional code is non-catastrophic. The concatenation of the expansion function F with G.sub.e is equivalent to a new convolutional encoder G.sub.d .ident.FG.sub.e of code rate r.sub.d less than the computational cutoff rate R.sub.comp of the equivalent channel, but without knowledge of the expansion function F or the artificial noise n, the transmitted information rate appears to exceed R.sub.comp. Thus, an opponent can only attempt to intercept the original message by decoding the code G.sub.e, but the information rate r.sub.e exceeding R.sub.comp will prevent this. The output of the encoder plus the noise sequence n is the cryptogram x which is transmitted over a noisy channel characterized by the additive noise sequence z. The corrupted version of the cryptogram y which is received at the receiving end is regarded as the output of the equivalent convolutional encoder G.sub.d plus noise e equivalent to the sum of the artificial noise n and the channel noise z. Therefore, the integrated channel decoder and deciphering system simply consists of a sequential decoder for G.sub.d =FG.sub.e.

Integrated encryption and channel coding technique

A spatial diversity satellite communications system adapted to provide its own error control. The diversity communications system utilizes transmitting station selection or burst switching in the up-link and both probabalistic and algebraic error control in the down link to increase message accuracy and to overcome precipitation attenuation. Specialized multiple level decision demodulators are employed to exploit the inherent redundancy supplied by having available multiple signals embodying identical messages. Information is fedback, once the system is operating, and is used to select the optimum transmitter site. However, all available receivers are utilized and the received messages are statistically evaluated to accurately reproduce the received message.

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Brookeville, MD, United States of America

Russell J Fang