Communications Satellite Corporation

Other

Shyue-Ching Lu

Lin-nan Lee

Russell J Fang

A method of protecting communications through encryption is disclosed which is particularly useful for multiplexed communications. On the transmit side, each message is multiplied by an enciphering key corresponding to its destination, and the products are all added together to form a cryptogram. Each receiving station divides the entire cryptogram by its own deciphering key to obtain the message intended for that station.

Multiple-destinational cryptosystem for broadcast networks

A public encryption key (c.sub.1, c.sub.2, r) in which r is the product of two relatively prime numbers, and in which c.sub.1 and r, as well as c.sub.2 and r, are relatively prime numbers, is used in an encryption algorithm

Simple and effective public-key cryptosystem

The strength of a cryptosystem is increased by providing multiple characters for one or more of the characters being encoded to thereby reduce redundancy.

Reduction of message redundancy by multiple substitution: a message

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.

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Chung-Li, Taiwan

Shyue-Ching Lu