Integrated encryption and channel coding technique

Abstract

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.