e that in DSSS the PN sequence is at a much greater clock 14.7.3 Specific cryptosystems frequency than the data stream. Each bit of the PN sequence is called a chip, and the clock rate of the PN generator is called the Many cryptosystems have been developed, both by government chip In practical systems the chip rate is a large integer multiple establishments and commercial organisations. The first public L of the databit rate. The bandwidth of the transmitted signal is standard system is the American National Bureau of Standards Data therefore L times greater than that of the data stream. Encryption Standard (DES), which is a block cipher with a 64-bit The DSSS signal gives LPI (a low probability of intercept) be- blocklength involving both substitution and transposition under the cause the total signal power is spread over a wide bandwidth and the control of a 56-bit key (NBS, 1977), the original proposal was for a signal is noise-like, making it hard to detect. In anti-jamming (AJ) 64-bit key and there is debate about whether the 56-bit key is secure applications, the transmitter introduces an unpredictable element enough. Many hardware and software implementations of this into the modulation of the signal, known also to the receiver but kept standard exist. secret from opponents, as in stream ciphering. This, together with The RSA algorithm (Rivest, 1978) is a public-key encryption the wide bandwidth of the transmitted signal, makes jamming more algorithm in which the recipient publishes an encipherment key N, difficult than for conventional signals. In CDMA applications, the consisting of the product of two primes, each of order 10 100 various transmitters which are sharing the channel use different together with another number E. The sender breaks the plaintext fixed PN sequences which are chosen so that their cross-correlation into blocks which can be represented by numbers less than N, then is low. After a receiver has correlated the received signal with its encrypts the blocks using a simple modulo-N arit