 but not the key. When a very 14.8 Spread spectrum systems high degree of security is required, it is also assumed that the eavesdropper has obtained (by other means) the plaintext corre- sponding to some of the intercepted ciphertext. 14.8.1 Applications In conventional or symmetric cryptosystems, it is easy to deduce the deciphering key from the enciphering key (they may even be the Spread spectrum systems (Proakis, 1989; Dixon, 1976) are used in same), so both must be kept secret. In public key cryptosystems, digital communications for: however, the enciphering key can be made public by the recipient; despite this, it is believed to be computationally infeasible for an Combating interference arising from jamming other users, or eavesdropper to work out the decipherment key. A further tech- self-interference due to multipath effects. nique, public key-distribution (Diffie, 1976) applies the same prin- 2. Making it difficult to detect the signal, to achieve covert oper- ciples to the secure distribution of keys for conventional ation. cryptosystems. 3. Making it difficult to demodulate the signal, to achieve privacy. In stream ciphers, a stream of plaintext symbols is enciphered symbol by symbol. Instead of a true one time pad, the stream of Signals in which spread spectrum techniques are used to make enciphering symbols is generated by a symbol generator under the detection difficult are referred to as Low Probability of Intercept control of the control of the enciphering key. A simple example for (LPI) signals. The use of spread spectrum techniques to allow binary data is the use of a pseudo random binary sequence (PRBS) several users to share a common channel is known as code division generator, initialised by the chosen key. This produces a stream of multiple access (CDMA) or spread spectrum multiple access Os and 1s, which are XORed with the databit stream. Clearly, (SSMA). In conventional approaches to multiple access (such as recovery of the original bit stream requires exactly the same oper- FDMA) the ch