annel transfer function may be very poor at some ation, so the decipherment key is the same as the encipherment key. frequencies and good at others. The advantage of using spread Such systems are widely used in communications; their advantages spectrum signals for CDMA and for combating multipath is that by include ease of encryption and decryption, and the fact that single using a much wider channel bandwidth, all users get a uniformly bit errors in the channel cause only single bit errors in the decrypted acceptable service, rather than a good service for some, and bad for plaintext. Disadvantages include the effort of initial synchronisation others. For this reason there is currently interest in spread spectrum and resynchronisation if synchronisation is lost, and the fact that systems for wireless LANs within buildings. there is no transposition element in this system. In block ciphers, the message is divided into n-bit blocks and the 14.8.2 Direct sequence spread spectrum blocks are input sequentially to the algorithm. At each stage the key (which is usually constant for the whole message) is also entered, In direct sequence spread spectrum (DSSS), the transmitter and and the encryptor typically uses both substitution and transposition receiver contain identical psuedo-random sequence generators pro- to produce consecutive blocks of ciphertext. In such systems an ducing a pseudo-noise (PN) signal. In the transmitter, the input data error in transmission usually results in corruption of the whole stream is XORed with the PN signal before transmission. In the received block. receiver the received signal is XORed with the PN stream to recover In all these systems, it is essential that the key is changed fre- the original data stream; this is equivalent to correlation with the quently; the security of an otherwise satisfactory cryptosystem may known PN sequence. There is an obvious analogy between this be compromised if the key is reused. process and stream ciphering (Section 14.8) but with the crucial differenc