Editing Stream cipher (section)

===Clock-controlled generators===
Normally LFSRs are stepped regularly. One approach to introducing non-linearity is to have the LFSR clocked irregularly, controlled by the output of a second LFSR. Such generators include the [[stop-and-go generator]], the [[alternating step generator]] and the [[shrinking generator]].

An [[alternating step generator]] comprises three LFSRs, which we will call LFSR0, LFSR1 and LFSR2 for convenience. The output of one of the registers decides which of the other two is to be used; for instance, if LFSR2 outputs a 0, LFSR0 is clocked, and if it outputs a 1, LFSR1 is clocked instead. The output is the exclusive OR of the last bit produced by LFSR0 and LFSR1. The initial state of the three LFSRs is the key.

The stop-and-go generator (Beth and Piper, 1984) consists of two LFSRs. One LFSR is clocked if the output of a second is a 1, otherwise it repeats its previous output. This output is then (in some versions) combined with the output of a third LFSR clocked at a regular rate.

The [[shrinking generator]] takes a different approach. Two LFSRs are used, both clocked regularly. If the output of the first LFSR is 1, the output of the second LFSR becomes the output of the generator. If the first LFSR outputs 0, however, the output of the second is discarded, and no bit is output by the generator. This mechanism suffers from timing attacks on the second generator, since the speed of the output is variable in a manner that depends on the second generator's state. This can be alleviated by buffering the output.