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A simple cryptanalysis of the self-shrinking generator with very short keystream for the case of unknown connection polynomial is provided. The expected complexity of this cryptanalysis is 21.5L when the length of the LFSR of the generator is L.

The authors propose clock-controlled chaotic keystream generators. For generating keystreams with periods long enough for cryptographic use while maintaining the good statistical properties of chaotic dynamics. These keystream generators may provide higher security than other proposed methods.

A new design of nonlinear combination generator is proposed. The nonlinear combining function employed is changed dynamically by permuting the variables in the nonlinear function. Simulated results show that the linear complexity of the resulting keystream is lower bounded by half the period of the keystream.

Cryptanalysis of the stream cipher 'Labyrinth', a cipher recently proposed by Lin and Shepherd (1997), is performed. Given only 2/sup 30/ known bits of keystream, the 119 bit key of Labyrinth is recovered in under a second of computation using a DEC Alpha.

Two keystream generators are described that use mutual clock control between a number of sequence generators. One is the T52e cipher, and the other uses three linear feedback shift registers. In the latter the register periods can lock together, giving an anomalously short period. Methods for avoiding this are described.

A cryptographic weakness in a pseudorandom binary sequence generator proposed in the literature as a keystream generator for stream cipher applications is pointed out. Some results obtained by computer simulation are presented.

A number of keystream generators can be attacked by guessing the contents of one shift register and then checking to see whether this guess is consistent with the observed keystream. Where the target register is n bits long, this gives an attack of complexity 2n−o(1). In the Letter a further optimisation is presented which appears to reduce the complexity to about 2n/2 in many cases of practical interest.

The generalised serial test for keystream sequences is compared with the poker test. It is suggested that the generalised serial test should be used in preference to the poker test.