An alternating step generator is a cryptographic pseudorandom number generator intended to be used in a stream cipher.

A typical alternating step generator can be constructed from three linear feedback shift registers, by using the output of one LFSR to clock one of the two other LFSRs independently. The result is that there's no usual LFSR period as would be generated by a single LFSR. Such an alternating step construction gains from a higher linear complexity and a potentially longer period. Yet, it's security is relative because the size of the shortest of the three LFSRs strongly influences the potential success of a reduced complexity attack. (see “Reduced Complexity Attacks on the Alternating Step Generator”)

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