RSA Labs FAQ -- Q91: What are the Shrinking and Self-Shrinking Generators?

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### Question 91. What are the Shrinking and Self-Shrinking Generators?

The shrinking generator was developed by Coppersmith, Krawczyk, and Mansour [CKM94]. It is a stream cipher based on the simple interaction between the outputs from two LFSRs (see Question 89). The bits of one output are used to determine whether the corresponding bits of the second output will be used as part of the overall keystream. The shrinking generator is simple and scaleable, and has good security properties. One drawback of the shrinking generator is that the output rate of the keystream will not be constant unless precautions are taken. A variant of the shrinking generator is the self-shrinking generator [MS95], where instead of using one output from one LFSR to "shrink" the output of another (as in the shrinking generator), the output of a single LFSR is used to extract bits from the same output. There are as yet no results on the cryptanalysis of either technique.