In cryptography, the shrinking generator is a form of pseudorandom number generator intended to be used in a stream cipher. It was published in Crypto 1993 by Don Coppersmith, Hugo Krawczyk, and Yishay Mansour.
The shrinking generator uses two linear feedback shift registers. One, called the A sequence, generates output bits, while the other, called the S sequence, controls their output. Both A and S are clocked; if the S bit is 1, then the A bit is output; if the S bit is 0, the A bit is discarded, nothing is output, and we clock the registers again. This has the disadvantage that the generator's output rate varies irregularly, and in a way that hints at the state of S; this problem can be overcome by buffering the output.
Despite this simplicity, the shrinking generator has remained remarkably resistant to cryptanalysis: there are currently no known attacks better than exhaustive search when the feedback polynomials are secret.
An interesting variant is the self-shrinking generator.
Read more about Shrinking Generator: An Implementation of A Shrinking Generator in Python
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