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Concept
Générateur par rétrécissement
Résumé
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 sequence, generates output bits, while the other, called the sequence, controls their output. Both and are clocked; if the bit is 1, then the bit is output; if the bit is 0, the bit is discarded, nothing is output, and the registers are clocked 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. The random sequence generated by LFSR can not guarantee the unpredictability in secure system and various methods have been proposed to improve its randomness
Despite this simplicity, there are currently no known attacks better than exhaustive search when the feedback polynomials are secret. If the feedback polynomials are known, however, the best known attack requires less than • bits of output.
A variant is the self-shrinking generator.
This example uses two Galois LFRSs to produce the output pseudorandom bitstream. The Python code can be used to encrypt and decrypt a file or any bytestream.
#!/usr/bin/env python3
import sys
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Crypto4o functions start here
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class GLFSR: """Galois linear-feedback shift register.""" def init(self, polynom, initial_value): print "Using polynom 0x%X, initial value: 0x%X." % (polynom, initial_value) self.polynom = polynom | 1 self.data = initial_value tmp = polynom self.mask = 1 while tmp != 0: if tmp & self.mask != 0: tmp ^= self.mask if tmp == 0: break self.maskSource officielle
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