Some Background
Santha and Vazirani proved that several bit streams with weak randomness can be combined to produce a higher-quality quasi-random bit stream. Even earlier, John von Neumann proved that a simple algorithm can remove a considerable amount of the bias in any bit stream which should be applied to each bit stream before using any variation of the Santha-Vazirani design. The field is termed entropy extraction and is the subject of active research (e.g., N Nisan, S Safra, R Shaltiel, A Ta-Shma, C Umans, D Zuckerman).
Read more about this topic: Cryptographically Secure Pseudorandom Number Generator
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