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82 lines
2.8 KiB
82 lines
2.8 KiB
13 years ago
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# Copyright (c) 2002-2012 Zooko Wilcox-O'Hearn
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# This file is part of pyutil; see README.rst for licensing terms.
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import warnings
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"""
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Cryptographically strong pseudo-random number generator based on SHA256.
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"""
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class SHA256Expander:
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"""
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Provide a cryptographically strong pseudo-random number generator based on
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SHA256. Hopefully this means that no attacker will be able to predict any
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bit of output that he hasn't seen, given that he doesn't know anything about
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the seed and given that he can see as many bits of output as he desires
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except for the bit that he is trying to predict. Further it is hoped that
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an attacker will not even be able to determine whether a given stream of
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random bytes was generated by this PRNG or by flipping a coin repeatedly.
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The safety of this technique has not been verified by a Real Cryptographer.
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... but it is similar to the PRNG in FIPS-186...
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The seed and counter are encoded in DJB's netstring format so that I
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don't have to think about the possibility of ambiguity.
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Note: I've since learned more about the theory of secure hash functions
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and the above is a strong assumption about a secure hash function. Use
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of this class should be considered deprecated and you should use a more
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well-analyzed KDF (such as the nascent standard HKDF) or stream cipher or
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whatever it is that you need.
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"""
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def __init__(self, seed=None):
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warnings.warn("deprecated", DeprecationWarning)
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if seed is not None:
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self.seed(seed)
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def seed(self, seed):
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import hashlib
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self.starth = hashlib.sha256('24:pyutil hash expansion v2,10:algorithm:,6:SHA256,6:value:,')
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seedlen = len(seed)
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seedlenstr = str(seedlen)
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self.starth.update(seedlenstr)
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self.starth.update(':')
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self.starth.update(seed)
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self.starth.update(',')
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self.avail = ""
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self.counter = 0
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def get(self, bytes):
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bytesleft = bytes
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res = []
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while bytesleft > 0:
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if len(self.avail) == 0:
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h = self.starth.copy()
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counterstr = str(self.counter)
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counterstrlen = len(counterstr)
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counterstrlenstr = str(counterstrlen)
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h.update(counterstrlenstr)
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h.update(':')
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h.update(counterstr)
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h.update(',')
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self.avail = h.digest()
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self.counter += 1
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numb = min(len(self.avail), bytesleft)
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(chunk, self.avail,) = (self.avail[:numb], self.avail[numb:],)
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res.append(chunk)
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bytesleft = bytesleft - numb
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resstr = ''.join(res)
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assert len(resstr) == bytes
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return resstr
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def sha256expand(inpstr, expbytes):
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return SHA256Expander(inpstr).get(expbytes)
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