129 lines
3.4 KiB
Python
129 lines
3.4 KiB
Python
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# -*- test-case-name: twisted.test.test_randbytes -*-
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# Copyright (c) Twisted Matrix Laboratories.
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# See LICENSE for details.
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"""
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Cryptographically secure random implementation, with fallback on normal random.
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"""
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import os
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import random
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import warnings
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getrandbits = getattr(random, "getrandbits", None)
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_fromhex = bytes.fromhex
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class SecureRandomNotAvailable(RuntimeError):
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"""
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Exception raised when no secure random algorithm is found.
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"""
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class SourceNotAvailable(RuntimeError):
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"""
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Internal exception used when a specific random source is not available.
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"""
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class RandomFactory:
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"""
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Factory providing L{secureRandom} and L{insecureRandom} methods.
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You shouldn't have to instantiate this class, use the module level
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functions instead: it is an implementation detail and could be removed or
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changed arbitrarily.
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"""
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# This variable is no longer used, and will eventually be removed.
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randomSources = ()
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getrandbits = getrandbits
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def _osUrandom(self, nbytes: int) -> bytes:
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"""
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Wrapper around C{os.urandom} that cleanly manage its absence.
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"""
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try:
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return os.urandom(nbytes)
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except (AttributeError, NotImplementedError) as e:
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raise SourceNotAvailable(e)
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def secureRandom(self, nbytes: int, fallback: bool = False) -> bytes:
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"""
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Return a number of secure random bytes.
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@param nbytes: number of bytes to generate.
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@type nbytes: C{int}
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@param fallback: Whether the function should fallback on non-secure
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random or not. Default to C{False}.
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@type fallback: C{bool}
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@return: a string of random bytes.
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@rtype: C{str}
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"""
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try:
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return self._osUrandom(nbytes)
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except SourceNotAvailable:
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pass
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if fallback:
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warnings.warn(
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"urandom unavailable - "
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"proceeding with non-cryptographically secure random source",
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category=RuntimeWarning,
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stacklevel=2,
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)
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return self.insecureRandom(nbytes)
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else:
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raise SecureRandomNotAvailable("No secure random source available")
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def _randBits(self, nbytes: int) -> bytes:
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"""
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Wrapper around C{os.getrandbits}.
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"""
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if self.getrandbits is not None:
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n = self.getrandbits(nbytes * 8)
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hexBytes = ("%%0%dx" % (nbytes * 2)) % n
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return _fromhex(hexBytes)
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raise SourceNotAvailable("random.getrandbits is not available")
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_maketrans = bytes.maketrans
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_BYTES = _maketrans(b"", b"")
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def _randModule(self, nbytes: int) -> bytes:
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"""
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Wrapper around the C{random} module.
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"""
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return b"".join([bytes([random.choice(self._BYTES)]) for i in range(nbytes)])
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def insecureRandom(self, nbytes: int) -> bytes:
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"""
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Return a number of non secure random bytes.
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@param nbytes: number of bytes to generate.
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@type nbytes: C{int}
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@return: a string of random bytes.
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@rtype: C{str}
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"""
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try:
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return self._randBits(nbytes)
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except SourceNotAvailable:
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pass
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return self._randModule(nbytes)
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factory = RandomFactory()
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secureRandom = factory.secureRandom
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insecureRandom = factory.insecureRandom
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del factory
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__all__ = ["secureRandom", "insecureRandom", "SecureRandomNotAvailable"]
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