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# -*- test-case-name: twisted.test.test_randbytes -*-
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.
"""
Cryptographically secure random implementation, with fallback on normal random.
"""
# System imports
import warnings, os, random
getrandbits = getattr(random, 'getrandbits', None)
class SecureRandomNotAvailable(RuntimeError):
"""
Exception raised when no secure random algorithm is found.
"""
class SourceNotAvailable(RuntimeError):
"""
Internal exception used when a specific random source is not available.
"""
class RandomFactory(object):
"""
Factory providing L{secureRandom} and L{insecureRandom} methods.
You shouldn't have to instantiate this class, use the module level
functions instead: it is an implementation detail and could be removed or
changed arbitrarily.
"""
# This variable is no longer used, and will eventually be removed.
randomSources = ()
getrandbits = getrandbits
def _osUrandom(self, nbytes):
"""
Wrapper around C{os.urandom} that cleanly manage its absence.
"""
try:
return os.urandom(nbytes)
except (AttributeError, NotImplementedError), e:
raise SourceNotAvailable(e)
def secureRandom(self, nbytes, fallback=False):
"""
Return a number of secure random bytes.
@param nbytes: number of bytes to generate.
@type nbytes: C{int}
@param fallback: Whether the function should fallback on non-secure
random or not. Default to C{False}.
@type fallback: C{bool}
@return: a string of random bytes.
@rtype: C{str}
"""
try:
return self._osUrandom(nbytes)
except SourceNotAvailable:
pass
if fallback:
warnings.warn(
"urandom unavailable - "
"proceeding with non-cryptographically secure random source",
category=RuntimeWarning,
stacklevel=2)
return self.insecureRandom(nbytes)
else:
raise SecureRandomNotAvailable("No secure random source available")
def _randBits(self, nbytes):
"""
Wrapper around C{os.getrandbits}.
"""
if self.getrandbits is not None:
n = self.getrandbits(nbytes * 8)
hexBytes = ("%%0%dx" % (nbytes * 2)) % n
return hexBytes.decode('hex')
raise SourceNotAvailable("random.getrandbits is not available")
def _randRange(self, nbytes):
"""
Wrapper around C{random.randrange}.
"""
bytes = ""
for i in xrange(nbytes):
bytes += chr(random.randrange(0, 255))
return bytes
def insecureRandom(self, nbytes):
"""
Return a number of non secure random bytes.
@param nbytes: number of bytes to generate.
@type nbytes: C{int}
@return: a string of random bytes.
@rtype: C{str}
"""
for src in ("_randBits", "_randRange"):
try:
return getattr(self, src)(nbytes)
except SourceNotAvailable:
pass
factory = RandomFactory()
secureRandom = factory.secureRandom
insecureRandom = factory.insecureRandom
del factory
__all__ = ["secureRandom", "insecureRandom", "SecureRandomNotAvailable"]
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