[New-bugs-announce] [issue18405] crypt.mksalt() result has unnecessarily low entropy

[Christian Heimes]

New submission from Christian Heimes:

crypt.mksalt() creates a salt with a lower entropy than possible. It uses random.SystemRandom().sample() to generate a salt string from the set of 64 chars (string.ascii_letters + string.digits + './'). SystemRandom() uses a CPRNG (good) but sample() returns n UNIQUE members of the set (very bad). sample() reduces the set possible chars by one for each salt char.

Suggested fix:

salt = base64.b64encode(os.urandom(salt_chars * 3 // 4), b"./").decode("ascii")

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assignee: christian.heimes
components: Library (Lib)
messages: 192683
nosy: christian.heimes, gregory.p.smith
priority: critical
severity: normal
stage: patch review
status: open
title: crypt.mksalt() result has unnecessarily low entropy
type: security
versions: Python 2.7, Python 3.2, Python 3.3, Python 3.4

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue18405>
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