[Python-Dev] Hash collision security issue (now public)

Christian Heimes lists at cheimes.de
Fri Jan 6 01:50:00 CET 2012

Am 06.01.2012 01:34, schrieb Nick Coghlan:
> On Fri, Jan 6, 2012 at 10:07 AM, Steven D'Aprano <steve at pearwood.info> wrote:
>> Surely the way to verify the behaviour is to run this from the shell:
>> python -c print(hash("abcde"))
>> twice, and see that the calls return different values. (Or have I
>> misunderstood the way the fix is going to work?)
>> In any case, I wouldn't want to rely on the presence of a flag in the sys
>> module to verify the behaviour, I'd want to see for myself that hash
>> collisions are no longer predictable.
> More directly, you can just check that the hash of the empty string is non-zero.
> So -1 for a flag in the sys module - "hash('') != 0" should serve as a
> sufficient check whether or not process-level string hash
> randomisation is in effect.

This might not work as we have to special case empty strings and perhaps
\0 strings, too. Otherwise we would give away the random seed to an
attacker if an attacker can somehow get hold of hash('') or hash(n * '\0').


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