On Tue, Feb 16, 2016 at 11:56:55AM -0800, Glenn Linderman wrote:
> On 2/16/2016 1:48 AM, Christoph Groth wrote:
> >Hello,
> >
> >Recent Python versions randomize the hashes of str, bytes and datetime
> >objects. I suppose that the choice of these three types is the result
> >of a compromise. Has this been discussed somewhere publicly?
>
> Search archives of this list... it was discussed at length.
There's a lot of discussion on the mailing list. I think that this is
the very start of it, in Dec 2011:
https://mail.python.org/pipermail/python-dev/2011-December/115116.html
and continuing into 2012, for example:
https://mail.python.org/pipermail/python-dev/2012-January/115577.html
https://mail.python.org/pipermail/python-dev/2012-January/115690.html
and a LOT more, spread over many different threads and subject lines.
You should also read the issue on the bug tracker:
http://bugs.python.org/issue13703
My recollection is that it was decided that only strings and bytes need
to have their hashes randomized, because only strings and bytes can be
used directly from user-input without first having a conversion step
with likely input range validation. In addition, changing the hash for
ints would break too much code for too little benefit: unlike strings,
where hash collision attacks on web apps are proven and easy, hash
collision attacks based on ints are more difficult and rare.
See also the comment here:
http://bugs.python.org/issue13703#msg151847
> >I'm not a web programmer, but don't web applications also use
> >dictionaries that are indexed by, say, tuples of integers?
>
> Sure, and that is the biggest part of the reason they were randomized.
But they aren't, as far as I can see:
[steve@ando 3.6]$ ./python -c "print(hash((23, 42, 99, 100)))"
1071302475
[steve@ando 3.6]$ ./python -c "print(hash((23, 42, 99, 100)))"
1071302475
Web apps can use dicts indexed by anything that they like, but unless
there is an actual attack, what does it matter? Guido makes a good point
about security here:
https://mail.python.org/pipermail/python-dev/2013-October/129181.html
> I think hashes of all types have been randomized, not _just_ the list
> you mentioned.
I'm pretty sure that's not actually the case. Using 3.6 from the repo
(admittedly not fully up to date though), I can see hash randomization
working for strings:
[steve@ando 3.6]$ ./python -c "print(hash('abc'))"
11601873
[steve@ando 3.6]$ ./python -c "print(hash('abc'))"
-2009889747
but not for ints:
[steve@ando 3.6]$ ./python -c "print(hash(42))"
42
[steve@ando 3.6]$ ./python -c "print(hash(42))"
42
which agrees with my recollection that only strings and bytes would be
randomized.
--
Steve
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