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

Antoine Pitrou solipsis at pitrou.net
Thu Dec 29 11:32:44 CET 2011

On Thu, 29 Dec 2011 04:04:17 +0100 Christian Heimes <lists at cheimes.de> wrote:

Am 29.12.2011 02:37, schrieb Jesse Noller: > Back up link for the PDF: > http://dl.dropbox.com/u/1374/200728C3EffectiveDoSonwebapplicationplatforms.pdf > > Ocert disclosure: > http://www.ocert.org/advisories/ocert-2011-003.html

>From http://www.nruns.com/downloads/advisory28122011.pdf --- Python uses a hash function which is very similar to DJBX33X, which can be broken using a meet-in-the-middle attack. It operates on register size and is thus different for 64 and 32 bit machines. While generating multi-collisions efficiently is also possible for the 64 bit version of the function, the resulting colliding strings are too large to be relevant for anything more than an academic attack. Plone as the most prominent Python web framework accepts 1 MB of POST data, which it parses in about 7 minutes of CPU time in the worst case. This gives an attacker with about 20 kbit/s the possibility to keep one Core Duo core constantly busy. If the attacker is in the position to have a Gigabit line available, he can keep about 50.000 Core Duo cores busy. --- If I remember correctly CPython uses the long for its hash function so 64bit Windows uses a 32bit hash.

Not anymore, Py_hash_t is currently aligned with Py_ssize_t.

Regards

Antoine.

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