[issue13703] Hash collision security issue

[Paul McMillan]

Paul McMillan <paul at mcmillan.ws> added the comment:

> Those who use or advocate a simple randomized starting hash (Perl, Ruby, perhaps MS, and the CCC presenters) are presuming that the randomized hash values are kept private. Indeed, they should be (and the docs could note this) unless an attacker has direct access to the interpreter.

Except that this is patently untrue. Anytime any programmer iterates
over a dictionary and returns the ordered result to the user in some
form, they're leaking information about the hash value. I hope you're
not suggesting that any programmer who is concerned about security
will make sure to sort the results of every iteration before making it
public in some fashion.

> I do not think we, as Python developers, should be concerned about esoteric timing attacks.

Timing attacks are less esoteric than you think they are. This issue
gets worse, not better, as the internet moves (for better or worse)
towards virtualized computing.

> And if hashing were randomized per process, and probes were randomly distributed among processes, and processes were periodically killed and restarted with new seeds, could such an attack get anywhere...

You're suggesting that in order for a Python application to be secure,
it's a requirement that we randomly kill and restart processes from
time to time? I thought we were going for a good solution here, not a
hacky workaround.

> We could also consider, for 3.3, making the output of hash() be different from the internal values used for dicts, perhaps by switching random seeds in hash(). So even if someone does return hash(x) values to potential attackers, they are not the values used in dicts. (This would require a slight change in the doc.)

This isn't a bad idea, but I'd be fine documenting that the output of
hash() shouldn't be made public.

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<http://bugs.python.org/issue13703>
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