[Python-Dev] Counting collisions for the win
mal at egenix.com
Mon Jan 23 22:55:47 CET 2012
Frank Sievertsen wrote:
> I'd still prefer to see a randomized hash()-function (at least for 3.3).
> But to protect against the attacks it would be sufficient to use
> randomization for collision resolution in dicts (and sets).
> What if we use a second (randomized) hash-function in case there
> are many collisions in ONE lookup. This hash-function is used only
> for collision resolution and is not cached.
This sounds a lot like what I'm referring to as universal hash function
in the discussion on the ticket:
However, I don't like the term "random" in there. It's better to make
the approach deterministic to avoid issues with not being able
to easily reproduce Python application runs for debugging purposes.
If you find that the data is manipulated, simply incrementing the
universal hash parameter and rehashing the dict with that parameter
should be enough to solve the issue (if not, which is highly unlikely,
the dict will simply reapply the fix). No randomness needed.
BTW: I attached a demo script to the ticket which demonstrates both
types of collisions using integers.
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