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

Alex Gaynor alex.gaynor at gmail.com
Thu Dec 29 02:51:21 CET 2011

A few thoughts on this:

a) This is not a new issue, I'm curious what the new interest is in it.

b) Whatever the solution to this is, it is not CPython specific, any decision should be reflected in the Python language spec IMO, if CPython has the semantic that dicts aren't vulnerable to hash collision then users will rely on this and another implementation having a different (valid) behavior opens up users to security issues.

c) I'm not convinced a randomized hash is appropriate for the default dict, for a number of reasons: it's a performance hit on every dict operations, using a per-process seed means you can't compile the hash of an obj at Python's compile time, a per-dict seed inhibits a bunch of other optimizations. These may not be relevant to CPython, but they are to PyPy and probably the invoke-dynamic work on Jython (pursuant to point b).

Therefore I think these should be considered application issues, since request limiting is difficult and error prone, I'd recommend the Python stdlib including a non-hash based map (such as a binary tree).

Alex

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