[Python-Dev] Hashing proposal: change only string-only dicts (original) (raw)

martin at v.loewis.de martin at v.loewis.de
Wed Jan 18 01:30:59 CET 2012

Zitat von Victor Stinner <victor.stinner at haypocalc.com>:

Each string would get two hashes: the "public" hash, which is constant across runs and bugfix releases, and the dict-hash, which is only used by the dictionary implementation, and only if all keys to the dict are strings. The distinction between secret (private, secure) and "public" hash (deterministic) is not clear to me.

It's not about privacy or security. It's about compatibility. The dict-hash is only used in the dict implementation, and never exposed, leaving the tp_hash unmodified.

Example: collections.UserDict implements hash() using hash(self.data).

Are you sure? I only see that used for UserString, not UserDict.

collections.abc.Set computes its hash using hash(x) of each item. Same question.

The hash of the Set should most certainly use the element's tp_hash. That is the hash of the objects, and it may collide for strings just fine due to the vulnerability.

If we need to use the secret hash, it should be exposed in Python.

It's not secret, just specific. I don't mind it being exposed. However, that would be a new feature, which cannot be added in a security fix or bug fix release.

Which function/method would be used? I suppose that we cannot add anything to stable releases like 2.7.

Right. Nor do I see any need to expose it. It fixes the vulnerability just fine without being exposed.

Regards, Martin

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