In http://mail.python.org/pipermail/python-dev/2011-December/115138.html, Christian Heimes pointed out that
... we don't have to alter the outcome of hash ... We just need to reduce the chance that an attacker can produce collisions in the dict (and set?)
I'll state it more strongly. hash probably should not change (at least for this), but we may want to consider a different conflict resolution strategy when the first slot is already filled. Remember that there was a fair amount of thought and timing effort put into selecting the current strategy; it is deliberately sub-optimal for random input, in order to do better with typical input. < http://hg.python.org/cpython/file/7010fa9bd190/Objects/dictnotes.txt > If there is a change, it would currently be needed in three places for each of set and dict (the lookdict functions and insertdict_clean). It may be worth adding some macros just to keep those six in sync. Once those macros are in place, that allows a compile-time switch. My personal opinion is that accepting *and parsing* enough data for this to be a problem is enough of an edge case that I don't want normal dicts slowed down at all for this; I would therefore prefer that the change be restricted to such a compile-time switch, with current behavior the default. http://hg.python.org/cpython/file/7010fa9bd190/Objects/dictobject.c#l571 583 for (perturb = hash; ep->me_key != NULL; perturb >>= PERTURB_SHIFT) { 584 i = (i << 2) + i + perturb + 1; PERTURB_SHIFT is already a private #define to 5; per dictnotes, 4 and 6 perform almost as well. Someone worried can easily make that change today, and be protected from "generic" anti-python attacks. I believe the salt suggestions have equivalent to replacing perturb = hash; with something like perturb = hash + salt; Changing i = (i << 2) + i + perturb + 1; would allow effectively replacing the initial hash, but risks spoiling performance in the non-adversary case. Would there be objections to replacing those two lines with something like: for (perturb = FIRST_PERTURB(hash, key); ep->me_key != NULL; perturb = NEXT_PERTURB(hash, key, perturb)) { i = NEXT_SLOT(i, perturb); The default macro definitions should keep things as they are #define FIRST_PERTURB(hash, key) hash #define NEXT_PERTURB(hash, key, perturb) perturb >> PERTURB_SHIFT #define NEXT_SLOT(i, perturb) (i << 2) + i + perturb + 1 while allowing #ifdefs for (slower but) safer things like adding a salt, or even using alternative hashes. -jJ