[issue13703] Hash collision security issue

[STINNER Victor]

STINNER Victor <victor.stinner at haypocalc.com> added the comment:

Christian Heimes proposes the following change in its randomhash branch (see issue #13704):

-    x = (Py_uhash_t) *p << 7;
+    x = Py_RndHashSeed + ((Py_uhash_t) *p << 7);
     for (i = 0; i < len; i++)
         x = (1000003U * x) ^ (Py_uhash_t) *p++;
     x ^= (Py_uhash_t) len;

This change doesn't add any security if the attacker can inject any string and retreive the hash value. You can retreive directly Py_RndHashSeed using:

Py_RndHashSeed = intmask((hash("a") ^ len("a") ^ ord("a")) * DIVIDE) - (ord("a") << 7)

where intmask() truncates to a long (x mod 2^(long bits)) and DIVIDE = 1/1000003 mod 2^(long bits). For example, DIVIDE=2021759595 for 32 bits long.

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue13703>
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