Zitat von Serhiy Storchaka <storch...@gmail.com>:
27.01.12 23:08, Frank Sievertsen написав(ла):
As already mentioned, the vulnerability of 64-bit Python rather
theoretical and not practical. The size of the hash makes the attack
is extremely unlikely.
Unfortunately this assumption is not correct. It works very good with
64bit-hashing.
It's much harder to create (efficiently) 64-bit hash-collisions.
But I managed to do so and created strings with
a length of 16 (6-bit)-characters (a-z, A-Z, 0-9, _, .). Even
14 characters would have been enough.
You need less than twice as many characters for the same effect as in
the 32bit-world.
The point is not the length of the string, but the size of string
space for inspection. To search for a string with a specified 64-bit
hash to iterate over 2 ** 64 strings.
I think you entirely missed the point of Frank's message. Despite your
analysis that it shall not be possible, Frank has *actually* computed
colliding strings, most likely also for a specified hash value.
Of course, to calculate the hash function to use secure, not
allowing "cut corners" and reduce computation time.
This issue wouldn't be that relevant if there wasn't a documented
algorithm to significantly reduce the number of tries you need to
make to produce a string with a desired hash value. My own implementation
would need 2**33 tries in the worst case (for a 64-bit hash value);
thanks to the birthday paradox, it's actually a significant chance
that the algorithm finds collisions even faster.
Regards,
Martin
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