Unfortunately it requires only a few seconds to compute enough 32bit
collisions on one core with no precomputed data.
Are you running the hash function "backward" to generate strings with
the same value, or you are more trying something like brute forcing?
If you try it brute force to hit a specific target, you'll only find
only one good string every 4 billion tries. That's why you first blow up
your target:
You start backward from an arbitrary target-value. You brute force for 3
characters, for example, this will give you 16 million intermediate
values from which you know that they'll end up in your target-value.
Those 16 million values are a huge target for now brute-forcing forward:
Every 256 tries you'll hit one of these values.
And how do you get the hash secret? You need it to run an attack.
I don't know. This was meant as an answer to the quoted text "So I hope
that computing collisions requires a lot of CPU time (is slow) to make
the attack ineffective with today computers.".
What I wanted to say is: The security relies on the fact that the
attacker can't guess the prefix, not that he can't precompute the values
and it takes hours or days to compute the collisions. If the prefix
leaks out of the application, then the rest is trivial and done in a few
seconds. The suffix is not important for the collision-prevention, but
it will probably make it much harder to guess the prefix.
I don't know an effective way to get the prefix either, (if the
application doesn't leak full hash(X) values).
Frank
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