On Fri Jun 29 01:13:26 2007, Joe Hildebrand wrote:
You're worried about the attack where someone generates a set of
features that has the same hash as the a different set of features.
In this case, the birthday attack doesn't help, since you only
get to pick one set of ciphertext.
Also, as I think I mentioned, the more structured the input text, the
harder it is to find a collision.
Let's assume that it's still possible to come up with a collision,
given sufficient computing power. Why would someone expend such
computing power to achieve this? We're talking weeks of work, here,
and even if it dropped to hours, there's a race involved - the
attacker would need to find a "spoof" set of capability data which
served whatever purpose was intended and matched the hash function's
output, *and* do so before the victim's client cached the legitimate
data.
That seems like the cost of such an attack outweighs the benefits, to
me. And that's just using a very cheap hash function. I actually
suspect that HMAC-MD4 would be sufficient, if it weren't for the fact
that MD4 implementations are pretty hard to find now. MD5 (and HMAC)
is everywhere, and cheap, so a good one to use.
Dave.
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