[Standards] Re: [jdev] XEP-0115: Entity Capabilities

[Dave Cridland]

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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