Jim Jewett wrote:
> 
>> BTW: If you set the limit N to e.g. 100 (which is reasonable given
>> Victor's and my tests),
> 
> Agreed.  Frankly, I think 5 would be more than reasonable so long as
> there is a fallback.
> 
>> the time it takes to process one of those
>> sets only takes 0.3 ms on my machine. That's hardly usable as basis
>> for an effective DoS attack.
> 
> So it would take around 3Mb to cause a minute's delay...

I'm not sure how you calculated that number.

Here's what I get: tale a dictionary with 100 integer collisions:
d = dict((x*(2**64 - 1), 1) for x in xrange(1, 100))

The repr(d) has 2713 bytes, which is a good approximation of how
much (string) data you have to send in order to trigger the
problem case.

If you can create 3333 distinct integer sequences, you'll get a
processing time of about 1 second on my slow dev machine. The
resulting dict will likely have a repr() of around
60*3333*2713 = 517MB.

So you need to send 517MB to cause my slow dev machine to consume
1 minute of CPU time. Today's servers are at least 10 times as fast as
my aging machine.

If you then take into account that the integer collision dictionary
is a very efficient collision example (size vs. effect), the attack
doesn't really sound practical anymore.