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.