2. While thinking more about this solution, I observed that if the attacker can have an "agent" sharing the same IP address with the victim (by agent I mean an entity that can communicate with the target web site and read back its response data), then the algorithms I suggested will not be effective. Note that an attacker can share IP address with the victim when both share a forward proxy (e.g. some universities and ISPs), or when the attacker and victim share the same machine (multi-user environment). Still, that narrows down the attack surface significantly.
It should be noted that this isn't as rare as just a few universities and ISPs. This also happens in lots of corporate networks (rogue user on the internal network), it happens with lots of internet cafe's, it happens with AOL (~5MM users) and it happens with TOR users. So while, yes, I agree it is better than nothing it is hardly a rock solid solution for anyone on a shared IP. -RSnake http://ha.ckers.org/ http://sla.ckers.org/ http://ha.ckers.org/fierce/
