The idea has been floated before, and I believe it to be the right solution to this problem. However, I have some suggested improvements:
1. The use of a "per boot" secret number allows an attacker to poll your machine to deduce the secret, and then attack you with that knowledge.
A solution to this problem is to use a rapidly changing secret, the pattern of which cannot be easily deduced, and a sliding window of acceptance. (If the hash doesn't match the current scheme, but matches the scheme we were using in the past N seconds, then accept the packet)
The change interval needs to be short enough that, by the time an attacker has been able to compute the next number, the window for accepting that has closed.
I figure that if you steal 4 to 12 bytes for mss storage, 2^20 is still enough possibilities that you can just rotate your secret every minutes and test against the old one for 30 seconds...
-matthew kaufman matthew@scruz.net
Yes.
ps. I've been meaning to write this entire scheme, with the enhancements I propose here, as a draft specification, but I keep getting interrupted by flooded phone rooms and the like this weekend. *sigh*
Hopefully there will be a working implementation of this by week's end. Jeff Weisberg has code which runs on sun3s and (soon, I hope) on other Suns under SunOS. This has always seemed to me to be the best way to do things, though an OS patch to go to hashed-entry into arrays of PCBs is a definite win to back-implement into SunOS (for example) in general. Avi