On Wed, 1 Sep 1999, Geoff Rehmet wrote: > After a bit of work on TCP sequence numbers, and generating initial > sequence numbers which are difficult to predict, I have put some > code together, which I belive makes the way in which FreeBSD > generates initial send sequence numbers more secure.
How do OpenBSD do it? > The patch I have developed is based on RFC1948, and also takes some ideas > from the way in which Linux calculates TCP ISS values. However, unlike > Linux, I am keeping the code true to RFC793's requirement for a > "fictitious" 250kHz clock. Instead of uising a cut-down MD4 transform, > (which is what Linux does), I have used a cut-down MD5 transform, with > round 4 removed. Just curious whether you have a reference for doing this or whether it was an ad-hoc change. Playing with cryptographic algorithms isn't usually a good idea unless you're sure, as I'm sure you know. > As with the Linux code, I am using 9 32-bit words of > random secret, which is rekeyed every 5 minutes. > The remainder of the sequence number generation is based on our existing > code. > > I have placed the diff in > > http://www.freebsd.org/~csgr/tcp_sequence.diff > > The diff was taken against -current as at approximately 0600 GMT > 1 September 1999. > > testing, comments would be appreciated. > > Once Mark has completed his work in devrandom, I will rather use > devrandom as a source of randomness. I'd expect Yarrow to be (perhaps quite a bit) slower than our existing PRNG - it's a more conservative design and uses primitives like SHA-1 (for yarrow-160). I don't know how much of an impact this would be for network performance. Kris To Unsubscribe: send mail to majord...@freebsd.org with "unsubscribe freebsd-hackers" in the body of the message
