Re: HalfSipHash Acceptable Usage

[Eric Dumazet]

On Tue, 2016-12-20 at 16:36 -0500, Theodore Ts'o wrote:
> On Mon, Dec 19, 2016 at 06:32:44PM +0100, Jason A. Donenfeld wrote:
> > 1) Anything that requires actual long-term security will use
> > SipHash2-4, with the 64-bit output and the 128-bit key. This includes
> > things like TCP sequence numbers. This seems pretty uncontroversial to
> > me. Seem okay to you?
> 
> Um, why do TCP sequence numbers need long-term security?  So long as
> you rekey every 5 minutes or so, TCP sequence numbers don't need any
> more security than that, since even if you break the key used to
> generate initial sequence numbers seven a minute or two later, any
> pending TCP connections will have timed out long before.
> 
> See the security analysis done in RFC 6528[1], where among other
> things, it points out why MD5 is acceptable with periodic rekeying,
> although there is the concern that this could break certain hueristics
> used when establishing new connections during the TIME-WAIT state.
> 
> [1] https://tools.ietf.org/html/rfc6528


We do not use rekeying for TCP ISN, not anymore after commit
6e5714eaf77d79ae1 (where we switched from MD4 to MD5 )

It might hurt some common cases and I do not believe it is mandated by a
current (ie not obsolete) RFC.

Our clock has a 64 ns resolution and 274 second period (commit
9b42c336d0641) (compared to 4 usec one in RFC 6528)

I do not see why SipHash, if faster than MD5 and more secure, would be a
problem.

Same for syncookies.

BTW, we probably should add a ratelimit on SYNACK retransmits,
because it seems that attackers understood linux kernels resist to
synfloods, and they (the bad guys) use reflection attacks.