Subject: Re: [PATCH] TCP: add option for silent port knocking with integrity protection

From: Christian Grothoff <[email-masked]>
Date: Thu, 12 Dec 2013 12:43:59 +0100

On 12/12/2013 11:19 AM, Jacob Appelbaum wrote:
> I think that generally, I would prefer if the code didn't use MD5 but
> otherwise, I don't see any real risk of adding an exploitable hole. It
> seems silly to disable it by default though - ideally, I'd like a sysctl
> to ensure that Tor could use this without making the user recompile
> their kernel. That is more of a pain than running a userspace helper, I
> think.
> 
> All the best,
> Jacob

Given that the output is truncated to 32 bits and that performance (SYN
flood) is also a concern, AND that the original TCP SQN generation is
also MD5-based (and we want to look the same), what disadvantage do you
see over MD5?  Given the truncation to 32 bits, I don't think a stronger
hash would do anything for us.

As for it being disabled by default, we did this with respect to
kernel submission guidelines which we understood said that features
should _initially_ always be submitted with disabled-by-default
(presumably so that until they have stabilized, nobody is harmed
unless they explicitly activate the code).

I don't see the point in having a sysctl, as applications have to
explicitly request it anyway.

-Christian