[Whonix-devel] Defeating TCP ISN skew attacks with SipHash or BLAKE2

Sun Jan 8 19:07:29 CET 2017

Hi I am a Whonix OS developer (an anonymity distro like TAILS)

I've seen your work on upstreaming SipHash and I think it improves 
security across the network stack in important ways.

An area we're focused on for anonymity reasons is TCP ISN generation 
which is susceptible to CPU load clock skews in predictable ways which 
allows unmasking of Tor clients and Hidden Services. We've looked at 
ways to mitigate this and the solution we thought of is rewriting the 
ISNs with a 32-bit output of a PRF to obfuscate the timer output so its 
not detectable by a passive network adversary. (I believe BLAKE2 is a 
great drop-in solution here: has 32byte output and is really fast).

Can you please help us with this by proposing direct upstream patches 
that changes the way ISNs work (or even as a network sysctl option if 
upstream is too resistant)?

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Relevant material:

events.ccc [.] 
de/congress/2006/Fahrplan/attachments/1211-23c3hotornotpres.pdf

phabricator.whonix [.] org/T543

*please remove the [.] to get working links.