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Date: Fri, 16 Dec 2016 22:31:15 +0100
From: "Jason A. Donenfeld" <>
Cc: George Spelvin <>, Andi Kleen <>, 
	David Miller <>, David Laight <>, 
	"Daniel J . Bernstein" <>, Eric Biggers <>, 
	Hannes Frederic Sowa <>, 
	Jean-Philippe Aumasson <>, 
	Linux Crypto Mailing List <>, LKML <>, 
	Andy Lutomirski <>, Netdev <>, 
	Tom Herbert <>, Linus Torvalds <>, 
	"Theodore Ts'o" <>, Vegard Nossum <>
Subject: Re: Re: [PATCH v5 1/4] siphash: add
 cryptographically secure PRF

Hi George,

On Fri, Dec 16, 2016 at 10:25 PM, George Spelvin
<> wrote:
> But yes, the sequence number is supposed to be (random base) + (timestamp).
> In the old days before Canter & Siegel when the internet was a nice place,
> people just used a counter that started at boot time.
> But then someone observed that I can start a connection to host X,
> see the sequence number it gives back to me, and thereby learn the
> seauence number it's using on its connections to host Y.
> And I can use that to inject forged data into an X-to-Y connection,
> without ever seeing a single byte of the traffic!  (If I *can* observe
> the traffic, of course, none of this makes the slightest difference.)
> So the random base was made a keyed hash of the endpoint identifiers.
> (Practically only the hosts matter, but generally the ports are thrown
> in for good measure.)  That way, the ISN that host X sends to me
> tells me nothing about the ISN it's using to talk to host Y.  Now the
> only way to inject forged data into the X-to-Y connection is to
> send 2^32 bytes, which is a little less practical.

Oh, okay, that is exactly what I thought was going on. I just thought
you were implying that jiffies could be moved inside the hash, which
then confused my understanding of how things should be. In any case,
thanks for the explanation.


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