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Message-ID: <20161216212528.26003.qmail@ns.sciencehorizons.net>
Date: 16 Dec 2016 16:25:28 -0500
From: "George Spelvin" <linux@...encehorizons.net>
To: Jason@...c4.com, linux@...encehorizons.net
Cc: ak@...ux.intel.com, davem@...emloft.net, David.Laight@...lab.com,
djb@...yp.to, ebiggers3@...il.com, hannes@...essinduktion.org,
jeanphilippe.aumasson@...il.com, kernel-hardening@...ts.openwall.com,
linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org,
luto@...capital.net, netdev@...r.kernel.org, tom@...bertland.com,
torvalds@...ux-foundation.org, tytso@....edu, vegard.nossum@...il.com
Subject: Re: [PATCH v5 1/4] siphash: add cryptographically secure PRF
Jason A. Donenfeld wrote:
> I saw that jiffies addition in there and was wondering what it was all
> about. It's currently added _after_ the siphash input, not before, to
> keep with how the old algorithm worked. I'm not sure if this is
> correct or if there's something wrong with that, as I haven't studied
> how it works. If that jiffies should be part of the siphash input and
> not added to the result, please tell me. Otherwise I'll keep things
> how they are to avoid breaking something that seems to be working.
Oh, geez, I didn't realize you didn't understand this code.
Full details at
https://en.wikipedia.org/wiki/TCP_sequence_prediction_attack
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.
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