Re: [Cfrg] A little room for AES-192 in TLS?

Ilari Liusvaara <ilariliusvaara@welho.com> Mon, 16 January 2017 19:14 UTC

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Date: Mon, 16 Jan 2017 21:14:16 +0200
From: Ilari Liusvaara <ilariliusvaara@welho.com>
To: Leonard den Ottolander <leonard-lists@den.ottolander.nl>
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References: <20170115205926.853FB60A6D@jupiter.mumble.net> <1484577818.5104.1.camel@quad> <D4A2A7CE.57FDF%john.mattsson@ericsson.com> <CABcZeBPGxT=9iiChy4PxD_zMHWcHU=AhCLoe7wEHHtryw2rfwg@mail.gmail.com> <D4A2B50D.7E040%kenny.paterson@rhul.ac.uk> <CAHOTMVJrHBn4AR7PCJ14xKYCVjdxF7SiswiOABX_g6A5gsQGDg@mail.gmail.com> <1484593651.5104.49.camel@quad>
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Subject: Re: [Cfrg] A little room for AES-192 in TLS?
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On Mon, Jan 16, 2017 at 08:07:31PM +0100, Leonard den Ottolander wrote:
> Hello Tony,
> 
> On Mon, 2017-01-16 at 10:09 -0800, Tony Arcieri wrote:
> > I would rate the chances of a related key attack against TLS as
> > "vanishingly small". The use of key derivation functions ensures keys will
> > not be related.
> 
> How about a scenario where an adversary is able to compromise the
> software in such a way that related keys are being generated
> occasionally and possibly even used for encryption of known plain text
> (protocol headers come to mind)? This scenario is assuming the adversary
> is not fully in control of the source code but is capable to inject
> subtle bugs "under the radar". Would AES-192 hold up better in such a
> scenario than AES-256?

The key derivations are interop-critical, so compromises like that would
be found real quick (since essentially nothing would work anymore).
 

And there is at least one place in TLS 1.2 that can be undetectably
compromised by a server in a way that lets adversary passively decrypt
most connections.


-Ilari