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Re: [TLS] draft-ietf-tls-tls13-21: TLS 1.3 record padding removal leaks padding size

Hubert Kario <hkario@redhat.com> Tue, 15 August 2017 17:49 UTC

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From: Hubert Kario <hkario@redhat.com>
To: Eric Rescorla <ekr@rtfm.com>
Cc: Ilari Liusvaara <ilariliusvaara@welho.com>, "tls@ietf.org" <tls@ietf.org>
Date: Tue, 15 Aug 2017 19:42:46 +0200
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Subject: Re: [TLS] draft-ietf-tls-tls13-21: TLS 1.3 record padding removal leaks padding size
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On Tuesday, 15 August 2017 17:28:22 CEST Eric Rescorla wrote:
> I generally agree with Ilari. To recap what I said on the PR:
> I think it would be fine to sharpen the point about padding leaking
> information and I'd take a short PR for that.

I've prepared https://github.com/tlswg/tls13-spec/pull/1074 with that in mind.

> I don't believe it's
> necessary either to require that it be constant time (for the reasons I
> indicated on-list and already documented in the spec) or to describe a
> specific algorithm, especially at this point on the document life cycle.
> 
> -Ekr
> 
> 
> 
> On Tue, Aug 15, 2017 at 6:54 AM, Ilari Liusvaara <ilariliusvaara@welho.com>
> 
> wrote:
> > On Tue, Aug 15, 2017 at 03:31:56PM +0200, Hubert Kario wrote:
> > > I've created a Pull Request that introduces requirement for constant
> > > time
> > > processing of padding and an example on how to do it:
> > > 
> > > https://github.com/tlswg/tls13-spec/pull/1073
> > 
> > -1
> > 
> > Except doing the depad in constant-time is useless if you just re-
> > introduce the timing leaks at the next step. Actually not introducing
> > timing leaks in TLS library requires special API for passing the data
> > to application... API that has had no reason to exist so far, and is
> > more complicated to use than current read or zerocopy callback APIs.
> > 
> > And even if you have such special API, it is extremely doubtful how
> > many applications could use it correctly. Constant-time processing of
> > variable-length data is extremely hard (LUCKY13 anyone?)
> > 
> > Oh, and then there are timing leaks when sending data too...
> > 
> > 
> > -Ilari
> > 
> > _______________________________________________
> > TLS mailing list
> > TLS@ietf.org
> > https://www.ietf.org/mailman/listinfo/tls


-- 
Regards,
Hubert Kario
Senior Quality Engineer, QE BaseOS Security team
Web: www.cz.redhat.com
Red Hat Czech s.r.o., Purkyňova 115, 612 00  Brno, Czech Republic

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