Re: [TLS] tls-unique

Simon Josefsson <simon@josefsson.org> Thu, 08 October 2015 11:20 UTC

Return-Path: <simon@josefsson.org>
X-Original-To: tls@ietfa.amsl.com
Delivered-To: tls@ietfa.amsl.com
Received: from localhost (ietfa.amsl.com [127.0.0.1]) by ietfa.amsl.com (Postfix) with ESMTP id CD3BE1B2BDF for <tls@ietfa.amsl.com>; Thu, 8 Oct 2015 04:20:32 -0700 (PDT)
X-Virus-Scanned: amavisd-new at amsl.com
X-Spam-Flag: NO
X-Spam-Score: -1.551
X-Spam-Level:
X-Spam-Status: No, score=-1.551 tagged_above=-999 required=5 tests=[BAYES_00=-1.9, HELO_EQ_SE=0.35, SPF_PASS=-0.001] autolearn=no
Received: from mail.ietf.org ([4.31.198.44]) by localhost (ietfa.amsl.com [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id ypq-VZKf2ftQ for <tls@ietfa.amsl.com>; Thu, 8 Oct 2015 04:20:30 -0700 (PDT)
Received: from duva.sjd.se (duva.sjd.se [IPv6:2001:9b0:1:1702::100]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ietfa.amsl.com (Postfix) with ESMTPS id 6C66E1B2BDC for <tls@ietf.org>; Thu, 8 Oct 2015 04:20:30 -0700 (PDT)
Received: from latte.josefsson.org ([155.4.17.3]) (authenticated bits=0) by duva.sjd.se (8.14.4/8.14.4/Debian-4) with ESMTP id t98BKQdu005651 (version=TLSv1/SSLv3 cipher=AES128-GCM-SHA256 bits=128 verify=NOT); Thu, 8 Oct 2015 13:20:27 +0200
Date: Thu, 8 Oct 2015 13:20:25 +0200
From: Simon Josefsson <simon@josefsson.org>
To: Eric Rescorla <ekr@rtfm.com>
Message-ID: <20151008132025.68eb7f3a@latte.josefsson.org>
In-Reply-To: <CABcZeBPqJXnOy7RPHZshiJqTrpj=+WbKXUHw9JyFsC1kN7s1gQ@mail.gmail.com>
References: <A1F63168-7736-452D-BC1B-23B665D81989@sn3rd.com> <87vbahu2r2.fsf@latte.josefsson.org> <CABcZeBNuSCB8i--TqYouiPyrPu6ZSunNeK40JHaO+DdBEnUL+A@mail.gmail.com> <87612hpsw8.fsf@latte.josefsson.org> <CABcZeBPqJXnOy7RPHZshiJqTrpj=+WbKXUHw9JyFsC1kN7s1gQ@mail.gmail.com>
X-Mailer: Claws Mail 3.11.1 (GTK+ 2.24.25; x86_64-pc-linux-gnu)
MIME-Version: 1.0
Content-Type: multipart/signed; micalg=pgp-sha256; boundary="Sig_/7A1Op_.bamSnNk4CMVhDO3K"; protocol="application/pgp-signature"
X-Virus-Scanned: clamav-milter 0.98.7 at duva.sjd.se
X-Virus-Status: Clean
Archived-At: <http://mailarchive.ietf.org/arch/msg/tls/kdxvUptArXe0W5OVIhw-CPTX0OM>
Cc: "tls@ietf.org" <tls@ietf.org>
Subject: Re: [TLS] tls-unique
X-BeenThere: tls@ietf.org
X-Mailman-Version: 2.1.15
Precedence: list
List-Id: "This is the mailing list for the Transport Layer Security working group of the IETF." <tls.ietf.org>
List-Unsubscribe: <https://www.ietf.org/mailman/options/tls>, <mailto:tls-request@ietf.org?subject=unsubscribe>
List-Archive: <https://mailarchive.ietf.org/arch/browse/tls/>
List-Post: <mailto:tls@ietf.org>
List-Help: <mailto:tls-request@ietf.org?subject=help>
List-Subscribe: <https://www.ietf.org/mailman/listinfo/tls>, <mailto:tls-request@ietf.org?subject=subscribe>
X-List-Received-Date: Thu, 08 Oct 2015 11:20:33 -0000

> > The introduction says:
> >
> >    There exists a TLS extension [I-D.ietf-tls-session-hash] that
> > modify TLS so that the definition of 'tls-unique' [RFC5929] has the
> > intended properties.  If widely implemented and deployed, the
> > channel binding type in this document would not offer any
> > additional protection.  The purpose of this document is to provide
> > an alternative channel binding that offers the intended properties
> > without requiring TLS protocol changes.  However, keep in mind that
> > TLS implementations needs to offer the appropriate APIs necessary
> > to be able to implement the channel binding described in this
> > document.
> >
> > I agree that one alternative is to require session_hash for all
> > connections.
> 
> 
> Given that you need to modify *some* software in any case, it seems
> like one ought to adopt session-hash.

The problem is if you want to resolve this at the application level and
don't have sufficient control over the TLS layer to influence it to
negotiate session-hash.  This is the situation for many SASL
applications.

If universal adoption of session-hash is a prio, then there is no
problem.  While RFC 7627 updates 5246 it does not talk a lot about what
it actually updates in 5246, or I missed it, and I haven't seen
session-hash functionality back-ported to deployed code.

My current approach works with or without session-hash negotiated, but
requires that you can get the session_hash value out of the TLS stack.

Another approach is to say that if session-hash is in use, it uses a
simple TLS exporter API call, and if it is not, it has to use TLS
exporter API on the session_hash value.  This would secure all cases.

> >   But then what is the problem with use of 'tls-unique'?
> 
> The general consensus is that 96 bits is too short.

I agree -- I used 256 bits.

/Simon