Re: [TLS] draft-ietf-tls-tls13-26 is vulnerable to externally set PSK identity enumeration

Hubert Kario <hkario@redhat.com> Thu, 15 March 2018 11:25 UTC

Return-Path: <hkario@redhat.com>
X-Original-To: tls@ietfa.amsl.com
Delivered-To: tls@ietfa.amsl.com
Received: from localhost (localhost [127.0.0.1]) by ietfa.amsl.com (Postfix) with ESMTP id D800E12895E; Thu, 15 Mar 2018 04:25:51 -0700 (PDT)
X-Virus-Scanned: amavisd-new at amsl.com
X-Spam-Flag: NO
X-Spam-Score: -4.201
X-Spam-Level:
X-Spam-Status: No, score=-4.201 tagged_above=-999 required=5 tests=[BAYES_00=-1.9, RCVD_IN_DNSWL_MED=-2.3, SPF_PASS=-0.001, T_RP_MATCHES_RCVD=-0.01, T_SPF_HELO_PERMERROR=0.01] autolearn=ham autolearn_force=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 t_rjLHkKAFDv; Thu, 15 Mar 2018 04:25:50 -0700 (PDT)
Received: from mx1.redhat.com (mx3-rdu2.redhat.com [66.187.233.73]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by ietfa.amsl.com (Postfix) with ESMTPS id 504F1128954; Thu, 15 Mar 2018 04:25:50 -0700 (PDT)
Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.rdu2.redhat.com [10.11.54.5]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 826788D77D; Thu, 15 Mar 2018 11:25:49 +0000 (UTC)
Received: from pintsize.usersys.redhat.com (unknown [10.43.21.223]) by smtp.corp.redhat.com (Postfix) with ESMTP id C80FDBDC3E; Thu, 15 Mar 2018 11:25:47 +0000 (UTC)
From: Hubert Kario <hkario@redhat.com>
To: Benjamin Kaduk <kaduk@mit.edu>
Cc: Ilari Liusvaara <ilariliusvaara@welho.com>, TLS WG <tls@ietf.org>, iesg@ietf.org
Date: Thu, 15 Mar 2018 12:25:38 +0100
Message-ID: <1937539.vLgMNW4b7C@pintsize.usersys.redhat.com>
In-Reply-To: <20180314201328.GF55987@kduck.kaduk.org>
References: <6112806.hxzZ6NivhB@pintsize.usersys.redhat.com> <2062943.8cTCpni5Dm@pintsize.usersys.redhat.com> <20180314201328.GF55987@kduck.kaduk.org>
MIME-Version: 1.0
Content-Type: multipart/signed; boundary="nextPart3355209.RsnACA0urF"; micalg="pgp-sha512"; protocol="application/pgp-signature"
X-Scanned-By: MIMEDefang 2.79 on 10.11.54.5
X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.2]); Thu, 15 Mar 2018 11:25:49 +0000 (UTC)
X-Greylist: inspected by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.2]); Thu, 15 Mar 2018 11:25:49 +0000 (UTC) for IP:'10.11.54.5' DOMAIN:'int-mx05.intmail.prod.int.rdu2.redhat.com' HELO:'smtp.corp.redhat.com' FROM:'hkario@redhat.com' RCPT:''
Archived-At: <https://mailarchive.ietf.org/arch/msg/tls/FpSGReHLhAtUHC1lmlpbhEzZtCU>
Subject: Re: [TLS] draft-ietf-tls-tls13-26 is vulnerable to externally set PSK identity enumeration
X-BeenThere: tls@ietf.org
X-Mailman-Version: 2.1.22
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, 15 Mar 2018 11:25:52 -0000

On Wednesday, 14 March 2018 21:13:29 CET Benjamin Kaduk wrote:
> On Wed, Mar 14, 2018 at 12:46:25PM +0100, Hubert Kario wrote:
> > On Wednesday, 14 March 2018 03:02:10 CET Benjamin Kaduk wrote:
> > > It seems like we get ourselves in trouble by allowing multiple
> > > external PSKs to be present.  If we allowed at most one external
> > > PSK in a given ClientHello, then aborting the handshake on binder
> > > failure would be the correct choice, as discovering a valid identity
> > > would require discovering a valid key/password as well.
> > 
> > but identity/binder may be invalid only because the server was restarted
> > and generated a new in-memory key; we don't want to abort connection in
> > such
> For an external PSK?  That hardly sounds like "external" to me...

not my fault that what we called just "PSK"s in TLS 1.2 is now "external PSKs" 
in TLS 1.3... I wanted to be unambiguous, so please, can we discuss the issue, 
not semiotics?

> > situation, continuing to a regular handshake is necessary then for good
> > user experience (and likely, even security, given the history of TLS
> > version fallbacks)
> > 
> > > Disallowing multiple external PSKs would make migration scenarios a
> > > little more annoying, but perhaps not fatally so.
> > 
> > not only migration, but session resumption and regular PSK at the same
> > time
> > too - for session resumption you may not do DH, while for initial
> > handshake
> > with PSK you may want to to gain PFS...
> > 
> > so as tempting as the removal of multiple PSKs from ClientHello is, I'm
> > afraid the fallout is far too large to do it
> 
> I did not say removal of multiple PSKs, rather removal of multiple
> *external* PSKs.

we do not have a reliable mechanism of differentiating between external and 
resumption PSKs while parsing Client Hello

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