Re: [TLS] SNI and Resumption/0-RTT

mrex@sap.com (Martin Rex) Fri, 21 October 2016 13:52 UTC

Return-Path: <mrex@sap.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 A7299129793 for <tls@ietfa.amsl.com>; Fri, 21 Oct 2016 06:52:11 -0700 (PDT)
X-Virus-Scanned: amavisd-new at amsl.com
X-Spam-Flag: NO
X-Spam-Score: -6.922
X-Spam-Level:
X-Spam-Status: No, score=-6.922 tagged_above=-999 required=5 tests=[BAYES_00=-1.9, RCVD_IN_DNSWL_HI=-5, RCVD_IN_MSPIKE_H3=-0.01, RCVD_IN_MSPIKE_WL=-0.01, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001] 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 5Eptj98vKIkK for <tls@ietfa.amsl.com>; Fri, 21 Oct 2016 06:52:10 -0700 (PDT)
Received: from smtpde01.smtp.sap-ag.de (smtpde01.smtp.sap-ag.de [155.56.68.170]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by ietfa.amsl.com (Postfix) with ESMTPS id 4E985129789 for <tls@ietf.org>; Fri, 21 Oct 2016 06:52:09 -0700 (PDT)
Received: from mail06.wdf.sap.corp (mail06.sap.corp [194.39.131.54]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtpde01.smtp.sap-ag.de (Postfix) with ESMTPS id 3t0nD72SW6z1J7X; Fri, 21 Oct 2016 15:52:07 +0200 (CEST)
X-purgate-ID: 152705::1477057927-00003836-97A9A340/0/0
X-purgate-size: 1289
X-purgate: clean
X-purgate: This mail is considered clean (visit http://www.eleven.de for further information)
X-purgate-Ad: Categorized by eleven eXpurgate (R) http://www.eleven.de
X-purgate-type: clean
X-SAP-SPAM-Status: clean
Received: from ld9781.wdf.sap.corp (ld9781.wdf.sap.corp [10.21.82.193]) by mail06.wdf.sap.corp (Postfix) with ESMTP id 3t0nD65W09zkqT4; Fri, 21 Oct 2016 15:52:06 +0200 (CEST)
Received: by ld9781.wdf.sap.corp (Postfix, from userid 10159) id B74981A565; Fri, 21 Oct 2016 15:52:06 +0200 (CEST)
In-Reply-To: <BY1PR0301MB0838DB70AF5B4667F9B1C3E98CD40@BY1PR0301MB0838.namprd03.prod.outlook.com>
To: Andrei Popov <Andrei.Popov@microsoft.com>
Date: Fri, 21 Oct 2016 15:52:06 +0200 (CEST)
X-Mailer: ELM [version 2.4ME+ PL125 (25)]
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit
Content-Type: text/plain; charset="US-ASCII"
Message-Id: <20161021135206.B74981A565@ld9781.wdf.sap.corp>
From: mrex@sap.com (Martin Rex)
Archived-At: <https://mailarchive.ietf.org/arch/msg/tls/wTk44A4qoLPI7DDvpMOXMF8XveM>
Cc: "tls@ietf.org" <tls@ietf.org>
Subject: Re: [TLS] SNI and Resumption/0-RTT
X-BeenThere: tls@ietf.org
X-Mailman-Version: 2.1.17
Precedence: list
Reply-To: mrex@sap.com
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: Fri, 21 Oct 2016 13:52:11 -0000

Andrei Popov wrote:
>
> Perhaps it's OK to resume a session with a different SNI if in this
> session the server has proved an identity that matches the new SNI.
> In order to enforce this, the server would have to cache (or save in
> the ticket) a list of identities it presented in each resumable session?

The current wording in rfc6066 may be slightly confusing about what is
acutally important and why.

The server ought to perform a full handshake whenever the full handshake
will result in selection & use of a _different_ TLS server certificate
than what was used for the original full handshake on a session resumption.
This is a direct consequence of the principle of least surprise.

This is also the most backwards-compatible behaviour when upgrading the
server from a does-not-support-SNI to a supports-SNI state/implementation.

You do *NOT* want to have session caching interfere with the
server certificate that a client gets to see, because that would
essentially result in not-quite-deterministic server behaviour.

Sometimes there may be bugs in client-side session caching,
and clients proposing the wrong session for resumption, and the server
doing a full handshake results in interoperable, deterministic and
secure behaviour.


-Martin