IETF Logo Mail Archive

Re: [TLS] rfc7366: is encrypt-then-mac implemented?

Manuel Pégourié-Gonnard <mpg@polarssl.org> Thu, 30 October 2014 09:19 UTC

Return-Path: <mpg@polarssl.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 CAC471AD056 for <tls@ietfa.amsl.com>; Thu, 30 Oct 2014 02:19:39 -0700 (PDT)
X-Virus-Scanned: amavisd-new at amsl.com
X-Spam-Flag: NO
X-Spam-Score: 0.397
X-Spam-Level:
X-Spam-Status: No, score=0.397 tagged_above=-999 required=5 tests=[BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, HELO_MISMATCH_COM=0.553, HOST_EQ_NL=1.545, MIME_8BIT_HEADER=0.3, 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 YYllff98DG6V for <tls@ietfa.amsl.com>; Thu, 30 Oct 2014 02:19:39 -0700 (PDT)
Received: from vps2.offspark.com (vps2.brainspark.nl [141.138.204.106]) (using TLSv1.2 with cipher DHE-RSA-AES128-SHA (128/128 bits)) (No client certificate requested) by ietfa.amsl.com (Postfix) with ESMTPS id 206501A6FC6 for <tls@ietf.org>; Thu, 30 Oct 2014 02:19:39 -0700 (PDT)
DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=polarssl.org; s=exim; h=Subject:Content-Transfer-Encoding:Content-Type:In-Reply-To:References:To:MIME-Version:From:Date:Message-ID; bh=IxlxlKbFrlg59nwHJsswCyuYMMxA8ib/Sr2X8sHm6mo=; b=Hs+WHtZo5TB3jaoOb2EtLxI/u1YAanW7BUIAVsPhT3bQw5JxGzlXHpw0hVmMnXOqrCUNXXzET02VxEZwmit5CbT/w1JuBbQXu/jTO+p9TWH+QQiAJX3E9xylDIRePReicqNvq0cIGWuVV2Yweg8LvRvD1XUCHaSTISx7zncbu4w=;
Received: from thue.elzevir.fr ([88.165.216.11] helo=[192.168.0.124]) by vps2.offspark.com with esmtpsa (TLS1.2:DHE_RSA_AES_128_CBC_SHA1:128) (Exim 4.80) (envelope-from <mpg@polarssl.org>) id 1XjltD-0002ct-0M; Thu, 30 Oct 2014 10:19:31 +0100
Message-ID: <545202A7.7050603@polarssl.org>
Date: Thu, 30 Oct 2014 10:19:35 +0100
From: Manuel Pégourié-Gonnard <mpg@polarssl.org>
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.2.0
MIME-Version: 1.0
To: Peter Gutmann <pgut001@cs.auckland.ac.nz>, "<tls@ietf.org>" <tls@ietf.org>
References: <9A043F3CF02CD34C8E74AC1594475C739B9DB460@uxcn10-5.UoA.auckland.ac.nz>
In-Reply-To: <9A043F3CF02CD34C8E74AC1594475C739B9DB460@uxcn10-5.UoA.auckland.ac.nz>
Content-Type: text/plain; charset="windows-1252"
Content-Transfer-Encoding: 7bit
X-SA-Exim-Connect-IP: 88.165.216.11
X-SA-Exim-Mail-From: mpg@polarssl.org
X-SA-Exim-Version: 4.2.1 (built Mon, 26 Dec 2011 16:24:06 +0000)
X-SA-Exim-Scanned: Yes (on vps2.offspark.com)
Archived-At: http://mailarchive.ietf.org/arch/msg/tls/uCqXIwnSC1mXlbkMFxFKpOtqbRk
Subject: Re: [TLS] rfc7366: is encrypt-then-mac implemented?
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: <http://www.ietf.org/mail-archive/web/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, 30 Oct 2014 09:19:39 -0000

On 29/10/2014 16:30, Peter Gutmann wrote:
> [...] In fact it doesn't even make any sense to include
> the MAC length, if you tell your code to MAC the length of the ciphertext plus
> the length of the MAC you'll end up MACing 20 or 32 bytes of whatever random
> data happens to be present at the end of the ciphertext.

Sure, I'd never think of doing that. But I can certainly tell my code to compute

  MAC_k( seq_num + type + version + length + encrypted_stuff )

with length != length_of(encrypted_stuff). After all, for the MAC function, what
we call "length" is just two meaningless bytes.

Manuel.

v2.23.0 | Report a Bug | By Email