Re: [TLS] AAED ciphers: AES-GCM vs AES-EAX/AES-CCM: a meta-analysis

Peter Gutmann <pgut001@cs.auckland.ac.nz> Tue, 06 January 2015 11:37 UTC

Return-Path: <pgut001@cs.auckland.ac.nz>
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 97D3C1AC3AC for <tls@ietfa.amsl.com>; Tue, 6 Jan 2015 03:37:20 -0800 (PST)
X-Virus-Scanned: amavisd-new at amsl.com
X-Spam-Flag: NO
X-Spam-Score: -4.21
X-Spam-Level:
X-Spam-Status: No, score=-4.21 tagged_above=-999 required=5 tests=[BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, RCVD_IN_DNSWL_MED=-2.3, T_RP_MATCHES_RCVD=-0.01] autolearn=ham
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 4UQiaBPvm-Qd for <tls@ietfa.amsl.com>; Tue, 6 Jan 2015 03:37:16 -0800 (PST)
Received: from mx2.auckland.ac.nz (mx2.auckland.ac.nz [130.216.125.245]) (using TLSv1 with cipher RC4-SHA (128/128 bits)) (No client certificate requested) by ietfa.amsl.com (Postfix) with ESMTPS id 3062C1AC3A9 for <tls@ietf.org>; Tue, 6 Jan 2015 03:37:15 -0800 (PST)
DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=auckland.ac.nz; i=@auckland.ac.nz; q=dns/txt; s=uoa; t=1420544237; x=1452080237; h=from:to:subject:date:message-id: content-transfer-encoding:mime-version; bh=M7jDATJfyy25kulHQVkwA9IGxrRnXHgSgjfBDxefQ0A=; b=j2ZGvdPmeLMQmU5Zkz2iW/NPIpXkLsePJm/zSjOMcW9eqnxptoTTruUU 4biglY/pJMrwN1boUq7FTbY1fcKl2u+hyW+yYwV3ujFGxj2wnQtrn7PgN dilGMssZzkvxRtJnMkFK5NNMKLc3N3bN1/zkNuC1dmMKUMXSqHCrcgs6z Y=;
X-IronPort-AV: E=Sophos;i="5.04,630,1406548800"; d="scan'208";a="299839510"
X-Ironport-HAT: MAIL-SERVERS - $RELAYED
X-Ironport-Source: 130.216.4.125 - Outgoing - Outgoing
Received: from uxchange10-fe3.uoa.auckland.ac.nz ([130.216.4.125]) by mx2-int.auckland.ac.nz with ESMTP/TLS/AES256-SHA; 07 Jan 2015 00:37:15 +1300
Received: from UXCN10-TDC05.UoA.auckland.ac.nz ([169.254.9.148]) by uxchange10-fe3.UoA.auckland.ac.nz ([169.254.143.234]) with mapi id 14.03.0174.001; Wed, 7 Jan 2015 00:37:13 +1300
From: Peter Gutmann <pgut001@cs.auckland.ac.nz>
To: "<tls@ietf.org>" <tls@ietf.org>
Thread-Topic: [TLS] AAED ciphers: AES-GCM vs AES-EAX/AES-CCM: a meta-analysis
Thread-Index: AdAppR1mLlQ4YHkETd2icvdmMpre+A==
Date: Tue, 6 Jan 2015 11:37:13 +0000
Message-ID: <9A043F3CF02CD34C8E74AC1594475C73AAF5268C@uxcn10-tdc05.UoA.auckland.ac.nz>
Accept-Language: en-NZ, en-GB, en-US
Content-Language: en-NZ
X-MS-Has-Attach:
X-MS-TNEF-Correlator:
x-originating-ip: [130.216.158.4]
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: quoted-printable
MIME-Version: 1.0
Archived-At: http://mailarchive.ietf.org/arch/msg/tls/V4crIKSWGDfX4inacGTwlxopMyg
Subject: Re: [TLS] AAED ciphers: AES-GCM vs AES-EAX/AES-CCM: a meta-analysis
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: Tue, 06 Jan 2015 11:37:21 -0000

"Paterson, Kenny" <Kenny.Paterson@rhul.ac.uk>; writes:

>It's good that nothing supports the truncated-MAC extension, because, in
>combination with TLS's support for variable length padding, it introduces a
>security vulnerability.

Ooh, good to know, thanks!  The truncated-MAC extension is actually kind of
silly, there's only one size allowed, 80 bits, which isn't really useful (you
save all of 48 bits for a 128-bit MAC) while for ridiculous-sized MACs like
HMAC-SHA256 and HMAC-SHA512 where you would want to truncate, say to 128 bits,
you can't do it.

>This is not an issue for any of the cihersuites proposed above, but it's
>something to be kept in mind when considering "exotic" ciphersuites.

Actually it does affect AES, since you can only truncate to 80 bits.

Peter.