[IPsec] One more comment to the draft-ietf-ipsecme-aes-ctr-ikev2-05.txt

Tero Kivinen <kivinen@iki.fi> Sun, 21 March 2010 22:41 UTC

Return-Path: <kivinen@iki.fi>
X-Original-To: ipsec@core3.amsl.com
Delivered-To: ipsec@core3.amsl.com
Received: from localhost (localhost [127.0.0.1]) by core3.amsl.com (Postfix) with ESMTP id 1D8BD3A67B4 for <ipsec@core3.amsl.com>; Sun, 21 Mar 2010 15:41:23 -0700 (PDT)
X-Virus-Scanned: amavisd-new at amsl.com
X-Spam-Flag: NO
X-Spam-Score: -0.465
X-Spam-Level:
X-Spam-Status: No, score=-0.465 tagged_above=-999 required=5 tests=[AWL=-1.596, BAYES_50=0.001, DNS_FROM_OPENWHOIS=1.13]
Received: from mail.ietf.org ([64.170.98.32]) by localhost (core3.amsl.com [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id 3CMaCJlV05Nw for <ipsec@core3.amsl.com>; Sun, 21 Mar 2010 15:41:22 -0700 (PDT)
Received: from mail.kivinen.iki.fi (fireball.acr.fi [83.145.195.1]) by core3.amsl.com (Postfix) with ESMTP id 9894C3A677D for <ipsec@ietf.org>; Sun, 21 Mar 2010 15:41:21 -0700 (PDT)
Received: from fireball.kivinen.iki.fi (localhost [127.0.0.1]) by mail.kivinen.iki.fi (8.14.3/8.14.3) with ESMTP id o2LMfS8g010839 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=NO); Mon, 22 Mar 2010 00:41:28 +0200 (EET)
Received: (from kivinen@localhost) by fireball.kivinen.iki.fi (8.14.3/8.12.11) id o2LMfQkM028699; Mon, 22 Mar 2010 00:41:26 +0200 (EET)
X-Authentication-Warning: fireball.kivinen.iki.fi: kivinen set sender to kivinen@iki.fi using -f
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
Message-ID: <19366.41110.787241.99409@fireball.kivinen.iki.fi>
Date: Mon, 22 Mar 2010 00:41:26 +0200
From: Tero Kivinen <kivinen@iki.fi>
To: ipsec@ietf.org
X-Mailer: VM 7.19 under Emacs 21.4.1
X-Edit-Time: 8 min
X-Total-Time: 8 min
Cc: sean.s.shen@gmail.com, ssmurthy.nittala@freescale.com, yumao9@gmail.com
Subject: [IPsec] One more comment to the draft-ietf-ipsecme-aes-ctr-ikev2-05.txt
X-BeenThere: ipsec@ietf.org
X-Mailman-Version: 2.1.9
Precedence: list
List-Id: Discussion of IPsec protocols <ipsec.ietf.org>
List-Unsubscribe: <https://www.ietf.org/mailman/listinfo/ipsec>, <mailto:ipsec-request@ietf.org?subject=unsubscribe>
List-Archive: <http://www.ietf.org/mail-archive/web/ipsec>
List-Post: <mailto:ipsec@ietf.org>
List-Help: <mailto:ipsec-request@ietf.org?subject=help>
List-Subscribe: <https://www.ietf.org/mailman/listinfo/ipsec>, <mailto:ipsec-request@ietf.org?subject=subscribe>
X-List-Received-Date: Sun, 21 Mar 2010 22:41:23 -0000

In the security considerations section there is a text saying:

>  Additionally, since AES has a 128-bit block size, regardless of the
>  mode employed, the ciphertext generated by AES encryption becomes
>  distinguishable from random values after 2^64 blocks are encrypted
>  with a single key.  Since IKEv2 is not likely to carry traffic in
>  such a high quantity compared with ESP, this won't be a big concern
>  here.  However, when a large amount of traffic appears in the future
>  or under abnormal circumstances, implementations SHOULD generate a
>  fresh key before 2^64 blocks are encrypted with the same key.

The last SHOULD is not really needed as IKEv2 message ID is 32-bits,
and the IKE SA MUST be closed (or rekeyed) before it wraps, thus at
most one IKE SA can have 2^32 messages, each consisting of at max 2^16
bytes, thus maximum number of bytes that may be transmitted over IKEv2
SA is 2^48 bytes. As this 2^48 bytes is much smaller than 2^64 blocks,
this paragraph is not an issue in IKEv2.

I would change the paragraph to be:

   Additionally, since AES has a 128-bit block size, regardless of the
   mode employed, the ciphertext generated by AES encryption becomes
   distinguishable from random values after 2^64 blocks are encrypted
   with a single key.  Since IKEv2 SA cannot carry that much of data,
   this issue is not a concern here.
-- 
kivinen@iki.fi