Re: [kitten] WGLC on draft-ietf-kitten-aes-cts-hmac-sha2-06

Viktor Dukhovni <viktor1dane@dukhovni.org> Wed, 15 April 2015 05:57 UTC

Return-Path: <viktor1dane@dukhovni.org>
X-Original-To: kitten@ietfa.amsl.com
Delivered-To: kitten@ietfa.amsl.com
Received: from localhost (ietfa.amsl.com [127.0.0.1]) by ietfa.amsl.com (Postfix) with ESMTP id 81A421B3031 for <kitten@ietfa.amsl.com>; Tue, 14 Apr 2015 22:57:41 -0700 (PDT)
X-Virus-Scanned: amavisd-new at amsl.com
X-Spam-Flag: NO
X-Spam-Score: -101.9
X-Spam-Level:
X-Spam-Status: No, score=-101.9 tagged_above=-999 required=5 tests=[BAYES_00=-1.9, USER_IN_WHITELIST=-100] 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 bFYOKCjeoDg2 for <kitten@ietfa.amsl.com>; Tue, 14 Apr 2015 22:57:40 -0700 (PDT)
Received: from mournblade.imrryr.org (mournblade.imrryr.org [38.117.134.19]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ietfa.amsl.com (Postfix) with ESMTPS id 24D2E1B30E3 for <kitten@ietf.org>; Tue, 14 Apr 2015 22:57:40 -0700 (PDT)
Received: by mournblade.imrryr.org (Postfix, from userid 1034) id E5405283031; Wed, 15 Apr 2015 05:57:38 +0000 (UTC)
Date: Wed, 15 Apr 2015 05:57:38 +0000
From: Viktor Dukhovni <viktor1dane@dukhovni.org>
To: kitten@ietf.org
Message-ID: <20150415055738.GT17637@mournblade.imrryr.org>
References: <alpine.GSO.1.10.1503301227280.22210@multics.mit.edu> <551D6C35.4080108@mit.edu> <alpine.GSO.1.10.1504081626110.22210@multics.mit.edu> <5525B044.8070509@mit.edu> <CAC2=hnfbLoRAQLwDQhL7pVYMS8kqfc1rAA6Ha1np1h1WnhT5aw@mail.gmail.com> <55271546.6020505@mit.edu>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <55271546.6020505@mit.edu>
User-Agent: Mutt/1.5.23 (2014-03-12)
Archived-At: <http://mailarchive.ietf.org/arch/msg/kitten/ODQE8PrbOZ6Rr50WJ_eqfZH3ovA>
Subject: Re: [kitten] WGLC on draft-ietf-kitten-aes-cts-hmac-sha2-06
X-BeenThere: kitten@ietf.org
X-Mailman-Version: 2.1.15
Precedence: list
Reply-To: kitten@ietf.org
List-Id: Common Authentication Technologies - Next Generation <kitten.ietf.org>
List-Unsubscribe: <https://www.ietf.org/mailman/options/kitten>, <mailto:kitten-request@ietf.org?subject=unsubscribe>
List-Archive: <http://www.ietf.org/mail-archive/web/kitten/>
List-Post: <mailto:kitten@ietf.org>
List-Help: <mailto:kitten-request@ietf.org?subject=help>
List-Subscribe: <https://www.ietf.org/mailman/listinfo/kitten>, <mailto:kitten-request@ietf.org?subject=subscribe>
X-List-Received-Date: Wed, 15 Apr 2015 05:57:41 -0000

On Thu, Apr 09, 2015 at 08:11:50PM -0400, Greg Hudson wrote:

> > In general, though, it's designed to make sense - you can't use SHA-256
> > to get a 192 bit key because a SHA-256 output has at most 128 bits of
> > security no matter how you slice it.
> 
> I think it very much does matter how you slice it.  For key derivation
> using an HMAC, collision resistance is not required, so it is entirely
> reasonable to use SHA-256 to generate a 192-bit key.  In fact, the key
> derivation function used in this draft (from SP 800-108 section 5.1) can
> work securely even if more keying material is required than the HMAC
> output size.

Yes.  A n-bit HMAC used as a key-derivation PRF is sound for deriving
n-bit keys.  The n/2 collision resistance is not in scope.  Combine
your nonces with the raw DH secret and key-purpose-specific salt,
and away you go.

-- 
	Viktor.