IETF Logo Mail Archive

[Cfrg] Prime order + twisty DH benefit (theoretical)

Dan Brown <dbrown@certicom.com> Mon, 21 July 2014 21:09 UTC

Return-Path: <dbrown@certicom.com>
X-Original-To: cfrg@ietfa.amsl.com
Delivered-To: cfrg@ietfa.amsl.com
Received: from localhost (ietfa.amsl.com [127.0.0.1]) by ietfa.amsl.com (Postfix) with ESMTP id EBECA1A0426 for <cfrg@ietfa.amsl.com>; Mon, 21 Jul 2014 14:09:54 -0700 (PDT)
X-Virus-Scanned: amavisd-new at amsl.com
X-Spam-Flag: NO
X-Spam-Score: -1.899
X-Spam-Level:
X-Spam-Status: No, score=-1.899 tagged_above=-999 required=5 tests=[BAYES_00=-1.9, HTML_MESSAGE=0.001] 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 hW4Ds05jit4a for <cfrg@ietfa.amsl.com>; Mon, 21 Jul 2014 14:09:53 -0700 (PDT)
Received: from smtp-p01.blackberry.com (smtp-p01.blackberry.com [208.65.78.88]) by ietfa.amsl.com (Postfix) with ESMTP id 587CE1A0334 for <cfrg@irtf.org>; Mon, 21 Jul 2014 14:09:53 -0700 (PDT)
Received: from xct107cnc.rim.net ([10.65.161.207]) by mhs211cnc.rim.net with ESMTP/TLS/AES128-SHA; 21 Jul 2014 17:09:52 -0400
Received: from XCT116CNC.rim.net (10.65.161.216) by XCT107CNC.rim.net (10.65.161.207) with Microsoft SMTP Server (TLS) id 14.3.174.1; Mon, 21 Jul 2014 17:09:51 -0400
Received: from XMB116CNC.rim.net ([fe80::45d:f4fe:6277:5d1b]) by XCT116CNC.rim.net ([::1]) with mapi id 14.03.0174.001; Mon, 21 Jul 2014 17:09:51 -0400
From: Dan Brown <dbrown@certicom.com>
To: "cfrg@irtf.org" <cfrg@irtf.org>
Thread-Topic: Prime order + twisty DH benefit (theoretical)
Thread-Index: Ac+lKBwZKLQiRF9VS3ietodFNgImmA==
Date: Mon, 21 Jul 2014 21:09:50 +0000
Message-ID: <20140721210944.6656149.59544.16939@certicom.com>
Accept-Language: en-CA, en-US
Content-Language: en-US
X-MS-Has-Attach: yes
X-MS-TNEF-Correlator:
Content-Type: multipart/signed; protocol="application/x-pkcs7-signature"; micalg="SHA1"; boundary="===============0621958836=="
MIME-Version: 1.0
Archived-At: http://mailarchive.ietf.org/arch/msg/cfrg/2n60JcsZ5vubmR43cg1DloUttsQ
Subject: [Cfrg] Prime order + twisty DH benefit (theoretical)
X-BeenThere: cfrg@irtf.org
X-Mailman-Version: 2.1.15
Precedence: list
List-Id: Crypto Forum Research Group <cfrg.irtf.org>
List-Unsubscribe: <http://www.irtf.org/mailman/options/cfrg>, <mailto:cfrg-request@irtf.org?subject=unsubscribe>
List-Archive: <http://www.irtf.org/mail-archive/web/cfrg/>
List-Post: <mailto:cfrg@irtf.org>
List-Help: <mailto:cfrg-request@irtf.org?subject=help>
List-Subscribe: <http://www.irtf.org/mailman/listinfo/cfrg>, <mailto:cfrg-request@irtf.org?subject=subscribe>
X-List-Received-Date: Mon, 21 Jul 2014 21:09:55 -0000

‎Sorry, if the following has been discussed previously. Curve25519 has cofactor 8 because it uses a different shape curve.

If cofactor multiplication DH is used, this gives the shared secrets about 3 bits of information theoretical bias. Of course, the kdf etc should hide that bias just fine, but it would be theoretically simpler to have a shared secret with potentially no bias, eg prime order for both curve and twist.

Best regards, 

-- Dan

v2.23.0 | Report a Bug | By Email