Re: [TLS] Wrapping up cached info

Martin Rex <> Mon, 17 May 2010 19:16 UTC

Return-Path: <>
Received: from localhost (localhost []) by (Postfix) with ESMTP id 0FE0F3A6AE9 for <>; Mon, 17 May 2010 12:16:43 -0700 (PDT)
X-Virus-Scanned: amavisd-new at
X-Spam-Flag: NO
X-Spam-Score: -9.031
X-Spam-Status: No, score=-9.031 tagged_above=-999 required=5 tests=[AWL=1.218, BAYES_00=-2.599, HELO_EQ_DE=0.35, RCVD_IN_DNSWL_HI=-8]
Received: from ([]) by localhost ( []) (amavisd-new, port 10024) with ESMTP id QIM9KmprvbiN for <>; Mon, 17 May 2010 12:16:42 -0700 (PDT)
Received: from ( []) by (Postfix) with ESMTP id D47C03A6AEA for <>; Mon, 17 May 2010 12:16:16 -0700 (PDT)
Received: from by (26) with ESMTP id o4HJG25w025684 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=NO); Mon, 17 May 2010 21:16:02 +0200 (MEST)
From: Martin Rex <>
Message-Id: <>
To: (Joseph Salowey)
Date: Mon, 17 May 2010 21:16:01 +0200 (MEST)
In-Reply-To: <> from "Joseph Salowey" at May 17, 10 09:29:44 am
MIME-Version: 1.0
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 8bit
X-Scanner: Virus Scanner virwal05
X-SAP: out
Subject: Re: [TLS] Wrapping up cached info
X-Mailman-Version: 2.1.9
Precedence: list
List-Id: "This is the mailing list for the Transport Layer Security working group of the IETF." <>
List-Unsubscribe: <>, <>
List-Archive: <>
List-Post: <>
List-Help: <>
List-Subscribe: <>, <>
X-List-Received-Date: Mon, 17 May 2010 19:16:43 -0000

Joseph Salowey wrote:
> I agree with Uri, that if you determine you need SHA-256 then you should
> plan for hash agility.  TLS 1.2 plans for hash agility.  
> What about Nico's proposal where a checksum is used to identify the
> cached data and the actual handshake contains the actual data hashed
> with the algorithm used in the PRF negotiated with the cipher suite? 
> This way we don't have to introduce hash agility into the extension, but
> we have cryptographic hash agility where it matters in the Finished
> computation.  Does it solve the problem?  

Yes, I think so.
This approach should solve the issue at the technical level.

Going more into detail, one would hash/mac only the data that got
actually replaced in the handshake, each prefixed by a (locally computed)
length field.