Re: [Cfrg] Message Digest Algorithm Choice for CMS with Ed448

Taylor R Campbell <campbell+cfrg@mumble.net> Mon, 14 November 2016 23:31 UTC

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From: Taylor R Campbell <campbell+cfrg@mumble.net>
To: Jim Schaad <ietf@augustcellars.com>
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Cc: IRTF CFRG <cfrg@irtf.org>, Russ Housley <housley@vigilsec.com>, "Scott Fluhrer \(sfluhrer\)" <sfluhrer@cisco.com>
Subject: Re: [Cfrg] Message Digest Algorithm Choice for CMS with Ed448
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   Date: Tue, 15 Nov 2016 08:10:08 +0900
   From: Jim Schaad <ietf@augustcellars.com>

   Please note that the following is how CMS works

   Sign( list of attributes )
   List of attributes contains a hash of the message along with other items
   such as a time, which signature algorithm, which hash algorithm, potentially
   which certificate(s) to use for verification.

   For this exercise, we are looking what to use for the hash of message, the
   sign operation is using EdDSA pure.

If you are limited to choosing a single fixed public hash function H
so that the message m figures into the rest of the system only via
H(m), then it is essential to choose H so that it has collision
resistance at the desired security level, e.g. SHAKE256-512 for a
256-bit security level.

(SHA3-512 does too but is excessively slow for that security level.
The CFRG's EdDSA draft uses SHAKE256 rather than SHA3-512 for this
reason.)