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

"Scott Fluhrer (sfluhrer)" <sfluhrer@cisco.com> Sun, 13 November 2016 04:20 UTC

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From: "Scott Fluhrer (sfluhrer)" <sfluhrer@cisco.com>
To: Russ Housley <housley@vigilsec.com>, IRTF CFRG <cfrg@irtf.org>
Thread-Topic: [Cfrg] Message Digest Algorithm Choice for CMS with Ed448
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Date: Sun, 13 Nov 2016 04:20:28 +0000
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Subject: Re: [Cfrg] Message Digest Algorithm Choice for CMS with Ed448
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> -----Original Message-----
> From: Cfrg [mailto:cfrg-bounces@irtf.org] On Behalf Of Russ Housley
> Sent: Saturday, November 12, 2016 10:55 PM
> To: IRTF CFRG
> Subject: [Cfrg] Message Digest Algorithm Choice for CMS with Ed448
> 
> The CURDLE WG is working on a document that specifies the conventions for
> using EdDSA with CMS [RFC5652].  See draft-ietf-curdle-cms-eddsa-
> signatures.
> 
> The most common case involves these steps:
> 
>    1.  Compute a message digest on the content.
> 
>    2.  Create a message-digest attribute that include the
>        result from 1.
> 
>    3.  Gather all of the attributes that will be signed, which
>        includes the attribute from 2.
> 
>    4.  Digitally sign the set of attributes.
> 
> For Ed448 (EdDSA with Curve448), step 4 uses SHAKE256.
> 
> SHAKE256 uses SHA3-512 internally.

Not precisely, a blackbox SHA3-512 implementation would be of little use to create a SHAKE256 implementation.  What is true that both are based on the Keccak permutation, and so parts of a nonblackbox SHA3-512 implementation could be reused in a SHAKE256 implementation.
> 
> What message digest algorithm should be used in step 1?
> 
> It seems that SHA3-512 would be a good choice to avoid having to implement
> more that one message digest algorithm to generate the signature or
> validate it.

How about SHAKE256, with the output limited to 256 bits?  SHAKE256 in that mode ought to have the same security properties as SHA3-256 (as they are the same except for different end-message padding).

> 
> Russ
> 
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