Re: [Dcrup] Is there anything this WG wants to do not yet in draft-ietf-dcrup-dkim-crypto-01 ?

"John R Levine" <johnl@taugh.com> Tue, 20 June 2017 16:50 UTC

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Date: Tue, 20 Jun 2017 12:50:07 -0400
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From: John R Levine <johnl@taugh.com>
To: Eric Rescorla <ekr@rtfm.com>
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Subject: Re: [Dcrup] Is there anything this WG wants to do not yet in draft-ietf-dcrup-dkim-crypto-01 ?
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> Giving this document a quick read I see several things I would change.

In case it's not clear, I do not purport to be any sort of crypto expert, 
so I'm hoping that you, r$, et al. help us to be sure that what I say 
makes sense.

> 1. You say you are using EdDSA with SHA-256? Does this mean you intend
>    to use the HashEdDSA variant see (
>    https://tools.ietf.org/rfcmarkup?doc=8032#section-4)?  If so you
>    should say so.

I think the answer is no.  DKIM signatures use two chained hashes.  First 
it hashes a canonicalized version of the message body, and inserts the 
base64 hash into the DKIM-Signature header.  Then it hashes a 
canonicalized version of the headers, signs that hash, and inserts the 
base64 signature of the second hash into the DKIM-Signatuer header.  To 
avoid infinite regress, the verification step pretends the signature 
wasn't in the header when verifying.

As far as I can tell, feeding the canonicalized header text into HashEdDSA 
gives the same result as feeding the hash into PureEdDSA.  Since the 
hashing in DKIM has been defined for a decade and we're not planning to 
change it (other than to deprecate SHA-1) I'd rather say this is a 
PureEdDSA signature of the existing hash.

> 2. I wouldn't specific generic EdDSA but rather EdDSA with a specific
>    curve. This is both for practical reasons (I don't want to have to
>    distinguish keys by len())  and for algorithmic reasons (you want to
>    use a stronger digest algorithm than SHA-256 with X448)

OK.  Looking at 8032, it appears I want to use Ed25519, right?

> 3. You shouldn't name the keys ecdh(fp) but rather eddsa(fp)  These keys
>    are not intended for use with key exchange but rather signature. The
>    document actually seems kinda confused on this point with the text
>    saying one thing and the table saying another.

OK.

Regards,
John Levine, johnl@taugh.com, Taughannock Networks, Trumansburg NY
Please consider the environment before reading this e-mail. https://jl.ly