Re: [openpgp] patch for EdDSA key packet formats

[NIIBE Yutaka <[e-mail]>]

Hello,

Taylor R Campbell <[e-mail]> wrote:
> The current text describing the EdDSA secret key packet format is
> wrong (it is not used as a scalar at all), and the text describing the
> public key packet format is confusing (can't find the notation Q
> anywhere, and it matches neither the EdDSA papers nor the CFRG EdDSA
> draft).

I agree these points.

But, I'm not sure the fixes in the patch are good enough.

In the patch:
> From: Taylor R Campbell <[e-mail]>
> Date: Wed, 11 Jan 2017 14:21:13 +0000
> Subject: [PATCH] Fix EdDSA secret key packet format with reference to CFRG
>  notation.
>
> What is stored is *not* a scalar; it is a b-bit secret input to a
> 2b-bit hash function that expands it into
>
> (a) the b-bit secret scalar a, giving the public key A = a B, where B
> is the standard base point; and
> (b) the b-bit nonce PRF key.
>
> While here, clarify EdDSA public key packet format with reference to
> CFRG notation too.

I agree these are correct and reference to CFRG notation is good.

Well, Simon's is now RFC8032: 
    https://datatracker.ietf.org/doc/rfc8032/

Here are my comments for the fixes.

> diff --git a/middle.mkd b/middle.mkd
> index 5182c7d..905bde1 100644
> --- a/middle.mkd
> +++ b/middle.mkd
> @@ -1936,8 +1936,9 @@ A version 4 packet contains:
>             - the octets representing a curve OID, defined in
>               section NN{FIXME};
>  
> -      - a MPI of an EC point representing a public key Q as described
> -        under EdDSA Point Format below.
> +      - a MPI, encoded as described under EdDSA Point Format, of an EC
> +        point A, in the notation of [](#I-D.irtf-cfrg-eddsa),
> +        Section 3.2 "Keys".

I think that the expression "an EC point A" would not be good.  In RFC
8032, Section 3.1 "Encoding" explains about ENC(), the little-endian
encoding, and Section 3.2 "Keys" says:

	The EdDSA public key is ENC(A).

... where A is an EC point.

We put the prefix 0x40 to ENC(A), it is not entirely same of
the notation of Section 3.2 "Keys".

Sorry, as I am not good in English, I don't have my own proposal.

>      Algorithm-Specific Fields for ECDH keys:
>  
> @@ -2034,8 +2035,8 @@ The packet contains:
>  
>      Algorithm-Specific Fields for EdDSA keys:
>  
> -      - MPI of an integer representing the secret key, which is a
> -        scalar of the public EC point.
> +      - an opaque octet string k, in the notation of
> +        [](#I-D.irtf-cfrg-eddsa), Section 3.2 "Keys".

Right.  It's not a scalar of the public EC point.  It is the one
which generates the scalar.

In my opinion, "MPI of an integer representing the secret key" is not
wrong.
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