Taylor R Campbell <campbell+ietf-openpgp(_at_)mumble(_dot_)net> wrote:
Right, but in RFC 8032, the letter `A' does mean a point on the curve.
Yes.
What my text says is that we encode the point A using the encoding
described below in the Section 13.3 `EdDSA Point Format'. This is not
the same as storing the RFC 8032 public key, which is the point A
encoded as ENC(A).
It may be that the Section 13.3 `EdDSA Point Format' encoding is
actually 0x40 || ENC(A), but I'm not sure offhand.
I believe this (0x40 || ENC(A)) is the `EdDSA Point Format' encoding.
You are probably right! I'm afraid I neglected to write notes when I
prepared the patch, so I forgot where in the code I should have cited
for that. Do you have a quick reference to the source code in
libgcrypt or gnupg that handles encoding this?
In GnuPG, secret keys are primarily handled by gpg-agent with libgcrypt.
And libgcrypt stores them in the format of SEXP [0]. So, GnuPG only
handles OpenPGP secret keys when importing/exporting in OpenPGP format.
For importing, we have the function parse_key in
gnupg/g10/parse-packet.c:2112 (in the release 2.1.18). When secret key
is not protected, it is handled by the mpi_read function (at line 2526).
Here, in the code, the expression is "pk->pkey[i]", where pk and pkey
stand for "public key", but this is because of historical reason, it
means secret key materials.
And then, it is handled by the function transfer_secret_keys in
gnupg/g10/import.c:1788. It composes SEXP around line 1917, by "%m"
format, which means MPI, to send gpg-agent.
Exporting is similar. In the function do_key in
gnupg/g10/build-packet.c:354, non-protected secret key is
written by gpg_mpi_write at line 478.
So, for the GnuPG implementation, secret key is handled as MPI. But I
think that this is basically due to its historical reason, when keys in
OpenPGP format were specified as MPI.
For specification, "an opaque octet string k" may be better. I support
this, personally.
On the other hand, I don't know why the ECC format of
04 || x || y
was described as MPI in RFC6637. For me, it is an octet string composed
by two MPIs and the prefix.
[0] SEXP --- (S-expressions)
http://people.csail.mit.edu/rivest/sexp.html
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