Adding ECC KEM

[NIIBE Yutaka]

Hello,

Werner Koch <wk at gnupg.org> wrote:
> However we also need to support BrainpoolP384r1 and P512r1 because they
> will likley be the default in GnuPG for Kyber (ML-KEM).  There are also
> request for other curves.

I will add other curves, too.

> Thus I wondered whether we really want to have a whole bunch of
> GCRY_KEM_* constants or whether it would be possible to define another
> parameter which can be shared by similar algorithms/curves.

Let's see by using the API.

While I added the ECC KEM API, I'm not sure if gpg-agent should use the
ECC KEM API for all of its uses of ECC.

Possibly, ECC KEM API will be only used for PQC.  In this case,
gpg-agent uses gcry_kem_* API for PQC hybrid, and keeps using gcry_pk_*
API for existing non-hybrid use of ECC.
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