Adding ECC KEM

[NIIBE Yutaka]

Hello,

Simon Josefsson <simon at josefsson.org> wrote:
> Nice!  Is this intended to be compatible with HPKE ECC KEM?
>
> https://www.rfc-editor.org/rfc/rfc9180.html#name-dh-based-kem-dhkem

Yes.  GCRY_KEM_DHKEM25519 is for DHKEM with X25519, HKDF, and SHA256
described in RFC 9180.

> Did you validate test vectors?

In my working branch of last year, I added test vectors from RFC 9180.

https://dev.gnupg.org/source/libgcrypt/browse/gniibe%252Fkem2/tests/t-kem.c;2f93e53f6525155b4c78419d55b35a35cde84907$349

It was tested at that time.  I plan to merge this test into master.
(Currently, tests/t-kem only has generate-encap-decap tests.)

So, the answer is: yes, I did, but not yet with master.  
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