gpg: Initial support for generating Kyber subkeys.

* common/openpgpdefs.h (PUBKEY_ALGO_KY768_25519): Remove.
(PUBKEY_ALGO_KY1024_448): Remove.
(PUBKEY_ALGO_KYBER): New.  Use them everywhere instead of the removed.

* g10/build-packet.c (gpg_mpi_write_nohdr): Rename to
(gpg_mpi_write_opaque_nohdr): this.  Change callers.
(gpg_mpi_write_opaque_32): New.
(do_key): Support Kyber keys using the revised format.
* g10/gpg.h (MAX_EXTERN_KEYPARM_BITS): New.
* g10/parse-packet.c (read_octet_string): Add arg nbytes so support
reading with a length prefix.  Adjust callers.
(parse_key): Parse Kyber public keys.
* g10/misc.c (pubkey_get_npkey): Support Kyber.
(pubkey_get_nskey): Ditto.

* g10/keyid.c (pubkey_string): Support dual algorithms.
(do_hash_public_key): Support Kyber.
(nbits_from_pk): Ditto.
(keygrip_from_pk): Return the Kyber part for the ECC+Kyber dual algo.

* g10/keygen.c (struct common_gen_cb_parm_s): Add genkey_result2.
Note that this callback is not yet used.
(ecckey_from_sexp): Add optional arg sexp2 and use it for Kyber.
Change callers.
(ecckey_from_sexp): Do not leak LIST in case of an error.
(common_gen): Add arg keyparms2, change callers, and support Kyber.
(gen_kyber): New.
(get_keysize_range): Support Kyber.
(fixup_keysize): Simplify and support Kyber.
(do_create): Handle Kyber.
(parse_key_parameter_part): Remove algo strings "ky768" and "ky1024"
and add a generic "kyber" with default parameters.
--

This uses a revised format which is more aligned with the usual
OpenPGP structure.  A lot of things are still missing.  For example
support for handling two keygrips and checking both of them in a -K
listing.  There is also only ky768_bp384 as fixed algorithm for now.
No passphrase for the Kyber part of the dual algorithm is on purpose.

A test was done using

  gpg --quick-gen-key pqc1 nistp256

and then running

  gpg -v --quick-add-key <fingerprint> kyber

which creates a v5 subkey on a v4 primary key.  A second test using

  gpg --quick-gen-key pqc2 Ed448

followed by a --quick-add-key created a v5 key with a v5 subkey.

GnuPG-bug-id: 6815

common/openpgpdefs.h

@@ -172,8 +172,7 @@ typedef enum
     PUBKEY_ALGO_ELGAMAL     = 20, /* Elgamal encrypt+sign (legacy).  */
     /*                        21     reserved by OpenPGP.            */
     PUBKEY_ALGO_EDDSA       = 22, /* EdDSA.                          */
-    PUBKEY_ALGO_KY768_25519 = 29, /* Kyber768 + X25519    (aka ML-KEM-768)   */
-    PUBKEY_ALGO_KY1024_448  = 30, /* Kyber1024 + X448     (aka ML-KEM-1024)  */
+    PUBKEY_ALGO_KYBER       = 29, /* Kyber                           */
     PUBKEY_ALGO_DIL3_25519  = 35, /* Dilithium3 + Ed25519 (aka ML-DSA-65)    */
     PUBKEY_ALGO_DIL5_448    = 36, /* Dilithium5 + Ed448   (aka ML-DSA-87)    */
     PUBKEY_ALGO_SPHINX_SHA2 = 41, /* SPHINX+-simple-SHA2  (aka SLH-DSA-SHA2) */