Honor OSSL_PKEY_PARAM_EC_POINT_CONVERSION_FORMAT as set and default to UNCOMPRESSED

Originally the code to im/export the EC pubkey was meant to be consumed
only by the im/export functions when crossing the provider boundary.
Having our providers exporting to a COMPRESSED format octet string made
sense to avoid memory waste, as it wasn't exposed outside the provider
API, and providers had all tools available to convert across the three
formats.

Later on, with #13139 deprecating the `EC_KEY_*` functions, more state
was added among the params imported/exported on an EC provider-native
key (including `OSSL_PKEY_PARAM_EC_POINT_CONVERSION_FORMAT`, although it
did not affect the format used to export `OSSL_PKEY_PARAM_PUB_KEY`).

Finally, in #14800, `EVP_PKEY_todata()` was introduced and prominently
exposed directly to users outside the provider API, and the choice of
COMPRESSED over UNCOMPRESSED as the default became less sensible in
light of usability, given the latter is more often needed by
applications and protocols.

This commit fixes it, by using `EC_KEY_get_conv_form()` to get the
point format from the internal state (an `EC_KEY` under the hood) of the
provider-side object, and using it on
`EVP_PKEY_export()`/`EVP_PKEY_todata()` to format
`OSSL_PKEY_PARAM_PUB_KEY`.
The default for an `EC_KEY` was already UNCOMPRESSED, and it is altered
if the user sets `OSSL_PKEY_PARAM_EC_POINT_CONVERSION_FORMAT` via
`EVP_PKEY_fromdata()`, `EVP_PKEY_set_params()`, or one of the
more specialized methods.

For symmetry, this commit also alters `ec_pkey_export_to()` in
`crypto/ec/ec_ameth.c`, part of the `EVP_PKEY_ASN1_METHOD` for legacy EC
keys: it exclusively used COMPRESSED format, and now it honors the
conversion format specified in the EC_KEY object being exported to a
provider when this function is called.

Expand documentation about `OSSL_PKEY_PARAM_PUB_KEY` and mention the
change in behavior for our providers.

Fixes #16595

(cherry picked from commit 926db476bc669fdcc4c4d2f1cb547060bdbfa153)

Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/19901)

1 files changed, 3 insertions, 1 deletions

diff --git a/crypto/ec/ec_ameth.c b/crypto/ec/ec_ameth.c
index 7dc2232e5d..d4348ff244 100644
--- a/crypto/ec/ec_ameth.c
+++ b/crypto/ec/ec_ameth.c
@@ -513,8 +513,10 @@ int ec_pkey_export_to(const EVP_PKEY *from, void *to_keydata,
 
     if (pub_point != NULL) {
         /* convert pub_point to a octet string according to the SECG standard */
+        point_conversion_form_t format = EC_KEY_get_conv_form(eckey);
+
         if ((pub_key_buflen = EC_POINT_point2buf(ecg, pub_point,
-                                                 POINT_CONVERSION_COMPRESSED,
+                                                 format,
                                                  &pub_key_buf, bnctx)) == 0
             || !OSSL_PARAM_BLD_push_octet_string(tmpl,
                                                  OSSL_PKEY_PARAM_PUB_KEY,