[openssl-users] Naive: how to generate EC public key from EC private key?

[Viktor Dukhovni]

> On Mar 17, 2016, at 6:32 PM, Blumenthal, Uri - 0553 - MITLL <uri at ll.mit.edu> wrote:
> 
> Oh, and I'd much prefer to stay at the EVP level, rather than invoke BIO primitives for this task.

Well you can work with http://openssl.org/docs/manmaster/crypto/EC_KEY_key2buf.html
to extract EC public key octets.  If you want an ASN.1 encoded "SPKI" object (i.e. an
X509_PUBKEY in OpenSSL) then you can use

	X509_PUBKEY *pk = NULL;
	unsigned char *buf = NULL;
	EVP_PKEY *key;

	key = ... ; /* Get a keypair */

	if (X509_PUBKEY_set(&pk, key) <= 0) {
		/* error */
	}

	len = i2d_X509_PUBKEY(pk, &buf);
	if (len < 0 || buf == NULL) {
		/* error */
	}
	
	/* buf contains ASN.1-encoded SPKI, use it */

	OPENSSL_free(buf);
	X509_PUBKEY_free(pk);
	EVP_PKEY_free(key); /* If no longer needed */

A shorter version of the above is possible via i2d_PUBKEY() which
handles the creation, encoding and destruction of the intermediate
X509_PUBKEY:

  int i2d_PUBKEY(EVP_PKEY *a, unsigned char **pp)
  {
    X509_PUBKEY *xpk = NULL;
    int ret;
    if (!a)
        return 0;
    if (!X509_PUBKEY_set(&xpk, a))
        return 0;
    ret = i2d_X509_PUBKEY(xpk, pp);
    X509_PUBKEY_free(xpk);
    return ret;
  }


Looks like we need documentation for X509_PUBKEY_set() and friends...
Any volunteers?

X509_PUBKEY_free();
X509_PUBKEY_get();
X509_PUBKEY_get0();
X509_PUBKEY_get0_param();
X509_PUBKEY_new();
X509_PUBKEY_set();
X509_PUBKEY_set0_param();
d2i_DSA_PUBKEY_bio();
d2i_DSA_PUBKEY_fp();
d2i_EC_PUBKEY_bio();
d2i_EC_PUBKEY_fp();
d2i_PUBKEY_bio();
d2i_PUBKEY_fp();
d2i_RSA_PUBKEY_bio();
d2i_RSA_PUBKEY_fp();
i2d_DSA_PUBKEY_bio();
i2d_DSA_PUBKEY_fp();
i2d_EC_PUBKEY_bio();
i2d_EC_PUBKEY_fp();
i2d_PUBKEY_bio();
i2d_PUBKEY_fp();
i2d_RSA_PUBKEY_bio();
i2d_RSA_PUBKEY_fp();

-- 
	Viktor.