Add derive method to pkey

Allows deriving shared secrets for use with symmetric encryption
algorithms such as is done for ECDH.

Closes wahern#213

src/openssl.c

@@ -4662,6 +4662,53 @@ static int pk_encrypt(lua_State *L) {
 
 	return auxL_error(L, auxL_EOPENSSL, "pkey:encrypt");
 } /* pk_encrypt() */
+
+static int pk_derive(lua_State *L) {
+	size_t outlen;
+	EVP_PKEY *key = checksimple(L, 1, PKEY_CLASS);
+	EVP_PKEY *peer = checksimple(L, 2, PKEY_CLASS);
+	EVP_PKEY_CTX *ctx;
+	BIO *bio;
+	BUF_MEM *buf;
+
+	bio = getbio(L);
+	BIO_get_mem_ptr(bio, &buf);
+
+	if (!(ctx = EVP_PKEY_CTX_new(key, NULL)))
+		goto sslerr;
+
+	if (EVP_PKEY_derive_init(ctx) <= 0)
+		goto sslerr;
+
+	if (EVP_PKEY_derive_set_peer(ctx, peer) <= 0)
+		goto sslerr;
+
+	if (EVP_PKEY_derive(ctx, NULL, &outlen) <= 0)
+		goto sslerr;
+
+	if (!BUF_MEM_grow_clean(buf, outlen))
+		goto sslerr;
+
+	if (EVP_PKEY_derive(ctx, (unsigned char*)buf->data, &outlen) <= 0)
+		goto sslerr;
+
+	EVP_PKEY_CTX_free(ctx);
+	ctx = NULL;
+
+	lua_pushlstring(L, buf->data, outlen);
+
+	BIO_reset(bio);
+
+	return 1;
+sslerr:
+	if (ctx) {
+		EVP_PKEY_CTX_free(ctx);
+		ctx = NULL;
+	}
+	BIO_reset(bio);
+
+	return auxL_error(L, auxL_EOPENSSL, "pkey:encrypt");
+} /* pk_derive() */
 #endif
 
 static int pk_sign(lua_State *L) {
@@ -5393,6 +5440,7 @@ static const auxL_Reg pk_methods[] = {
 	{ "toPEM",         &pk_toPEM },
 	{ "tostring",      &pk__tostring },
 	{ "verify",        &pk_verify },
+	{ "derive",        &pk_derive },
 	{ NULL,            NULL },
 };