New function mbedtls_ecp_keypair_get_group_id

Add a simple function to get the group id from a key object.

This information is available via mbedtls_ecp_export, but that function
consumes a lot of memory, which is a waste if all you need is to identify
the curve.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

include/mbedtls/ecp.h

@@ -1323,6 +1323,18 @@ int mbedtls_ecp_check_pub_priv(
     const mbedtls_ecp_keypair *pub, const mbedtls_ecp_keypair *prv,
     int (*f_rng)(void *, unsigned char *, size_t), void *p_rng);
 
+/** \brief          Query the group that a key pair belongs to.
+ *
+ * \param key       The key pair to query.
+ *
+ * \return          The group ID for the group registered in the key pair
+ *                  object.
+ *                  This is \c MBEDTLS_ECP_DP_NONE if no group has been set
+ *                  in the key pair object.
+ */
+mbedtls_ecp_group_id mbedtls_ecp_keypair_get_group_id(
+    const mbedtls_ecp_keypair *key);
+
 /**
  * \brief           This function exports generic key-pair parameters.
  *

library/ecp.c

@@ -3357,6 +3357,12 @@ cleanup:
 }
 #endif /* MBEDTLS_ECP_C */
 
+mbedtls_ecp_group_id mbedtls_ecp_keypair_get_group_id(
+    const mbedtls_ecp_keypair *key)
+{
+    return key->grp.id;
+}
+
 /*
  * Export generic key-pair parameters.
  */

tests/suites/test_suite_ecp.function

@@ -1030,6 +1030,7 @@ void mbedtls_ecp_gen_key(int id)
                                     &mbedtls_test_rnd_pseudo_rand,
                                     &rnd_info) == 0);
 
+    TEST_EQUAL(mbedtls_ecp_keypair_get_group_id(&key), id);
     TEST_ASSERT(mbedtls_ecp_check_pubkey(&key.grp, &key.Q) == 0);
     TEST_ASSERT(mbedtls_ecp_check_privkey(&key.grp, &key.d) == 0);
 
@@ -1052,6 +1053,7 @@ void mbedtls_ecp_read_key(int grp_id, data_t *in_key, int expected, int canonica
     TEST_ASSERT(ret == expected);
 
     if (expected == 0) {
+        TEST_EQUAL(mbedtls_ecp_keypair_get_group_id(&key), grp_id);
         ret = mbedtls_ecp_check_privkey(&key.grp, &key.d);
         TEST_ASSERT(ret == 0);
 
@@ -1233,6 +1235,10 @@ void ecp_export(int id, char *Qx, char *Qy, char *d, int expected_ret, int inval
         TEST_EQUAL(mbedtls_ecp_point_cmp(&key.Q, &export_Q), 0);
         TEST_EQUAL(mbedtls_mpi_cmp_mpi(&key.d, &export_d), 0);
         TEST_EQUAL(mbedtls_ecp_group_cmp(&key.grp, &export_grp), 0);
+
+        /* Check consistency with the group id */
+        TEST_EQUAL(export_grp.id,
+                   mbedtls_ecp_keypair_get_group_id(&key));
     }
 
 exit: