Fix ecdh_get_params with mismatching group

If mbedtls_ecdh_get_params is called with keys belonging to
different groups, make it return an error the second time, rather than
silently interpret the first key as being on the second curve.

This makes the non-regression test added by the previous commit pass.

Author: gilles-peskine-arm

library/ecdh.c

@@ -442,8 +442,21 @@ int mbedtls_ecdh_get_params( mbedtls_ecdh_context *ctx,
     ECDH_VALIDATE_RET( side == MBEDTLS_ECDH_OURS ||
                        side == MBEDTLS_ECDH_THEIRS );
 
-    if( ( ret = mbedtls_ecdh_setup( ctx, key->grp.id ) ) != 0 )
-        return( ret );
+    if( ctx->grp.id == MBEDTLS_ECP_DP_NONE )
+    {
+        /* This is the first call to get_params(). Set up the context
+         * for use with the group. */
+        if( ( ret = mbedtls_ecdh_setup( ctx, key->grp.id ) ) != 0 )
+            return( ret );
+    }
+    else
+    {
+        /* This is not the first call to get_params(). Check that the
+         * current key's group is the same as the context's, which was set
+         * from the first key's group. */
+        if( ctx->grp.id != key->grp.id )
+            return( MBEDTLS_ERR_ECP_BAD_INPUT_DATA );
+    }
 
 #if defined(MBEDTLS_ECDH_LEGACY_CONTEXT)
     return( ecdh_get_params_internal( ctx, key, side ) );