Add quasi-reduction function for ecp

Signed-off-by: Gabor Mezei <gabor.mezei@arm.com>

library/ecp.c

@@ -79,6 +79,8 @@
 
 #include "bn_mul.h"
 #include "ecp_invasive.h"
+#include "ecp_internal.h"
+#include "bignum_core.h"
 
 #include <string.h>
 
@@ -1029,6 +1031,20 @@ cleanup:
     return ret;
 }
 
+int mbedtls_ecp_quasi_reduction(mbedtls_mpi_uint *X,
+                                const mbedtls_mpi_mod_modulus *N)
+{
+    if (N->limbs == 0) {
+        return MBEDTLS_ERR_ECP_BAD_INPUT_DATA;
+    }
+
+    mbedtls_mpi_uint c = mbedtls_mpi_core_sub(X, X, N->p, N->limbs);
+
+    (void) mbedtls_mpi_core_add_if(X, N->p, N->limbs, (unsigned) c);
+
+    return 0;
+}
+
 /*
  * Fast mod-p functions expect their argument to be in the 0..p^2 range.
  *

library/ecp_internal.h

@@ -0,0 +1,34 @@
+/**
+ * \file ecp_internal.h
+ *
+ * \brief Function declarations for internal functions of elliptic curve
+ * point arithmetic.
+ */
+/**
+ *  Copyright The Mbed TLS Contributors
+ *  SPDX-License-Identifier: Apache-2.0
+ *
+ *  Licensed under the Apache License, Version 2.0 (the "License"); you may
+ *  not use this file except in compliance with the License.
+ *  You may obtain a copy of the License at
+ *
+ *  http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ *  WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ */
+
+#ifndef MBEDTLS_ECP_INTERNAL_H
+#define MBEDTLS_ECP_INTERNAL_H
+
+#include "common.h"
+#include "mbedtls/bignum.h"
+#include "bignum_mod.h"
+
+int mbedtls_ecp_quasi_reduction(mbedtls_mpi_uint *X,
+                                const mbedtls_mpi_mod_modulus *N);
+
+#endif /* MBEDTLS_ECP_INTERNAL_H */