mbedtls: Add mbedtls_ecc_group_to_psa()

We'd like to enable Mbed TLS's PK module in using TF-M's PSA
implementation, even if it doesn't expose the same set of PSA extensions
as Mbed TLS's PSA implementation. To do this, we add
mbedtls_ecc_group_to_psa() in its own header available when using the
latest TF-M.

Add mbedtls_ecc_group_to_psa(), one of Mbed TLS's PSA compatibility
helpers, for internal use by the Mbed TLS PK module. Without this
conversion function, the Mbed TLS PK module is unable to use any PSA
implementation other than one which provides a compatible set of PSA
extensions.

platform/FEATURE_EXPERIMENTAL_API/FEATURE_PSA/TARGET_TFM/TARGET_TFM_LATEST/include/mbedtls_ecc_group_to_psa.h

@@ -0,0 +1,114 @@
+/**
+ * \file mbedtls_ecc_group_to_psa.h
+ *
+ * Excerpted from Mbed TLS for internal use by Mbed TLS's PK module to
+ * interface with generic PSA Crypto implementations.
+ *
+ */
+/*
+ *  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_ECC_GROUP_TO_PSA_H
+#define MBEDTLS_ECC_GROUP_TO_PSA_H
+
+//#include "mbedtls/platform_util.h"
+
+//#include "crypto_compat.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/** \defgroup psa_tls_helpers TLS helper functions
+ * @{
+ */
+
+#if defined(MBEDTLS_ECP_C)
+#include <mbedtls/ecp.h>
+
+/** Convert an ECC curve identifier from the Mbed TLS encoding to PSA.
+ *
+ * \note This function is provided solely for the convenience of
+ *       Mbed TLS and may be removed at any time without notice.
+ *
+ * \param grpid         An Mbed TLS elliptic curve identifier
+ *                      (`MBEDTLS_ECP_DP_xxx`).
+ * \param[out] bits     On success, the bit size of the curve.
+ *
+ * \return              The corresponding PSA elliptic curve identifier
+ *                      (`PSA_ECC_FAMILY_xxx`).
+ * \return              \c 0 on failure (\p grpid is not recognized).
+ */
+static inline psa_ecc_family_t mbedtls_ecc_group_to_psa( mbedtls_ecp_group_id grpid,
+                                                        size_t *bits )
+{
+    switch( grpid )
+    {
+        case MBEDTLS_ECP_DP_SECP192R1:
+            *bits = 192;
+            return( PSA_ECC_FAMILY_SECP_R1 );
+        case MBEDTLS_ECP_DP_SECP224R1:
+            *bits = 224;
+            return( PSA_ECC_FAMILY_SECP_R1 );
+        case MBEDTLS_ECP_DP_SECP256R1:
+            *bits = 256;
+            return( PSA_ECC_FAMILY_SECP_R1 );
+        case MBEDTLS_ECP_DP_SECP384R1:
+            *bits = 384;
+            return( PSA_ECC_FAMILY_SECP_R1 );
+        case MBEDTLS_ECP_DP_SECP521R1:
+            *bits = 521;
+            return( PSA_ECC_FAMILY_SECP_R1 );
+        case MBEDTLS_ECP_DP_BP256R1:
+            *bits = 256;
+            return( PSA_ECC_FAMILY_BRAINPOOL_P_R1 );
+        case MBEDTLS_ECP_DP_BP384R1:
+            *bits = 384;
+            return( PSA_ECC_FAMILY_BRAINPOOL_P_R1 );
+        case MBEDTLS_ECP_DP_BP512R1:
+            *bits = 512;
+            return( PSA_ECC_FAMILY_BRAINPOOL_P_R1 );
+        case MBEDTLS_ECP_DP_CURVE25519:
+            *bits = 255;
+            return( PSA_ECC_FAMILY_MONTGOMERY );
+        case MBEDTLS_ECP_DP_SECP192K1:
+            *bits = 192;
+            return( PSA_ECC_FAMILY_SECP_K1 );
+        case MBEDTLS_ECP_DP_SECP224K1:
+            *bits = 224;
+            return( PSA_ECC_FAMILY_SECP_K1 );
+        case MBEDTLS_ECP_DP_SECP256K1:
+            *bits = 256;
+            return( PSA_ECC_FAMILY_SECP_K1 );
+        case MBEDTLS_ECP_DP_CURVE448:
+            *bits = 448;
+            return( PSA_ECC_FAMILY_MONTGOMERY );
+        default:
+            *bits = 0;
+            return( 0 );
+    }
+}
+
+#endif /* MBEDTLS_ECP_C */
+
+/**@}*/
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* MBEDTLS_ECC_GROUP_TO_PSA_H */