psa: Add storage implementation for files

Add new functions, psa_load_persistent_key(),
psa_free_persistent_key_data(), and psa_save_persistent_key(), for
managing persistent keys. These functions load to or save from our
internal representation of key slots. Serialization is a concern of the
storage backend implementation and doesn't abstraction-leak into the
lifetime management code.

An initial implementation for files is provided. Additional storage
backends can implement this interface for other storage types.

library/psa_crypto_storage_backend.h

@@ -0,0 +1,112 @@
+/**
+ * \file psa_crypto_storage_backend.h
+ *
+ * \brief PSA cryptography module: Mbed TLS key storage backend
+ */
+/*
+ *  Copyright (C) 2018, ARM Limited, All Rights Reserved
+ *  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.
+ *
+ *  This file is part of mbed TLS (https://tls.mbed.org)
+ */
+
+#ifndef PSA_CRYPTO_STORAGE_BACKEND_H
+#define PSA_CRYPTO_STORAGE_BACKEND_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/* Include the Mbed TLS configuration file, the way Mbed TLS does it
+ * in each of its header files. */
+#if defined(MBEDTLS_CONFIG_FILE)
+#include MBEDTLS_CONFIG_FILE
+#else
+#include "mbedtls/config.h"
+#endif
+
+#include "psa/crypto.h"
+#include "psa_crypto_storage.h"
+#include <stdint.h>
+
+/**
+ * \brief Load persistent data for the given key slot number.
+ *
+ * This function reads data from a storage backend and returns the data in a
+ * buffer.
+ *
+ * \param key              Slot number whose content is to be loaded. This must
+ *                         be a key slot whose lifetime is set to persistent.
+ * \param[out] data        Buffer where the data is to be written.
+ * \param data_size        Size of the \c data buffer in bytes.
+ *
+ * \retval PSA_SUCCESS
+ * \retval PSA_ERROR_STORAGE_FAILURE
+ */
+psa_status_t psa_crypto_storage_load( const psa_key_slot_t key, uint8_t *data,
+                                      size_t data_size );
+
+/**
+ * \brief Store persistent data for the given key slot number.
+ *
+ * This function stores the given data buffer to a persistent storage.
+ *
+ * \param key           Slot number whose content is to be stored.
+ * \param[in] data      Buffer containing the data to be stored.
+ * \param data_length   The number of bytes
+ *                      that make up the data.
+ *
+ * \retval PSA_SUCCESS
+ * \retval PSA_ERROR_INSUFFICIENT_STORAGE
+ * \retval PSA_ERROR_STORAGE_FAILURE
+ */
+psa_status_t psa_crypto_storage_store( const psa_key_slot_t key,
+                                       const uint8_t *data,
+                                       size_t data_length );
+
+/**
+ * \brief Checks if persistent data is stored for the given key slot number
+ *
+ * This function checks if any key data or metadata exists for the key slot in
+ * the persistent storage.
+ *
+ * \param key           Slot number whose content is to be checked.
+ *
+ * \retval 0
+ *         No persistent data present for slot number
+ * \retval 1
+ *         Persistent data present for slot number
+ */
+int psa_is_key_present_in_storage( const psa_key_slot_t key );
+
+/**
+ * \brief Get data length for given key slot number.
+ *
+ * \param key              Slot number whose stored data length is to be obtained.
+ * \param[out] data_length The number of bytes
+ *                         that make up the data.
+ *
+ * \retval PSA_SUCCESS
+ * \retval PSA_ERROR_STORAGE_FAILURE
+ */
+psa_status_t psa_crypto_storage_get_data_length( const psa_key_slot_t key,
+                                                 size_t *data_length );
+
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* PSA_CRYPTO_STORAGE_H */