Add a utils.h file that contains common functions

The new header contains common information across various mbed TLS
modules and avoids code duplication. To start, utils.h currently only
contains the mbedtls_zeroize() function.

include/mbedtls/utils.h

@@ -0,0 +1,39 @@
+/**
+ * \file utils.h
+ *
+ * \brief mbed TLS utility functions
+ *
+ *  Copyright (C) 2017, 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 MBEDTLS_UTILS_H
+#define MBEDTLS_UTILS_H
+
+#include <stddef.h>
+
+/**
+ * \brief       Securely zeroize a buffer
+ *
+ * \param buf   Buffer to be zeroized
+ * \param len   Length of the buffer in bytes
+ *
+ * \note        This implementation should never be optimized out by the
+ *              compiler
+ */
+void mbedtls_zeroize( void *buf, size_t len );
+
+#endif /* MBEDTLS_UTILS_H */

library/CMakeLists.txt

@@ -57,6 +57,7 @@ set(src_crypto
     version.c
     version_features.c
     xtea.c
+    utils.c
 )
 
 set(src_x509

library/Makefile

@@ -65,7 +65,8 @@ OBJS_CRYPTO=	aes.o		aesni.o		arc4.o		\
 		ripemd160.o	rsa_internal.o	rsa.o  		\
 		sha1.o		sha256.o	sha512.o	\
 		threading.o	timing.o	version.o	\
-		version_features.o		xtea.o
+		version_features.o		xtea.o	\
+		utils.o
 
 OBJS_X509=	certs.o		pkcs11.o	x509.o		\
 		x509_create.o	x509_crl.o	x509_crt.o	\

library/utils.c

@@ -0,0 +1,33 @@
+/*
+ *  mbedtls utility functions
+ *
+ *  Copyright (C) 2017, 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)
+ */
+
+#include "mbedtls/utils.h"
+
+#include <stddef.h>
+
+/* This implementation should never be optimized out by the compiler */
+void mbedtls_zeroize( void *buf, size_t len )
+{
+    volatile unsigned char *p = (unsigned char *)buf;
+
+    while( len-- )
+        *p++ = 0;
+}