Add an HKDF (RFC 5869) implementation
diff --git a/library/hkdf.c b/library/hkdf.c
new file mode 100644
index 0000000..d2e55e8
--- /dev/null
+++ b/library/hkdf.c
@@ -0,0 +1,180 @@
+/*
+ * HKDF implementation -- RFC 5869
+ *
+ * Copyright (C) 2016-2018, 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)
+ */
+#if !defined(MBEDTLS_CONFIG_FILE)
+#include "mbedtls/config.h"
+#else
+#include MBEDTLS_CONFIG_FILE
+#endif
+
+#if defined(MBEDTLS_HKDF_C)
+
+#include <string.h>
+#include "mbedtls/hkdf.h"
+#include "mbedtls/platform_util.h"
+
+int mbedtls_hkdf( const mbedtls_md_info_t *md, const unsigned char *salt,
+ size_t salt_len, const unsigned char *ikm, size_t ikm_len,
+ const unsigned char *info, size_t info_len,
+ unsigned char *okm, size_t okm_len )
+{
+ int ret;
+ unsigned char prk[MBEDTLS_MD_MAX_SIZE];
+
+ ret = mbedtls_hkdf_extract( md, salt, salt_len, ikm, ikm_len, prk );
+
+ if( ret == 0 )
+ {
+ ret = mbedtls_hkdf_expand( md, prk, mbedtls_md_get_size( md ),
+ info, info_len, okm, okm_len );
+ }
+
+ mbedtls_platform_zeroize( prk, sizeof( prk ) );
+
+ return( ret );
+}
+
+int mbedtls_hkdf_extract( const mbedtls_md_info_t *md,
+ const unsigned char *salt, size_t salt_len,
+ const unsigned char *ikm, size_t ikm_len,
+ unsigned char *prk )
+{
+ unsigned char null_salt[MBEDTLS_MD_MAX_SIZE] = { '\0' };
+
+ if( salt == NULL )
+ {
+ size_t hash_len;
+
+ hash_len = mbedtls_md_get_size( md );
+
+ if( hash_len == 0 )
+ {
+ return MBEDTLS_ERR_HKDF_BAD_INPUT_DATA;
+ }
+
+ salt = null_salt;
+ salt_len = hash_len;
+ }
+
+ return( mbedtls_md_hmac( md, salt, salt_len, ikm, ikm_len, prk ) );
+}
+
+int mbedtls_hkdf_expand( const mbedtls_md_info_t *md, const unsigned char *prk,
+ size_t prk_len, const unsigned char *info,
+ size_t info_len, unsigned char *okm, size_t okm_len )
+{
+ size_t hash_len;
+ size_t where = 0;
+ size_t n;
+ size_t t_len = 0;
+ size_t i;
+ int ret = 0;
+ mbedtls_md_context_t ctx;
+ unsigned char t[MBEDTLS_MD_MAX_SIZE];
+
+ if( okm == NULL )
+ {
+ return( MBEDTLS_ERR_HKDF_BAD_INPUT_DATA );
+ }
+
+ hash_len = mbedtls_md_get_size( md );
+
+ if( prk_len < hash_len || hash_len == 0 )
+ {
+ return( MBEDTLS_ERR_HKDF_BAD_INPUT_DATA );
+ }
+
+ if( info == NULL )
+ {
+ info = (const unsigned char *) "";
+ info_len = 0;
+ }
+
+ n = okm_len / hash_len;
+
+ if( (okm_len % hash_len) != 0 )
+ {
+ n++;
+ }
+
+ if( n > 255 )
+ {
+ return( MBEDTLS_ERR_HKDF_BAD_INPUT_DATA );
+ }
+
+ mbedtls_md_init( &ctx );
+
+ if( (ret = mbedtls_md_setup( &ctx, md, 1) ) != 0 )
+ {
+ goto exit;
+ }
+
+ /* RFC 5869 Section 2.3. */
+ for( i = 1; i <= n; i++ )
+ {
+ size_t num_to_copy;
+ unsigned char c = i & 0xff;
+
+ ret = mbedtls_md_hmac_starts( &ctx, prk, prk_len );
+ if( ret != 0 )
+ {
+ goto exit;
+ }
+
+ ret = mbedtls_md_hmac_update( &ctx, t, t_len );
+ if( ret != 0 )
+ {
+ goto exit;
+ }
+
+ ret = mbedtls_md_hmac_update( &ctx, info, info_len );
+ if( ret != 0 )
+ {
+ goto exit;
+ }
+
+ /* The constant concatenated to the end of each t(n) is a single octet.
+ * */
+ ret = mbedtls_md_hmac_update( &ctx, &c, 1 );
+ if( ret != 0 )
+ {
+ goto exit;
+ }
+
+ ret = mbedtls_md_hmac_finish( &ctx, t );
+ if( ret != 0 )
+ {
+ goto exit;
+ }
+
+ num_to_copy = i != n ? hash_len : okm_len - where;
+ memcpy( okm + where, t, num_to_copy );
+ where += hash_len;
+ t_len = hash_len;
+ }
+
+exit:
+ mbedtls_md_free( &ctx );
+ mbedtls_platform_zeroize( t, sizeof( t ) );
+
+ return( ret );
+}
+
+#endif /* MBEDTLS_HKDF_C */
