1 /**
2 * MD API multi-part HMAC demonstration.
3 *
4 * This programs computes the HMAC of two messages using the multi-part API.
5 *
6 * This is a companion to psa/hmac_demo.c, doing the same operations with the
7 * legacy MD API. The goal is that comparing the two programs will help people
8 * migrating to the PSA Crypto API.
9 *
10 * When it comes to multi-part HMAC operations, the `mbedtls_md_context`
11 * serves a dual purpose (1) hold the key, and (2) save progress information
12 * for the current operation. With PSA those roles are held by two disinct
13 * objects: (1) a psa_key_id_t to hold the key, and (2) a psa_operation_t for
14 * multi-part progress.
15 *
16 * This program and its companion psa/hmac_demo.c illustrate this by doing the
17 * same sequence of multi-part HMAC computation with both APIs; looking at the
18 * two side by side should make the differences and similarities clear.
19 */
20
21 /*
22 * Copyright The Mbed TLS Contributors
23 * SPDX-License-Identifier: Apache-2.0
24 *
25 * Licensed under the Apache License, Version 2.0 (the "License"); you may
26 * not use this file except in compliance with the License.
27 * You may obtain a copy of the License at
28 *
29 * http://www.apache.org/licenses/LICENSE-2.0
30 *
31 * Unless required by applicable law or agreed to in writing, software
32 * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
33 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
34 * See the License for the specific language governing permissions and
35 * limitations under the License.
36 */
37
38 /* First include Mbed TLS headers to get the Mbed TLS configuration and
39 * platform definitions that we'll use in this program. Also include
40 * standard C headers for functions we'll use here. */
41 #include "mbedtls/build_info.h"
42
43 #include "mbedtls/md.h"
44
45 #include "mbedtls/platform_util.h" // for mbedtls_platform_zeroize
46
47 #include <stdlib.h>
48 #include <stdio.h>
49
50 /* If the build options we need are not enabled, compile a placeholder. */
51 #if !defined(MBEDTLS_MD_C)
main(void)52 int main(void)
53 {
54 printf("MBEDTLS_MD_C not defined\r\n");
55 return 0;
56 }
57 #else
58
59 /* The real program starts here. */
60
61 /* Dummy inputs for HMAC */
62 const unsigned char msg1_part1[] = { 0x01, 0x02 };
63 const unsigned char msg1_part2[] = { 0x03, 0x04 };
64 const unsigned char msg2_part1[] = { 0x05, 0x05 };
65 const unsigned char msg2_part2[] = { 0x06, 0x06 };
66
67 /* Dummy key material - never do this in production!
68 * This example program uses SHA-256, so a 32-byte key makes sense. */
69 const unsigned char key_bytes[32] = { 0 };
70
71 /* Print the contents of a buffer in hex */
print_buf(const char * title,unsigned char * buf,size_t len)72 void print_buf(const char *title, unsigned char *buf, size_t len)
73 {
74 printf("%s:", title);
75 for (size_t i = 0; i < len; i++) {
76 printf(" %02x", buf[i]);
77 }
78 printf("\n");
79 }
80
81 /* Run an Mbed TLS function and bail out if it fails.
82 * A string description of the error code can be recovered with:
83 * programs/util/strerror <value> */
84 #define CHK(expr) \
85 do \
86 { \
87 ret = (expr); \
88 if (ret != 0) \
89 { \
90 printf("Error %d at line %d: %s\n", \
91 ret, \
92 __LINE__, \
93 #expr); \
94 goto exit; \
95 } \
96 } while (0)
97
98 /*
99 * This function demonstrates computation of the HMAC of two messages using
100 * the multipart API.
101 */
hmac_demo(void)102 int hmac_demo(void)
103 {
104 int ret;
105 const mbedtls_md_type_t alg = MBEDTLS_MD_SHA256;
106 unsigned char out[MBEDTLS_MD_MAX_SIZE]; // safe but not optimal
107
108 mbedtls_md_context_t ctx;
109
110 mbedtls_md_init(&ctx);
111
112 /* prepare context and load key */
113 // the last argument to setup is 1 to enable HMAC (not just hashing)
114 const mbedtls_md_info_t *info = mbedtls_md_info_from_type(alg);
115 CHK(mbedtls_md_setup(&ctx, info, 1));
116 CHK(mbedtls_md_hmac_starts(&ctx, key_bytes, sizeof(key_bytes)));
117
118 /* compute HMAC(key, msg1_part1 | msg1_part2) */
119 CHK(mbedtls_md_hmac_update(&ctx, msg1_part1, sizeof(msg1_part1)));
120 CHK(mbedtls_md_hmac_update(&ctx, msg1_part2, sizeof(msg1_part2)));
121 CHK(mbedtls_md_hmac_finish(&ctx, out));
122 print_buf("msg1", out, mbedtls_md_get_size(info));
123
124 /* compute HMAC(key, msg2_part1 | msg2_part2) */
125 CHK(mbedtls_md_hmac_reset(&ctx)); // prepare for new operation
126 CHK(mbedtls_md_hmac_update(&ctx, msg2_part1, sizeof(msg2_part1)));
127 CHK(mbedtls_md_hmac_update(&ctx, msg2_part2, sizeof(msg2_part2)));
128 CHK(mbedtls_md_hmac_finish(&ctx, out));
129 print_buf("msg2", out, mbedtls_md_get_size(info));
130
131 exit:
132 mbedtls_md_free(&ctx);
133 mbedtls_platform_zeroize(out, sizeof(out));
134
135 return ret;
136 }
137
main(void)138 int main(void)
139 {
140 int ret;
141
142 CHK(hmac_demo());
143
144 exit:
145 return ret == 0 ? EXIT_SUCCESS : EXIT_FAILURE;
146 }
147
148 #endif
149
