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 OR GPL-2.0-or-later
24 */
25
26 /* First include Mbed TLS headers to get the Mbed TLS configuration and
27 * platform definitions that we'll use in this program. Also include
28 * standard C headers for functions we'll use here. */
29 #include "mbedtls/build_info.h"
30
31 #include "mbedtls/md.h"
32
33 #include "mbedtls/platform_util.h" // for mbedtls_platform_zeroize
34
35 #include <stdlib.h>
36 #include <stdio.h>
37
38 /* If the build options we need are not enabled, compile a placeholder. */
39 #if !defined(MBEDTLS_MD_C)
main(void)40 int main(void)
41 {
42 printf("MBEDTLS_MD_C not defined\r\n");
43 return 0;
44 }
45 #else
46
47 /* The real program starts here. */
48
49 /* Dummy inputs for HMAC */
50 const unsigned char msg1_part1[] = { 0x01, 0x02 };
51 const unsigned char msg1_part2[] = { 0x03, 0x04 };
52 const unsigned char msg2_part1[] = { 0x05, 0x05 };
53 const unsigned char msg2_part2[] = { 0x06, 0x06 };
54
55 /* Dummy key material - never do this in production!
56 * This example program uses SHA-256, so a 32-byte key makes sense. */
57 const unsigned char key_bytes[32] = { 0 };
58
59 /* Print the contents of a buffer in hex */
print_buf(const char * title,unsigned char * buf,size_t len)60 void print_buf(const char *title, unsigned char *buf, size_t len)
61 {
62 printf("%s:", title);
63 for (size_t i = 0; i < len; i++) {
64 printf(" %02x", buf[i]);
65 }
66 printf("\n");
67 }
68
69 /* Run an Mbed TLS function and bail out if it fails.
70 * A string description of the error code can be recovered with:
71 * programs/util/strerror <value> */
72 #define CHK(expr) \
73 do \
74 { \
75 ret = (expr); \
76 if (ret != 0) \
77 { \
78 printf("Error %d at line %d: %s\n", \
79 ret, \
80 __LINE__, \
81 #expr); \
82 goto exit; \
83 } \
84 } while (0)
85
86 /*
87 * This function demonstrates computation of the HMAC of two messages using
88 * the multipart API.
89 */
hmac_demo(void)90 int hmac_demo(void)
91 {
92 int ret;
93 const mbedtls_md_type_t alg = MBEDTLS_MD_SHA256;
94 unsigned char out[MBEDTLS_MD_MAX_SIZE]; // safe but not optimal
95
96 mbedtls_md_context_t ctx;
97
98 mbedtls_md_init(&ctx);
99
100 /* prepare context and load key */
101 // the last argument to setup is 1 to enable HMAC (not just hashing)
102 const mbedtls_md_info_t *info = mbedtls_md_info_from_type(alg);
103 CHK(mbedtls_md_setup(&ctx, info, 1));
104 CHK(mbedtls_md_hmac_starts(&ctx, key_bytes, sizeof(key_bytes)));
105
106 /* compute HMAC(key, msg1_part1 | msg1_part2) */
107 CHK(mbedtls_md_hmac_update(&ctx, msg1_part1, sizeof(msg1_part1)));
108 CHK(mbedtls_md_hmac_update(&ctx, msg1_part2, sizeof(msg1_part2)));
109 CHK(mbedtls_md_hmac_finish(&ctx, out));
110 print_buf("msg1", out, mbedtls_md_get_size(info));
111
112 /* compute HMAC(key, msg2_part1 | msg2_part2) */
113 CHK(mbedtls_md_hmac_reset(&ctx)); // prepare for new operation
114 CHK(mbedtls_md_hmac_update(&ctx, msg2_part1, sizeof(msg2_part1)));
115 CHK(mbedtls_md_hmac_update(&ctx, msg2_part2, sizeof(msg2_part2)));
116 CHK(mbedtls_md_hmac_finish(&ctx, out));
117 print_buf("msg2", out, mbedtls_md_get_size(info));
118
119 exit:
120 mbedtls_md_free(&ctx);
121 mbedtls_platform_zeroize(out, sizeof(out));
122
123 return ret;
124 }
125
main(void)126 int main(void)
127 {
128 int ret;
129
130 CHK(hmac_demo());
131
132 exit:
133 return ret == 0 ? EXIT_SUCCESS : EXIT_FAILURE;
134 }
135
136 #endif
137
