programs/hash/md_hmac_demo.c

/**
 * MD API multi-part HMAC demonstration.
 *
 * This programs computes the HMAC of two messages using the multi-part API.
 *
 * This is a companion to psa/hmac_demo.c, doing the same operations with the
 * legacy MD API. The goal is that comparing the two programs will help people
 * migrating to the PSA Crypto API.
 *
 * When it comes to multi-part HMAC operations, the `mbedtls_md_context`
 * serves a dual purpose (1) hold the key, and (2) save progress information
 * for the current operation. With PSA those roles are held by two disinct
 * objects: (1) a psa_key_id_t to hold the key, and (2) a psa_operation_t for
 * multi-part progress.
 *
 * This program and its companion psa/hmac_demo.c illustrate this by doing the
 * same sequence of multi-part HMAC computation with both APIs; looking at the
 * two side by side should make the differences and similarities clear.
 */

/*
 *  Copyright The Mbed TLS Contributors
 *  SPDX-License-Identifier: Apache-2.0 OR GPL-2.0-or-later
 */

/* First include Mbed TLS headers to get the Mbed TLS configuration and
 * platform definitions that we'll use in this program. Also include
 * standard C headers for functions we'll use here. */
#include "mbedtls/build_info.h"

#include "mbedtls/md.h"

#include "mbedtls/platform_util.h" // for mbedtls_platform_zeroize

#include <stdlib.h>
#include <stdio.h>

/* If the build options we need are not enabled, compile a placeholder. */
#if !defined(MBEDTLS_MD_C)
int main(void)
{
    printf("MBEDTLS_MD_C not defined\r\n");
    return 0;
}
#else

/* The real program starts here. */

/* Dummy inputs for HMAC */
const unsigned char msg1_part1[] = { 0x01, 0x02 };
const unsigned char msg1_part2[] = { 0x03, 0x04 };
const unsigned char msg2_part1[] = { 0x05, 0x05 };
const unsigned char msg2_part2[] = { 0x06, 0x06 };

/* Dummy key material - never do this in production!
 * This example program uses SHA-256, so a 32-byte key makes sense. */
const unsigned char key_bytes[32] = { 0 };

/* Print the contents of a buffer in hex */
void print_buf(const char *title, unsigned char *buf, size_t len)
{
    printf("%s:", title);
    for (size_t i = 0; i < len; i++) {
        printf(" %02x", buf[i]);
    }
    printf("\n");
}

/* Run an Mbed TLS function and bail out if it fails.
 * A string description of the error code can be recovered with:
 * programs/util/strerror <value> */
#define CHK(expr)                                             \
    do                                                          \
    {                                                           \
        ret = (expr);                                         \
        if (ret != 0)                                          \
        {                                                       \
            printf("Error %d at line %d: %s\n",                \
                   ret,                                        \
                   __LINE__,                                   \
                   #expr);                                    \
            goto exit;                                          \
        }                                                       \
    } while (0)

/*
 * This function demonstrates computation of the HMAC of two messages using
 * the multipart API.
 */
int hmac_demo(void)
{
    int ret;
    const mbedtls_md_type_t alg = MBEDTLS_MD_SHA256;
    unsigned char out[MBEDTLS_MD_MAX_SIZE]; // safe but not optimal

    mbedtls_md_context_t ctx;

    mbedtls_md_init(&ctx);

    /* prepare context and load key */
    // the last argument to setup is 1 to enable HMAC (not just hashing)
    const mbedtls_md_info_t *info = mbedtls_md_info_from_type(alg);
    CHK(mbedtls_md_setup(&ctx, info, 1));
    CHK(mbedtls_md_hmac_starts(&ctx, key_bytes, sizeof(key_bytes)));

    /* compute HMAC(key, msg1_part1 | msg1_part2) */
    CHK(mbedtls_md_hmac_update(&ctx, msg1_part1, sizeof(msg1_part1)));
    CHK(mbedtls_md_hmac_update(&ctx, msg1_part2, sizeof(msg1_part2)));
    CHK(mbedtls_md_hmac_finish(&ctx, out));
    print_buf("msg1", out, mbedtls_md_get_size(info));

    /* compute HMAC(key, msg2_part1 | msg2_part2) */
    CHK(mbedtls_md_hmac_reset(&ctx));     // prepare for new operation
    CHK(mbedtls_md_hmac_update(&ctx, msg2_part1, sizeof(msg2_part1)));
    CHK(mbedtls_md_hmac_update(&ctx, msg2_part2, sizeof(msg2_part2)));
    CHK(mbedtls_md_hmac_finish(&ctx, out));
    print_buf("msg2", out, mbedtls_md_get_size(info));

exit:
    mbedtls_md_free(&ctx);
    mbedtls_platform_zeroize(out, sizeof(out));

    return ret;
}

int main(void)
{
    int ret;

    CHK(hmac_demo());

exit:
    return ret == 0 ? EXIT_SUCCESS : EXIT_FAILURE;
}

#endif