library/hkdf.c

/*
 *  HKDF implementation -- RFC 5869
 *
 *  Copyright The Mbed TLS Contributors
 *  SPDX-License-Identifier: Apache-2.0 OR GPL-2.0-or-later
 */
#include "common.h"

#if defined(MBEDTLS_HKDF_C)

#include <string.h>
#include "mbedtls/hkdf.h"
#include "mbedtls/platform_util.h"
#include "mbedtls/error.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 = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
    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;

        if (salt_len != 0) {
            return MBEDTLS_ERR_HKDF_BAD_INPUT_DATA;
        }

        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++;
    }

    /*
     * Per RFC 5869 Section 2.3, okm_len must not exceed
     * 255 times the hash length
     */
    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;
    }

    memset(t, 0, hash_len);

    /*
     * Compute T = T(1) | T(2) | T(3) | ... | T(N)
     * Where T(N) is defined in 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 */