crypto/hkdf/hkdf.c

/* Copyright (c) 2014, Google Inc.
 *
 * Permission to use, copy, modify, and/or distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
 * SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
 * OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
 * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. */

#include <openssl/hkdf.h>

#include <assert.h>

#include <openssl/err.h>
#include <openssl/hmac.h>


int HKDF(uint8_t *out_key, size_t out_len,
         const EVP_MD *digest,
         const uint8_t *secret, size_t secret_len,
         const uint8_t *salt, size_t salt_len,
         const uint8_t *info, size_t info_len) {
  /* https://tools.ietf.org/html/rfc5869#section-2.2 */
  const size_t digest_len = EVP_MD_size(digest);
  uint8_t prk[EVP_MAX_MD_SIZE], previous[EVP_MAX_MD_SIZE];
  size_t n, done = 0;
  unsigned i, prk_len;
  int ret = 0;
  HMAC_CTX hmac;

  /* If salt is not given, HashLength zeros are used. However, HMAC does that
   * internally already so we can ignore it.*/

  /* Expand key material to desired length. */
  n = (out_len + digest_len - 1) / digest_len;
  if (out_len + digest_len < out_len || n > 255) {
    OPENSSL_PUT_ERROR(HKDF, HKDF, HKDF_R_OUTPUT_TOO_LARGE);
    return 0;
  }

  HMAC_CTX_init(&hmac);

  /* Extract input keying material into pseudorandom key |prk|. */
  if (HMAC(digest, salt, salt_len, secret, secret_len, prk, &prk_len) == NULL) {
    goto out;
  }
  assert(prk_len == digest_len);

  if (!HMAC_Init_ex(&hmac, prk, prk_len, digest, NULL)) {
    goto out;
  }

  for (i = 0; i < n; i++) {
    uint8_t ctr = i + 1;
    size_t todo;

    if (i != 0 && (!HMAC_Init_ex(&hmac, NULL, 0, NULL, NULL) ||
                   !HMAC_Update(&hmac, previous, digest_len))) {
      goto out;
    }
    if (!HMAC_Update(&hmac, info, info_len) ||
        !HMAC_Update(&hmac, &ctr, 1) ||
        !HMAC_Final(&hmac, previous, NULL)) {
      goto out;
    }

    todo = digest_len;
    if (done + todo > out_len) {
      todo = out_len - done;
    }
    memcpy(out_key + done, previous, todo);
    done += todo;
  }

  ret = 1;

out:
  HMAC_CTX_cleanup(&hmac);
  if (ret != 1) {
    OPENSSL_PUT_ERROR(HKDF, HKDF, ERR_R_HMAC_LIB);
  }
  return ret;
}