1/* Copyright (c) 2014, Google Inc.
2 *
3 * Permission to use, copy, modify, and/or distribute this software for any
4 * purpose with or without fee is hereby granted, provided that the above
5 * copyright notice and this permission notice appear in all copies.
6 *
7 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
8 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
9 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
10 * SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
11 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
12 * OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
13 * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. */
14
15#include <openssl/hkdf.h>
16
17#include <assert.h>
18#include <string.h>
19
20#include <openssl/err.h>
21#include <openssl/hmac.h>
22
23#include "../internal.h"
24
25
26int HKDF(uint8_t *out_key, size_t out_len, const EVP_MD *digest,
27         const uint8_t *secret, size_t secret_len, const uint8_t *salt,
28         size_t salt_len, const uint8_t *info, size_t info_len) {
29  // https://tools.ietf.org/html/rfc5869#section-2
30  uint8_t prk[EVP_MAX_MD_SIZE];
31  size_t prk_len;
32
33  if (!HKDF_extract(prk, &prk_len, digest, secret, secret_len, salt,
34                    salt_len) ||
35      !HKDF_expand(out_key, out_len, digest, prk, prk_len, info, info_len)) {
36    return 0;
37  }
38
39  return 1;
40}
41
42int HKDF_extract(uint8_t *out_key, size_t *out_len, const EVP_MD *digest,
43                 const uint8_t *secret, size_t secret_len, const uint8_t *salt,
44                 size_t salt_len) {
45  // https://tools.ietf.org/html/rfc5869#section-2.2
46
47  // If salt is not given, HashLength zeros are used. However, HMAC does that
48  // internally already so we can ignore it.
49  unsigned len;
50  if (HMAC(digest, salt, salt_len, secret, secret_len, out_key, &len) == NULL) {
51    OPENSSL_PUT_ERROR(HKDF, ERR_R_HMAC_LIB);
52    return 0;
53  }
54  *out_len = len;
55  assert(*out_len == EVP_MD_size(digest));
56  return 1;
57}
58
59int HKDF_expand(uint8_t *out_key, size_t out_len, const EVP_MD *digest,
60                const uint8_t *prk, size_t prk_len, const uint8_t *info,
61                size_t info_len) {
62  // https://tools.ietf.org/html/rfc5869#section-2.3
63  const size_t digest_len = EVP_MD_size(digest);
64  uint8_t previous[EVP_MAX_MD_SIZE];
65  size_t n, done = 0;
66  unsigned i;
67  int ret = 0;
68  HMAC_CTX hmac;
69
70  // Expand key material to desired length.
71  n = (out_len + digest_len - 1) / digest_len;
72  if (out_len + digest_len < out_len || n > 255) {
73    OPENSSL_PUT_ERROR(HKDF, HKDF_R_OUTPUT_TOO_LARGE);
74    return 0;
75  }
76
77  HMAC_CTX_init(&hmac);
78  if (!HMAC_Init_ex(&hmac, prk, prk_len, digest, NULL)) {
79    goto out;
80  }
81
82  for (i = 0; i < n; i++) {
83    uint8_t ctr = i + 1;
84    size_t todo;
85
86    if (i != 0 && (!HMAC_Init_ex(&hmac, NULL, 0, NULL, NULL) ||
87                   !HMAC_Update(&hmac, previous, digest_len))) {
88      goto out;
89    }
90    if (!HMAC_Update(&hmac, info, info_len) ||
91        !HMAC_Update(&hmac, &ctr, 1) ||
92        !HMAC_Final(&hmac, previous, NULL)) {
93      goto out;
94    }
95
96    todo = digest_len;
97    if (done + todo > out_len) {
98      todo = out_len - done;
99    }
100    OPENSSL_memcpy(out_key + done, previous, todo);
101    done += todo;
102  }
103
104  ret = 1;
105
106out:
107  HMAC_CTX_cleanup(&hmac);
108  if (ret != 1) {
109    OPENSSL_PUT_ERROR(HKDF, ERR_R_HMAC_LIB);
110  }
111  return ret;
112}
113