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/* Copyright 2016 The Chromium OS Authors. All rights reserved.
* Use of this source code is governed by a BSD-style license that can be
* found in the LICENSE file.
*/
/* An implementation of HKDF as per RFC 5869. */
#include "dcrypto.h"
#include "internal.h"
static int hkdf_extract(uint8_t *PRK, const uint8_t *salt, size_t salt_len,
const uint8_t *IKM, size_t IKM_len)
{
struct hmac_sha256_ctx ctx;
if (PRK == NULL)
return 0;
if (salt == NULL && salt_len > 0)
return 0;
if (IKM == NULL && IKM_len > 0)
return 0;
HMAC_SHA256_hw_init(&ctx, salt, salt_len);
HMAC_SHA256_update(&ctx, IKM, IKM_len);
memcpy(PRK, HMAC_SHA256_hw_final(&ctx), SHA256_DIGEST_SIZE);
return 1;
}
static int hkdf_expand(uint8_t *OKM, size_t OKM_len, const uint8_t *PRK,
const uint8_t *info, size_t info_len)
{
uint8_t count = 1;
const uint8_t *T = OKM;
size_t T_len = 0;
uint32_t num_blocks = (OKM_len / SHA256_DIGEST_SIZE) +
(OKM_len % SHA256_DIGEST_SIZE ? 1 : 0);
if (OKM == NULL || OKM_len == 0)
return 0;
if (PRK == NULL)
return 0;
if (info == NULL && info_len > 0)
return 0;
if (num_blocks > 255)
return 0;
while (OKM_len > 0) {
struct hmac_sha256_ctx ctx;
const size_t block_size = OKM_len < SHA256_DIGEST_SIZE ?
OKM_len : SHA256_DIGEST_SIZE;
HMAC_SHA256_hw_init(&ctx, PRK, SHA256_DIGEST_SIZE);
HMAC_SHA256_update(&ctx, T, T_len);
HMAC_SHA256_update(&ctx, info, info_len);
HMAC_SHA256_update(&ctx, &count, sizeof(count));
memcpy(OKM, HMAC_SHA256_hw_final(&ctx), block_size);
T += T_len;
T_len = SHA256_DIGEST_SIZE;
count += 1;
OKM += block_size;
OKM_len -= block_size;
}
return 1;
}
int DCRYPTO_hkdf(uint8_t *OKM, size_t OKM_len,
const uint8_t *salt, size_t salt_len,
const uint8_t *IKM, size_t IKM_len,
const uint8_t *info, size_t info_len)
{
int result;
uint8_t PRK[SHA256_DIGEST_SIZE];
if (!hkdf_extract(PRK, salt, salt_len, IKM, IKM_len))
return 0;
result = hkdf_expand(OKM, OKM_len, PRK, info, info_len);
always_memset(PRK, 0, sizeof(PRK));
return result;
}
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