“The Keyed-Hash Message Authentication Code” --nist
Reference link: https://csrc.nist.gov/csrc/media/publications/fips/198/archive/2002-03-06/documents/fips-198a.pdf
HMAC specification:
Flow chart:
flow chart:
openssl implementation:
/*
* Copyright 1995-2016 The OpenSSL Project Authors. All Rights Reserved.
*
* Licensed under the OpenSSL license (the "License"). You may not use
* this file except in compliance with the License. You can obtain a copy
* in the file LICENSE in the source distribution or at
* https://www.openssl.org/source/license.html
*/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "internal/cryptlib.h"
#include <openssl/hmac.h>
#include <openssl/opensslconf.h>
#include "hmac_lcl.h"
int HMAC_Init_ex(HMAC_CTX *ctx, const void *key, int len,
const EVP_MD *md, ENGINE *impl)
{
int i, j, reset = 0;
unsigned char pad[HMAC_MAX_MD_CBLOCK];
/* If we are changing MD then we must have a key */
if (md != NULL && md != ctx->md && (key == NULL || len < 0))
return 0;
if (md != NULL) {
reset = 1;
ctx->md = md;
} else if (ctx->md) {
md = ctx->md;
} else {
return 0;
}
if (key != NULL) {
reset = 1;
j = EVP_MD_block_size(md);
OPENSSL_assert(j <= (int)sizeof(ctx->key));
if (j < len) {
if (!EVP_DigestInit_ex(ctx->md_ctx, md, impl))
goto err;
if (!EVP_DigestUpdate(ctx->md_ctx, key, len))
goto err;
if (!EVP_DigestFinal_ex(ctx->md_ctx, ctx->key,
&ctx->key_length))
goto err;
} else {
if (len < 0 || len > (int)sizeof(ctx->key))
return 0;
memcpy(ctx->key, key, len);
ctx->key_length = len;
}
if (ctx->key_length != HMAC_MAX_MD_CBLOCK)
memset(&ctx->key[ctx->key_length], 0,
HMAC_MAX_MD_CBLOCK - ctx->key_length);
}
if (reset) {
for (i = 0; i < HMAC_MAX_MD_CBLOCK; i++)
pad[i] = 0x36 ^ ctx->key[i];
if (!EVP_DigestInit_ex(ctx->i_ctx, md, impl))
goto err;
if (!EVP_DigestUpdate(ctx->i_ctx, pad, EVP_MD_block_size(md)))
goto err;
for (i = 0; i < HMAC_MAX_MD_CBLOCK; i++)
pad[i] = 0x5c ^ ctx->key[i];
if (!EVP_DigestInit_ex(ctx->o_ctx, md, impl))
goto err;
if (!EVP_DigestUpdate(ctx->o_ctx, pad, EVP_MD_block_size(md)))
goto err;
}
if (!EVP_MD_CTX_copy_ex(ctx->md_ctx, ctx->i_ctx))
goto err;
return 1;
err:
return 0;
}
#if OPENSSL_API_COMPAT < 0x10100000L
int HMAC_Init(HMAC_CTX *ctx, const void *key, int len, const EVP_MD *md)
{
if (key && md)
HMAC_CTX_reset(ctx);
return HMAC_Init_ex(ctx, key, len, md, NULL);
}
#endif
int HMAC_Update(HMAC_CTX *ctx, const unsigned char *data, size_t len)
{
if (!ctx->md)
return 0;
return EVP_DigestUpdate(ctx->md_ctx, data, len);
}
int HMAC_Final(HMAC_CTX *ctx, unsigned char *md, unsigned int *len)
{
unsigned int i;
unsigned char buf[EVP_MAX_MD_SIZE];
if (!ctx->md)
goto err;
if (!EVP_DigestFinal_ex(ctx->md_ctx, buf, &i))
goto err;
if (!EVP_MD_CTX_copy_ex(ctx->md_ctx, ctx->o_ctx))
goto err;
if (!EVP_DigestUpdate(ctx->md_ctx, buf, i))
goto err;
if (!EVP_DigestFinal_ex(ctx->md_ctx, md, len))
goto err;
return 1;
err:
return 0;
}
size_t HMAC_size(const HMAC_CTX *ctx)
{
return EVP_MD_size((ctx)->md);
}
HMAC_CTX *HMAC_CTX_new(void)
{
HMAC_CTX *ctx = OPENSSL_zalloc(sizeof(HMAC_CTX));
if (ctx != NULL) {
if (!HMAC_CTX_reset(ctx)) {
HMAC_CTX_free(ctx);
return NULL;
}
}
return ctx;
}
static void hmac_ctx_cleanup(HMAC_CTX *ctx)
{
EVP_MD_CTX_reset(ctx->i_ctx);
EVP_MD_CTX_reset(ctx->o_ctx);
EVP_MD_CTX_reset(ctx->md_ctx);
ctx->md = NULL;
ctx->key_length = 0;
OPENSSL_cleanse(ctx->key, sizeof(ctx->key));
}
void HMAC_CTX_free(HMAC_CTX *ctx)
{
if (ctx != NULL) {
hmac_ctx_cleanup(ctx);
EVP_MD_CTX_free(ctx->i_ctx);
EVP_MD_CTX_free(ctx->o_ctx);
EVP_MD_CTX_free(ctx->md_ctx);
OPENSSL_free(ctx);
}
}
int HMAC_CTX_reset(HMAC_CTX *ctx)
{
hmac_ctx_cleanup(ctx);
if (ctx->i_ctx == NULL)
ctx->i_ctx = EVP_MD_CTX_new();
if (ctx->i_ctx == NULL)
goto err;
if (ctx->o_ctx == NULL)
ctx->o_ctx = EVP_MD_CTX_new();
if (ctx->o_ctx == NULL)
goto err;
if (ctx->md_ctx == NULL)
ctx->md_ctx = EVP_MD_CTX_new();
if (ctx->md_ctx == NULL)
goto err;
ctx->md = NULL;
return 1;
err:
hmac_ctx_cleanup(ctx);
return 0;
}
int HMAC_CTX_copy(HMAC_CTX *dctx, HMAC_CTX *sctx)
{
if (!HMAC_CTX_reset(dctx))
goto err;
if (!EVP_MD_CTX_copy_ex(dctx->i_ctx, sctx->i_ctx))
goto err;
if (!EVP_MD_CTX_copy_ex(dctx->o_ctx, sctx->o_ctx))
goto err;
if (!EVP_MD_CTX_copy_ex(dctx->md_ctx, sctx->md_ctx))
goto err;
memcpy(dctx->key, sctx->key, HMAC_MAX_MD_CBLOCK);
dctx->key_length = sctx->key_length;
dctx->md = sctx->md;
return 1;
err:
hmac_ctx_cleanup(dctx);
return 0;
}
unsigned char *HMAC(const EVP_MD *evp_md, const void *key, int key_len,
const unsigned char *d, size_t n, unsigned char *md,
unsigned int *md_len)
{
HMAC_CTX *c = NULL;
static unsigned char m[EVP_MAX_MD_SIZE];
static const unsigned char dummy_key[1] = {'\0'};
if (md == NULL)
md = m;
if ((c = HMAC_CTX_new()) == NULL)
goto err;
/* For HMAC_Init_ex, NULL key signals reuse. */
if (key == NULL && key_len == 0) {
key = dummy_key;
}
if (!HMAC_Init_ex(c, key, key_len, evp_md, NULL))
goto err;
if (!HMAC_Update(c, d, n))
goto err;
if (!HMAC_Final(c, md, md_len))
goto err;
HMAC_CTX_free(c);
return md;
err:
HMAC_CTX_free(c);
return NULL;
}
void HMAC_CTX_set_flags(HMAC_CTX *ctx, unsigned long flags)
{
EVP_MD_CTX_set_flags(ctx->i_ctx, flags);
EVP_MD_CTX_set_flags(ctx->o_ctx, flags);
EVP_MD_CTX_set_flags(ctx->md_ctx, flags);
}
const EVP_MD *HMAC_CTX_get_md(const HMAC_CTX *ctx)
{
return ctx->md;
}