Standard from nist
“Recommendation for Block Cipher Modes of Operation: The CMAC Mode for Authentication” --nist
Reference link: https://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-38b.pdf
Based on the message authentication code of the block cipher cbc mode, the last block (may need to be filled with 10...0) is XORed with the subkey and then participates in the CBC mode encryption.
Subkey generation:
Message authentication code generation:
work flow chart:
openssl implementation:
/*
* Copyright 2010-2016 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/cmac.h>
struct CMAC_CTX_st {
/* Cipher context to use */
EVP_CIPHER_CTX *cctx;
/* Keys k1 and k2 */
unsigned char k1[EVP_MAX_BLOCK_LENGTH];
unsigned char k2[EVP_MAX_BLOCK_LENGTH];
/* Temporary block */
unsigned char tbl[EVP_MAX_BLOCK_LENGTH];
/* Last (possibly partial) block */
unsigned char last_block[EVP_MAX_BLOCK_LENGTH];
/* Number of bytes in last block: -1 means context not initialised */
int nlast_block;
};
/* Make temporary keys K1 and K2 */
static void make_kn(unsigned char *k1, const unsigned char *l, int bl)
{
int i;
unsigned char c = l[0], carry = c >> 7, cnext;
/* Shift block to left, including carry */
for (i = 0; i < bl - 1; i++, c = cnext)
k1[i] = (c << 1) | ((cnext = l[i + 1]) >> 7);
/* If MSB set fixup with R */
k1[i] = (c << 1) ^ ((0 - carry) & (bl == 16 ? 0x87 : 0x1b));
}
CMAC_CTX *CMAC_CTX_new(void)
{
CMAC_CTX *ctx;
ctx = OPENSSL_malloc(sizeof(*ctx));
if (ctx == NULL)
return NULL;
ctx->cctx = EVP_CIPHER_CTX_new();
if (ctx->cctx == NULL) {
OPENSSL_free(ctx);
return NULL;
}
ctx->nlast_block = -1;
return ctx;
}
void CMAC_CTX_cleanup(CMAC_CTX *ctx)
{
EVP_CIPHER_CTX_free(ctx->cctx);
OPENSSL_cleanse(ctx->tbl, EVP_MAX_BLOCK_LENGTH);
OPENSSL_cleanse(ctx->k1, EVP_MAX_BLOCK_LENGTH);
OPENSSL_cleanse(ctx->k2, EVP_MAX_BLOCK_LENGTH);
OPENSSL_cleanse(ctx->last_block, EVP_MAX_BLOCK_LENGTH);
ctx->nlast_block = -1;
}
EVP_CIPHER_CTX *CMAC_CTX_get0_cipher_ctx(CMAC_CTX *ctx)
{
return ctx->cctx;
}
void CMAC_CTX_free(CMAC_CTX *ctx)
{
if (!ctx)
return;
CMAC_CTX_cleanup(ctx);
OPENSSL_free(ctx);
}
int CMAC_CTX_copy(CMAC_CTX *out, const CMAC_CTX *in)
{
int bl;
if (in->nlast_block == -1)
return 0;
if (!EVP_CIPHER_CTX_copy(out->cctx, in->cctx))
return 0;
bl = EVP_CIPHER_CTX_block_size(in->cctx);
memcpy(out->k1, in->k1, bl);
memcpy(out->k2, in->k2, bl);
memcpy(out->tbl, in->tbl, bl);
memcpy(out->last_block, in->last_block, bl);
out->nlast_block = in->nlast_block;
return 1;
}
int CMAC_Init(CMAC_CTX *ctx, const void *key, size_t keylen,
const EVP_CIPHER *cipher, ENGINE *impl)
{
static const unsigned char zero_iv[EVP_MAX_BLOCK_LENGTH] = { 0 };
/* All zeros means restart */
if (!key && !cipher && !impl && keylen == 0) {
/* Not initialised */
if (ctx->nlast_block == -1)
return 0;
if (!EVP_EncryptInit_ex(ctx->cctx, NULL, NULL, NULL, zero_iv))
return 0;
memset(ctx->tbl, 0, EVP_CIPHER_CTX_block_size(ctx->cctx));
ctx->nlast_block = 0;
return 1;
}
/* Initialise context */
if (cipher && !EVP_EncryptInit_ex(ctx->cctx, cipher, impl, NULL, NULL))
return 0;
/* Non-NULL key means initialisation complete */
if (key) {
int bl;
if (!EVP_CIPHER_CTX_cipher(ctx->cctx))
return 0;
if (!EVP_CIPHER_CTX_set_key_length(ctx->cctx, keylen))
return 0;
if (!EVP_EncryptInit_ex(ctx->cctx, NULL, NULL, key, zero_iv))
return 0;
bl = EVP_CIPHER_CTX_block_size(ctx->cctx);
if (!EVP_Cipher(ctx->cctx, ctx->tbl, zero_iv, bl))
return 0;
make_kn(ctx->k1, ctx->tbl, bl);
make_kn(ctx->k2, ctx->k1, bl);
OPENSSL_cleanse(ctx->tbl, bl);
/* Reset context again ready for first data block */
if (!EVP_EncryptInit_ex(ctx->cctx, NULL, NULL, NULL, zero_iv))
return 0;
/* Zero tbl so resume works */
memset(ctx->tbl, 0, bl);
ctx->nlast_block = 0;
}
return 1;
}
int CMAC_Update(CMAC_CTX *ctx, const void *in, size_t dlen)
{
const unsigned char *data = in;
size_t bl;
if (ctx->nlast_block == -1)
return 0;
if (dlen == 0)
return 1;
bl = EVP_CIPHER_CTX_block_size(ctx->cctx);
/* Copy into partial block if we need to */
if (ctx->nlast_block > 0) {
size_t nleft;
nleft = bl - ctx->nlast_block;
if (dlen < nleft)
nleft = dlen;
memcpy(ctx->last_block + ctx->nlast_block, data, nleft);
dlen -= nleft;
ctx->nlast_block += nleft;
/* If no more to process return */
if (dlen == 0)
return 1;
data += nleft;
/* Else not final block so encrypt it */
if (!EVP_Cipher(ctx->cctx, ctx->tbl, ctx->last_block, bl))
return 0;
}
/* Encrypt all but one of the complete blocks left */
while (dlen > bl) {
if (!EVP_Cipher(ctx->cctx, ctx->tbl, data, bl))
return 0;
dlen -= bl;
data += bl;
}
/* Copy any data left to last block buffer */
memcpy(ctx->last_block, data, dlen);
ctx->nlast_block = dlen;
return 1;
}
int CMAC_Final(CMAC_CTX *ctx, unsigned char *out, size_t *poutlen)
{
int i, bl, lb;
if (ctx->nlast_block == -1)
return 0;
bl = EVP_CIPHER_CTX_block_size(ctx->cctx);
*poutlen = (size_t)bl;
if (!out)
return 1;
lb = ctx->nlast_block;
/* Is last block complete? */
if (lb == bl) {
for (i = 0; i < bl; i++)
out[i] = ctx->last_block[i] ^ ctx->k1[i];
} else {
ctx->last_block[lb] = 0x80;
if (bl - lb > 1)
memset(ctx->last_block + lb + 1, 0, bl - lb - 1);
for (i = 0; i < bl; i++)
out[i] = ctx->last_block[i] ^ ctx->k2[i];
}
if (!EVP_Cipher(ctx->cctx, out, out, bl)) {
OPENSSL_cleanse(out, bl);
return 0;
}
return 1;
}
int CMAC_resume(CMAC_CTX *ctx)
{
if (ctx->nlast_block == -1)
return 0;
/*
* The buffer "tbl" contains the last fully encrypted block which is the
* last IV (or all zeroes if no last encrypted block). The last block has
* not been modified since CMAC_final(). So reinitialising using the last
* decrypted block will allow CMAC to continue after calling
* CMAC_Final().
*/
return EVP_EncryptInit_ex(ctx->cctx, NULL, NULL, NULL, ctx->tbl);
}