NAME¶

EVP_MAC, EVP_MAC_fetch, EVP_MAC_up_ref, EVP_MAC_free, EVP_MAC_is_a, EVP_MAC_get0_name, EVP_MAC_names_do_all, EVP_MAC_get0_description, EVP_MAC_get0_provider, EVP_MAC_get_params, EVP_MAC_gettable_params, EVP_MAC_CTX, EVP_MAC_CTX_new, EVP_MAC_CTX_free, EVP_MAC_CTX_dup, EVP_MAC_CTX_get0_mac, EVP_MAC_CTX_get_params, EVP_MAC_CTX_set_params, EVP_MAC_CTX_get_mac_size, EVP_MAC_CTX_get_block_size, EVP_Q_mac, EVP_MAC_init, EVP_MAC_update, EVP_MAC_final, EVP_MAC_finalXOF, EVP_MAC_gettable_ctx_params, EVP_MAC_settable_ctx_params, EVP_MAC_CTX_gettable_params, EVP_MAC_CTX_settable_params, EVP_MAC_do_all_provided - EVP MAC routines

SYNOPSIS¶

 #include <openssl/evp.h>
 typedef struct evp_mac_st EVP_MAC;
 typedef struct evp_mac_ctx_st EVP_MAC_CTX;
 EVP_MAC *EVP_MAC_fetch(OSSL_LIB_CTX *libctx, const char *algorithm,
                        const char *properties);
 int EVP_MAC_up_ref(EVP_MAC *mac);
 void EVP_MAC_free(EVP_MAC *mac);
 int EVP_MAC_is_a(const EVP_MAC *mac, const char *name);
 const char *EVP_MAC_get0_name(const EVP_MAC *mac);
 int EVP_MAC_names_do_all(const EVP_MAC *mac,
                          void (*fn)(const char *name, void *data),
                          void *data);
 const char *EVP_MAC_get0_description(const EVP_MAC *mac);
 const OSSL_PROVIDER *EVP_MAC_get0_provider(const EVP_MAC *mac);
 int EVP_MAC_get_params(EVP_MAC *mac, OSSL_PARAM params[]);
 EVP_MAC_CTX *EVP_MAC_CTX_new(EVP_MAC *mac);
 void EVP_MAC_CTX_free(EVP_MAC_CTX *ctx);
 EVP_MAC_CTX *EVP_MAC_CTX_dup(const EVP_MAC_CTX *src);
 EVP_MAC *EVP_MAC_CTX_get0_mac(EVP_MAC_CTX *ctx);
 int EVP_MAC_CTX_get_params(EVP_MAC_CTX *ctx, OSSL_PARAM params[]);
 int EVP_MAC_CTX_set_params(EVP_MAC_CTX *ctx, const OSSL_PARAM params[]);
 size_t EVP_MAC_CTX_get_mac_size(EVP_MAC_CTX *ctx);
 size_t EVP_MAC_CTX_get_block_size(EVP_MAC_CTX *ctx);
 unsigned char *EVP_Q_mac(OSSL_LIB_CTX *libctx, const char *name, const char *propq,
                          const char *subalg, const OSSL_PARAM *params,
                          const void *key, size_t keylen,
                          const unsigned char *data, size_t datalen,
                          unsigned char *out, size_t outsize, size_t *outlen);
 int EVP_MAC_init(EVP_MAC_CTX *ctx, const unsigned char *key, size_t keylen,
                  const OSSL_PARAM params[]);
 int EVP_MAC_update(EVP_MAC_CTX *ctx, const unsigned char *data, size_t datalen);
 int EVP_MAC_final(EVP_MAC_CTX *ctx,
                   unsigned char *out, size_t *outl, size_t outsize);
 int EVP_MAC_finalXOF(EVP_MAC_CTX *ctx, unsigned char *out, size_t outsize);
 const OSSL_PARAM *EVP_MAC_gettable_params(const EVP_MAC *mac);
 const OSSL_PARAM *EVP_MAC_gettable_ctx_params(const EVP_MAC *mac);
 const OSSL_PARAM *EVP_MAC_settable_ctx_params(const EVP_MAC *mac);
 const OSSL_PARAM *EVP_MAC_CTX_gettable_params(EVP_MAC_CTX *ctx);
 const OSSL_PARAM *EVP_MAC_CTX_settable_params(EVP_MAC_CTX *ctx);
 void EVP_MAC_do_all_provided(OSSL_LIB_CTX *libctx,
                              void (*fn)(EVP_MAC *mac, void *arg),
                              void *arg);

DESCRIPTION¶

These types and functions help the application to calculate MACs of different types and with different underlying algorithms if there are any.

MACs are a bit complex insofar that some of them use other algorithms for actual computation. HMAC uses a digest, and CMAC uses a cipher. Therefore, there are sometimes two contexts to keep track of, one for the MAC algorithm itself and one for the underlying computation algorithm if there is one.

To make things less ambiguous, this manual talks about a "context" or "MAC context", which is to denote the MAC level context, and about a "underlying context", or "computation context", which is to denote the context for the underlying computation algorithm if there is one.

PARAMETERS¶

Parameters are identified by name as strings, and have an expected data type and maximum size. OpenSSL has a set of macros for parameter names it expects to see in its own MAC implementations. Here, we show all three, the OpenSSL macro for the parameter name, the name in string form, and a type description.

The standard parameter names are:

"key" (OSSL_MAC_PARAM_KEY) <octet string>

Its value is the MAC key as an array of bytes.

For MACs that use an underlying computation algorithm, the algorithm must be set first, see parameter names "algorithm" below.

"iv" (OSSL_MAC_PARAM_IV) <octet string>

Some MAC implementations (GMAC) require an IV, this parameter sets the IV.

"custom" (OSSL_MAC_PARAM_CUSTOM) <octet string>

Some MAC implementations (KMAC, BLAKE2) accept a Customization String, this parameter sets the Customization String. The default value is the empty string.

"salt" (OSSL_MAC_PARAM_SALT) <octet string>

This option is used by BLAKE2 MAC.

"xof" (OSSL_MAC_PARAM_XOF) <integer>

It's a simple flag, the value 0 or 1 are expected.

This option is used by KMAC.

"digest-noinit" (OSSL_MAC_PARAM_DIGEST_NOINIT) <integer>

A simple flag to set the MAC digest to not initialise the implementation specific data. The value 0 or 1 is expected.

This option is used by HMAC.

"digest-oneshot" (OSSL_MAC_PARAM_DIGEST_ONESHOT) <integer>

A simple flag to set the MAC digest to be a oneshot operation. The value 0 or 1 is expected.

This option is used by HMAC.

"properties" (OSSL_MAC_PARAM_PROPERTIES) <UTF8 string>

"digest" (OSSL_MAC_PARAM_DIGEST) <UTF8 string>

"cipher" (OSSL_MAC_PARAM_CIPHER) <UTF8 string>

For MAC implementations that use an underlying computation cipher or digest, these parameters set what the algorithm should be.

The value is always the name of the intended algorithm, or the properties.

Note that not all algorithms may support all digests. HMAC does not support variable output length digests such as SHAKE128 or SHAKE256.

"size" (OSSL_MAC_PARAM_SIZE) <unsigned integer>

For MAC implementations that support it, set the output size that EVP_MAC_final() should produce. The allowed sizes vary between MAC implementations, but must never exceed what can be given with a size_t.

"tls-data-size" (OSSL_MAC_PARAM_TLS_DATA_SIZE) <unsigned integer>

This parameter is only supported by HMAC. If set then special handling is activated for calculating the MAC of a received mac-then-encrypt TLS record where variable length record padding has been used (as in the case of CBC mode ciphersuites). The value represents the total length of the record that is having the MAC calculated including the received MAC and the record padding.

When used EVP_MAC_update must be called precisely twice. The first time with the 13 bytes of TLS "header" data, and the second time with the entire record including the MAC itself and any padding. The entire record length must equal the value passed in the "tls-data-size" parameter. The length passed in the datalen parameter to EVP_MAC_update() should be equal to the length of the record after the MAC and any padding has been removed.

All these parameters should be used before the calls to any of EVP_MAC_init(), EVP_MAC_update() and EVP_MAC_final() for a full computation. Anything else may give undefined results.

NOTES¶

The MAC life-cycle is described in life_cycle-mac(7). In the future, the transitions described there will be enforced. When this is done, it will not be considered a breaking change to the API.

The usage of the parameter names "custom", "iv" and "salt" correspond to the names used in the standard where the algorithm was defined.

Some MAC algorithms store internal state that cannot be extracted during re-initalization. For example GMAC cannot extract an IV from the underlying CIPHER context, and so calling EVP_MAC_init() on an EVP_MAC object after EVP_MAC_final() has been called cannot reset its cipher state to what it was when the IV was initially generated. For such instances, an OSSL_MAC_PARAM_IV parameter must be passed with each call to EVP_MAC_init().

RETURN VALUES¶

EVP_MAC_fetch() returns a pointer to a newly fetched EVP_MAC, or NULL if allocation failed.

EVP_MAC_up_ref() returns 1 on success, 0 on error.

EVP_MAC_names_do_all() returns 1 if the callback was called for all names. A return value of 0 means that the callback was not called for any names.

EVP_MAC_free() returns nothing at all.

EVP_MAC_is_a() returns 1 if the given method can be identified with the given name, otherwise 0.

EVP_MAC_get0_name() returns a name of the MAC, or NULL on error.

EVP_MAC_get0_provider() returns a pointer to the provider for the MAC, or NULL on error.

EVP_MAC_CTX_new() and EVP_MAC_CTX_dup() return a pointer to a newly created EVP_MAC_CTX, or NULL if allocation failed.

EVP_MAC_CTX_free() returns nothing at all.

EVP_MAC_CTX_get_params() and EVP_MAC_CTX_set_params() return 1 on success, 0 on error.

EVP_Q_mac() returns a pointer to the computed MAC value, or NULL on error.

EVP_MAC_init(), EVP_MAC_update(), EVP_MAC_final(), and EVP_MAC_finalXOF() return 1 on success, 0 on error.

EVP_MAC_CTX_get_mac_size() returns the expected output size, or 0 if it isn't set. If it isn't set, a call to EVP_MAC_init() will set it.

EVP_MAC_CTX_get_block_size() returns the block size, or 0 if it isn't set. If it isn't set, a call to EVP_MAC_init() will set it.

EVP_MAC_do_all_provided() returns nothing at all.

EXAMPLES¶

  #include <stdlib.h>
  #include <stdio.h>
  #include <string.h>
  #include <stdarg.h>
  #include <unistd.h>
  #include <openssl/evp.h>
  #include <openssl/err.h>
  #include <openssl/params.h>
  int main() {
      EVP_MAC *mac = EVP_MAC_fetch(NULL, getenv("MY_MAC"), NULL);
      const char *cipher = getenv("MY_MAC_CIPHER");
      const char *digest = getenv("MY_MAC_DIGEST");
      const char *key = getenv("MY_KEY");
      EVP_MAC_CTX *ctx = NULL;
      unsigned char buf[4096];
      size_t read_l;
      size_t final_l;
      size_t i;
      OSSL_PARAM params[3];
      size_t params_n = 0;
      if (cipher != NULL)
          params[params_n++] =
              OSSL_PARAM_construct_utf8_string("cipher", (char*)cipher, 0);
      if (digest != NULL)
          params[params_n++] =
              OSSL_PARAM_construct_utf8_string("digest", (char*)digest, 0);
      params[params_n] = OSSL_PARAM_construct_end();
      if (mac == NULL
          || key == NULL
          || (ctx = EVP_MAC_CTX_new(mac)) == NULL
          || !EVP_MAC_init(ctx, (const unsigned char *)key, strlen(key),
                           params))
          goto err;
      while ( (read_l = read(STDIN_FILENO, buf, sizeof(buf))) > 0) {
          if (!EVP_MAC_update(ctx, buf, read_l))
              goto err;
      }
      if (!EVP_MAC_final(ctx, buf, &final_l, sizeof(buf)))
          goto err;
      printf("Result: ");
      for (i = 0; i < final_l; i++)
          printf("%02X", buf[i]);
      printf("\n");
      EVP_MAC_CTX_free(ctx);
      EVP_MAC_free(mac);
      exit(0);
   err:
      EVP_MAC_CTX_free(ctx);
      EVP_MAC_free(mac);
      fprintf(stderr, "Something went wrong\n");
      ERR_print_errors_fp(stderr);
      exit (1);
  }

A run of this program, called with correct environment variables, can look like this:

  $ MY_MAC=cmac MY_KEY=secret0123456789 MY_MAC_CIPHER=aes-128-cbc \
    LD_LIBRARY_PATH=. ./foo < foo.c
  Result: C5C06683CD9DDEF904D754505C560A4E

(in this example, that program was stored in foo.c and compiled to ./foo)

SEE ALSO¶

property(7) OSSL_PARAM(3), EVP_MAC-BLAKE2(7), EVP_MAC-CMAC(7), EVP_MAC-GMAC(7), EVP_MAC-HMAC(7), EVP_MAC-KMAC(7), EVP_MAC-Siphash(7), EVP_MAC-Poly1305(7), provider-mac(7), life_cycle-mac(7)

HISTORY¶

These functions were added in OpenSSL 3.0.

COPYRIGHT¶

Copyright 2018-2024 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (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>.