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=head1 NAME
EVP_MAC, EVP_MAC_fetch, EVP_MAC_up_ref, EVP_MAC_free,
EVP_MAC_is_a, EVP_MAC_number, EVP_MAC_names_do_all,
EVP_MAC_provider, EVP_MAC_get_params, EVP_MAC_gettable_params,
EVP_MAC_CTX_mac, EVP_MAC_CTX_get_params, EVP_MAC_CTX_set_params,
EVP_MAC_size, EVP_MAC_init, EVP_MAC_update, EVP_MAC_final,
EVP_MAC_gettable_ctx_params, EVP_MAC_settable_ctx_params,
EVP_MAC_do_all_provided - EVP MAC routines
#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(OPENSSL_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);
int EVP_MAC_number(const EVP_MAC *mac);
void EVP_MAC_names_do_all(const EVP_MAC *mac,
void (*fn)(const char *name, void *data),
void *data);
const OSSL_PROVIDER *EVP_MAC_provider(const EVP_MAC *mac);
int EVP_MAC_get_params(EVP_MAC *mac, OSSL_PARAM params[]);
void EVP_MAC_CTX_free(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_size(EVP_MAC_CTX *ctx);
int EVP_MAC_init(EVP_MAC_CTX *ctx);
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);
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);
void EVP_MAC_do_all_provided(OPENSSL_CTX *libctx,
void (*fn)(EVP_MAC *mac, void *arg),
void *arg);
These types and functions help the application to calculate MACs of
different types and with different underlying algorithms if there are
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.
=head2 Types
B<EVP_MAC> is a type that holds the implementation of a MAC.
B<EVP_MAC_CTX> is a context type that holds internal MAC information
as well as a reference to a computation context, for those MACs that
rely on an underlying computation algorithm.
=head2 Algorithm implementation fetching
EVP_MAC_fetch() fetches an implementation of a MAC I<algorithm>, given
a library context I<libctx> and a set of I<properties>.
See L<provider(7)/Fetching algorithms> for further information.
The returned value must eventually be freed with
EVP_MAC_up_ref() increments the reference count of an already fetched
EVP_MAC_free() frees a fetched algorithm.
NULL is a valid parameter, for which this function is a no-op.
=head2 Context manipulation functions
EVP_MAC_CTX_new() creates a new context for the MAC type I<mac>.
The created context can then be used with most other functions
described here.
EVP_MAC_CTX_free() frees the contents of the context, including an
underlying context if there is one, as well as the context itself.
NULL is a valid parameter, for which this function is a no-op.
EVP_MAC_CTX_dup() duplicates the I<src> context and returns a newly allocated
EVP_MAC_CTX_mac() returns the B<EVP_MAC> associated with the context
=head2 Computing functions
EVP_MAC_init() sets up the underlying context with information given
through diverse controls.
This should be called before calling EVP_MAC_update() and
EVP_MAC_update() adds I<datalen> bytes from I<data> to the MAC input.
EVP_MAC_final() does the final computation and stores the result in
the memory pointed at by I<out> of size I<outsize>, and sets the number
of bytes written in I<*outl> at.
If I<out> is NULL or I<outsize> is too small, then no computation
is made.
To figure out what the output length will be and allocate space for it
dynamically, simply call with I<out> being NULL and I<outl>
pointing at a valid location, then allocate space and make a second
call with I<out> pointing at the allocated space.
EVP_MAC_get_params() retrieves details about the implementation
The set of parameters given with I<params> determine exactly what
parameters should be retrieved.
Note that a parameter that is unknown in the underlying context is
simply ignored.
EVP_MAC_CTX_get_params() retrieves chosen parameters, given the
context I<ctx> and its underlying context.
The set of parameters given with I<params> determine exactly what
parameters should be retrieved.
Note that a parameter that is unknown in the underlying context is
simply ignored.
EVP_MAC_CTX_set_params() passes chosen parameters to the underlying
context, given a context I<ctx>.
The set of parameters given with I<params> determine exactly what
parameters are passed down.
Note that a parameter that is unknown in the underlying context is
simply ignored.
Also, what happens when a needed parameter isn't passed down is
defined by the implementation.
EVP_MAC_gettable_params(), EVP_MAC_gettable_ctx_params() and
EVP_MAC_settable_ctx_params() get a constant B<OSSL_PARAM> array that
describes the retrievable and settable parameters, i.e. parameters that
can be used with EVP_MAC_get_params(), EVP_MAC_CTX_get_params()
and EVP_MAC_CTX_set_params(), respectively.
See L<OSSL_PARAM(3)> for the use of B<OSSL_PARAM> as parameter descriptor.
=head2 Information functions
EVP_MAC_size() returns the MAC output size for the given context.
EVP_MAC_is_a() checks if the given I<mac> is an implementation of an
algorithm that's identifiable with I<name>.
EVP_MAC_provider() returns the provider that holds the implementation
of the given I<mac>.
EVP_MAC_do_all_provided() traverses all MAC implemented by all activated
providers in the given library context I<libctx>, and for each of the
implementations, calls the given function I<fn> with the implementation method
and the given I<arg> as argument.
EVP_MAC_number() returns the internal dynamic number assigned to
EVP_MAC_names_do_all() traverses all names for I<mac>, and calls
I<fn> with each name and I<data>.
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:
=over 4
=item "key" (B<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.
=item "iv" (B<OSSL_MAC_PARAM_IV>) <octet string>
Some MAC implementations require an IV, this parameter sets the IV.
=item "custom" (B<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.
=item "salt" (B<OSSL_MAC_PARAM_SALT>) <octet string>
This option is used by BLAKE2 MAC.
=item "xof" (B<OSSL_MAC_PARAM_XOF>) <integer>
It's a simple flag, the value 0 or 1 are expected.
This option is used by KMAC.
=item "flags" (B<OSSL_MAC_PARAM_FLAGS>) <integer>
These will set the MAC flags to the given numbers.
Some MACs do not support this option.
=item "properties" (B<OSSL_MAC_PARAM_PROPERTIES>) <UTF8 string>
=item "digest" (B<OSSL_MAC_PARAM_DIGEST>) <UTF8 string>
=item "cipher" (B<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.
=item "size" (B<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 B<size_t>.
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
Anything else may give undefined results.
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_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_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_MAC_init(), EVP_MAC_update(), and EVP_MAC_final() return 1 on success, 0
on error.
EVP_MAC_size() returns the expected output size, or 0 if it isn't
If it isn't set, a call to EVP_MAC_init() should get it set.
EVP_MAC_do_all_provided() returns nothing at all.
#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");
unsigned char buf[4096];
ssize_t read_l;
size_t final_l;
size_t i;
OSSL_PARAM params[4];
size_t params_n = 0;
if (cipher != NULL)
params[params_n++] =
OSSL_PARAM_construct_utf8_string("cipher", cipher, 0, NULL);
if (digest != NULL)
params[params_n++] =
OSSL_PARAM_construct_utf8_string("digest", digest, 0, NULL);
params[params_n++] =
OSSL_PARAM_construct_octet_string("key", key, strlen(key), NULL);
params[params_n] = OSSL_PARAM_construct_end();
if (mac == NULL
|| key == NULL
|| (ctx = EVP_MAC_CTX_new(mac)) == NULL
|| EVP_MAC_CTX_set_params(ctx, params) <= 0)
goto err;
if (!EVP_MAC_init(ctx))
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))
goto err;
printf("Result: ");
for (i = 0; i < final_l; i++)
printf("%02X", buf[i]);
fprintf(stderr, "Something went wrong\n");
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 F<foo.c> and compiled to
=head1 SEE ALSO
=head1 HISTORY
These functions were added in OpenSSL 3.0.
Copyright 2018-2019 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