=pod

=head1 NAME

EVP_MAC, EVP_MAC_CTX, EVP_MAC_CTX_new, EVP_MAC_CTX_new_id, EVP_MAC_CTX_free,
EVP_MAC_CTX_copy, EVP_MAC_CTX_mac, EVP_MAC_size, EVP_MAC_init, EVP_MAC_update,
EVP_MAC_final, EVP_MAC_ctrl, EVP_MAC_vctrl, EVP_MAC_ctrl_str,
EVP_MAC_str2ctrl, EVP_MAC_hex2ctrl, EVP_MAC_nid, EVP_MAC_name,
EVP_get_macbyname, EVP_get_macbynid, EVP_get_macbyobj - EVP MAC routines

=head1 SYNOPSIS

 #include <openssl/evp.h>

 typedef struct evp_mac_st EVP_MAC;
 typedef struct evp_mac_ctx_st EVP_MAC_CTX;

 EVP_MAC_CTX *EVP_MAC_CTX_new(const EVP_MAC *mac);
 EVP_MAC_CTX *EVP_MAC_CTX_new_id(int nid);
 void EVP_MAC_CTX_free(EVP_MAC_CTX *ctx);
 int EVP_MAC_CTX_copy(EVP_MAC_CTX *dest, EVP_MAC_CTX *src);
 const EVP_MAC *EVP_MAC_CTX_mac(EVP_MAC_CTX *ctx);
 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 *poutlen);
 int EVP_MAC_ctrl(EVP_MAC_CTX *ctx, int cmd, ...);
 int EVP_MAC_vctrl(EVP_MAC_CTX *ctx, int cmd, va_list args);
 int EVP_MAC_ctrl_str(EVP_MAC_CTX *ctx, const char *type, const char *value);
 int EVP_MAC_str2ctrl(EVP_MAC_CTX *ctx, int cmd, const char *value);
 int EVP_MAC_hex2ctrl(EVP_MAC_CTX *ctx, int cmd, const char *value);
 int EVP_MAC_nid(const EVP_MAC *mac);
 const char *EVP_MAC_name(const EVP_MAC *mac);
 const EVP_MAC *EVP_get_macbyname(const char *name);
 const EVP_MAC *EVP_get_macbynid(int nid);
 const EVP_MAC *EVP_get_macbyobj(const ASN1_OBJECT *o);

=head1 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.

=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 Context manipulation functions

EVP_MAC_CTX_new() creates a new context for the MAC type C<mac>.
EVP_MAC_CTX_new_id() creates a new context for the numerical MAC
identity <nid>.
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.
B<NULL> is a valid parameter, for which this function is a no-op.

EVP_MAC_CTX_copy() makes a deep copy of the C<src> context to the
C<dest> context.
The C<dest> context I<must> have been created before calling this
function.

EVP_MAC_CTX_mac() returns the B<EVP_MAC> associated with the context
C<ctx>.

=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_final().

EVP_MAC_reset() resets the computation for the given context.
This may not be supported by the MAC implementation.

EVP_MAC_update() adds C<datalen> bytes from C<data> to the MAC input.

EVP_MAC_final() does the final computation and stores the result in
the memory pointed at by C<out>, and sets its size in the B<size_t>
the C<poutlen> points at.
If C<out> is B<NULL>, then no computation is made.
To figure out what the output length will be and allocate space for it
dynamically, simply call with C<out> being B<NULL> and C<poutlen>
pointing at a valid location, then allocate space and make a second
call with C<out> pointing at the allocated space.

EVP_MAC_ctrl() is used to manipulate or get information on aspects of
the MAC which may vary depending on the MAC algorithm or its
implementation.
This includes the MAC key, and for MACs that use other algorithms to
do their computation, this is also the way to tell it which one to
use.
This functions takes variable arguments, the exact expected arguments
depend on C<cmd>.
EVP_MAC_ctrl() can be called both before and after EVP_MAC_init(), but
the effect will depend on what control is being use.
See </CONTROLS> below for a description of standard controls.

EVP_MAC_vctrl() is the variant of EVP_MAC_ctrl() that takes a
C<va_list> argument instead of variadic arguments.

EVP_MAC_ctrl_str() is an alternative to EVP_MAC_ctrl() to control the
MAC implementation as E<lt> C<type>, C<value> E<gt> pairs.
The MAC implementation documentation should specify what control type
strings are accepted.

EVP_MAC_str2ctrl() and EVP_MAC_hex2ctrl() are helper functions to
control the MAC implementation with raw strings or with strings
containing hexadecimal numbers.
The latter are decoded into bitstrings that are sent on to
EVP_MAC_ctrl().

=head2 Information functions

EVP_MAC_size() returns the MAC output size for the given context.

EVP_MAC_nid() returns the numeric identity of the given MAC implementation.

EVP_MAC_name() returns the name of the given MAC implementation.

=head2 Object database functions

EVP_get_macbyname() fetches a MAC implementation from the object
database by name.

EVP_get_macbynid() fetches a MAC implementation from the object
database by numeric identity.

EVP_get_macbyobj() fetches a MAC implementation from the object
database by ASN.1 OBJECT (i.e. an encoded OID).

=head1 CONTROLS

The standard controls are:

=over 4

=item B<EVP_MAC_CTRL_SET_KEY>

This control expects two arguments: C<unsigned char *key>, C<size_t keylen>

These will set the MAC key from the given string of the given length.
The string may be any bitstring, and can contain NUL bytes.

For MACs that use an underlying computation algorithm, the algorithm
I<must> be set first, see B<EVP_MAC_CTRL_SET_ENGINE>,
B<EVP_MAC_CTRL_SET_MD> and B<EVP_MAC_CTRL_SET_CIPHER> below.

=item B<EVP_MAC_CTRL_SET_FLAGS>

This control expects one arguments: C<unsigned long flags>

These will set the MAC flags to the given numbers.
Some MACs do not support this option.

=item B<EVP_MAC_CTRL_SET_ENGINE>

=item B<EVP_MAC_CTRL_SET_MD>

=item B<EVP_MAC_CTRL_SET_CIPHER>

For MAC implementations that use an underlying computation algorithm,
these controls set what the algorithm should be, and the engine that
implements the algorithm if needed.

B<EVP_MAC_CTRL_SET_ENGINE> takes one argument: C<ENGINE *>

B<EVP_MAC_CTRL_SET_MD> takes one argument: C<EVP_MD *>

B<EVP_MAC_CTRL_SET_CIPHER> takes one argument: C<EVP_CIPHER *>

=item B<EVP_MAC_CTRL_SET_SIZE>

For MAC implementations that support it, set the output size that
EVP_MAC_final() should produce.
The allowed sizes vary between MAC implementations.

=back

All these control 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.

=head1 NOTES

EVP_get_macbynid(), EVP_get_macbyobj() and EVP_MAC_name() are
implemented as a macro.

=head1 RETURN VALUES

EVP_MAC_CTX_new() and EVP_MAC_CTX_new_id() 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_copy(), EVP_MAC_reset(), EVP_MAC_init(), EVP_MAC_update(),
and EVP_MAC_final() return 1 on success, 0 on error.

EVP_MAC_ctrl(), EVP_MAC_ctrl_str(), EVP_MAC_str2ctrl() and
EVP_MAC_hex2ctrl() return 1 on success and 0 or a negative value on
error.
In particular, the value -2 indicates that the given control type
isn't supported by the MAC implementation.

EVP_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() should get it set.

EVP_MAC_nid() returns the numeric identity for the given C<mac>.

EVP_MAC_name() returns the name for the given C<mac>, if it has been
added to the object database.

EVP_add_mac() returns 1 if the given C<mac> was successfully added to
the object database, otherwise 0.

EVP_get_macbyname(), EVP_get_macbynid() and EVP_get_macbyobj() return
the request MAC implementation, if it exists in the object database,
otherwise B<NULL>.

=head1 EXAMPLE

  #include <stdlib.h>
  #include <stdio.h>
  #include <string.h>
  #include <stdarg.h>
  #include <unistd.h>

  #include <openssl/evp.h>
  #include <openssl/err.h>

  int ctrl_ign_unsupported(EVP_MAC_CTX *ctx, int cmd, ...)
  {
      va_list args;
      int rv;

      va_start(args, cmd);
      rv = EVP_MAC_vctrl(ctx, cmd, args);
      va_end(args);

      if (rv == -2)
          rv = 1;       /* Ignore unsupported, pretend it worked fine */

      return rv;
  }

  int main() {
      const EVP_MAC *mac =
          EVP_get_macbyname(getenv("MY_MAC"));
      const EVP_CIPHER *cipher =
          EVP_get_cipherbyname(getenv("MY_MAC_CIPHER"));
      const EVP_MD *digest =
          EVP_get_digestbyname(getenv("MY_MAC_DIGEST"));
      const char *key = getenv("MY_KEY");
      EVP_MAC_CTX *ctx = NULL;

      unsigned char buf[4096];
      ssize_t read_l;
      size_t final_l;

      size_t i;

      if (mac == NULL
          || key == NULL
          || (ctx = EVP_MAC_CTX_new(mac)) == NULL
          || (cipher != NULL
              && !ctrl_ign_unsupported(ctx, EVP_MAC_CTRL_SET_CIPHER, cipher))
          || (digest != NULL
              && !ctrl_ign_unsupported(ctx, EVP_MAC_CTRL_SET_MD, digest))
          || EVP_MAC_ctrl(ctx, EVP_MAC_CTRL_SET_KEY, key, strlen(key)) <= 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]);
      printf("\n");

      EVP_MAC_CTX_free(ctx);
      exit(0);

   err:
      EVP_MAC_CTX_free(ctx);
      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: ECCAAFF041B22A2299EB90A1B53B6D45

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

=head1 SEE ALSO

L<EVP_MAC_CMAC(7)>,
L<EVP_MAC_HMAC(7)>

=head1 COPYRIGHT

Copyright 2018 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
L<https://www.openssl.org/source/license.html>.

=cut
