=pod

=head1 NAME

HMAC, HMAC_CTX_init, HMAC_Init, HMAC_Init_ex, HMAC_Update, HMAC_Final, HMAC_CTX_cleanup - HMAC message authentication code

=head1 SYNOPSIS

 #include <openssl/hmac.h>

 unsigned char *HMAC(const EVP_MD *evp_md, const void *key,
               int key_len, const unsigned char *d, int n,
               unsigned char *md, unsigned int *md_len);

 int HMAC_CTX_init(HMAC_CTX *ctx);

 int HMAC_Init(HMAC_CTX *ctx, const void *key, int key_len,
               const EVP_MD *md);
 int HMAC_Init_ex(HMAC_CTX *ctx, const void *key, int key_len,
               	   const EVP_MD *md, ENGINE *impl);
 int HMAC_Update(HMAC_CTX *ctx, const unsigned char *data, int len);
 int HMAC_Final(HMAC_CTX *ctx, unsigned char *md, unsigned int *len);

 void HMAC_CTX_cleanup(HMAC_CTX *ctx);

=head1 DESCRIPTION

HMAC is a MAC (message authentication code), i.e. a keyed hash
function used for message authentication, which is based on a hash
function.

HMAC() computes the message authentication code of the B<n> bytes at
B<d> using the hash function B<evp_md> and the key B<key> which is
B<key_len> bytes long.

It places the result in B<md> (which must have space for the output of
the hash function, which is no more than B<EVP_MAX_MD_SIZE> bytes).
If B<md> is NULL, the digest is placed in a static array.  The size of
the output is placed in B<md_len>, unless it is B<NULL>.

B<evp_md> can be EVP_sha1(), EVP_ripemd160() etc.

HMAC_CTX_init() initialises a B<HMAC_CTX> before first use. It must be
called.

HMAC_CTX_cleanup() erases the key and other data from the B<HMAC_CTX>
and releases any associated resources. It must be called when an
B<HMAC_CTX> is no longer required.

The following functions may be used if the message is not completely
stored in memory:

HMAC_Init() initializes a B<HMAC_CTX> structure to use the hash
function B<evp_md> and the key B<key> which is B<key_len> bytes
long. It is deprecated and only included for backward compatibility
with OpenSSL 0.9.6b.

HMAC_Init_ex() initializes or reuses a B<HMAC_CTX> structure to use
the function B<evp_md> and key B<key>. Either can be NULL, in which
case the existing one will be reused. HMAC_CTX_init() must have been
called before the first use of an B<HMAC_CTX> in this
function. B<N.B. HMAC_Init() had this undocumented behaviour in
previous versions of OpenSSL - failure to switch to HMAC_Init_ex() in
programs that expect it will cause them to stop working>.

HMAC_Update() can be called repeatedly with chunks of the message to
be authenticated (B<len> bytes at B<data>).

HMAC_Final() places the message authentication code in B<md>, which
must have space for the hash function output.

=head1 RETURN VALUES

HMAC() returns a pointer to the message authentication code or NULL if
an error occurred.

HMAC_CTX_init(), HMAC_Init_ex(), HMAC_Update() and HMAC_Final() return 1
for success or 0 if an error occurred.

HMAC_CTX_cleanup() do not return values.

=head1 CONFORMING TO

RFC 2104

=head1 SEE ALSO

L<sha(3)>, L<evp(3)>

=head1 HISTORY

HMAC_CTX_init() did not return a value in OpenSSL versions before 1.1.

HMAC_Init_ex(), HMAC_Update() and HMAC_Final() did not return values in
versions of OpenSSL before 1.0.0.

=cut