=pod

=head1 NAME

HMAC, HMAC_Init, HMAC_Update, HMAC_Final, HMAC_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);

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

 void HMAC_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.
B<key> and B<evp_md> may be B<NULL> if a key and hash function have
been set in a previous call to HMAC_Init() for that B<HMAC_CTX>.

HMAC_cleanup() erases the key and other data from the B<HMAC_CTX>.

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.

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.

HMAC_Init(), HMAC_Update(), HMAC_Final() and HMAC_cleanup() do not
return values.

=head1 CONFORMING TO

RFC 2104

=head1 SEE ALSO

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

=head1 HISTORY

HMAC(), HMAC_Init(), HMAC_Update(), HMAC_Final() and HMAC_cleanup()
are available since SSLeay 0.9.0.

=cut