NAME
HMAC, HMAC_Init, HMAC_Update, HMAC_Final, HMAC_cleanup — HMAC message authentication code
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_CTX_init(HMAC_CTX *ctx);
void HMAC_Init(HMAC_CTX
*ctx, const void *key, int key_len, const EVP_MD *md);
void
HMAC_Init_ex(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_CTX_cleanup(HMAC_CTX
*ctx);
void HMAC_cleanup(HMAC_CTX *ctx);
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 n bytes
at d using the hash function evp_md and the
key key which is key_len bytes
long.
It places the result in md (which must
have space for the output of the hash function, which is no more
than EVP_MAX_MD_SIZE bytes). If md is
NULL, the digest is placed in a static array. The size of the output
is placed in md_len, unless it is NULL.
evp_md can be EVP_sha1(), EVP_ripemd160()
etc. key and evp_md may
be NULL if a key and hash function have been
set in a previous call to HMAC_Init() for that HMAC_CTX.
HMAC_CTX_init() initialises a HMAC_CTX before
first use. It must be called.
HMAC_CTX_cleanup() erases the key and other data from the HMAC_CTX and
releases any associated resources. It must be called when an HMAC_CTX is
no longer required.
HMAC_cleanup() is an alias for HMAC_CTX_cleanup() included
for back compatibility with 0.9.6b, it is deprecated.
The following functions may be used if the message is not
completely stored in memory:
HMAC_Init() initializes a HMAC_CTX structure
to use the hash function evp_md and the key key which
is 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 HMAC_CTX structure
to use the function evp_md and key 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 HMAC_CTX in
this function. 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 (len bytes at data).
HMAC_Final() places the message authentication code in md,
which must have space for the hash function output.
RETURN VALUES
HMAC() returns a pointer to the message authentication code.
HMAC_CTX_init(), HMAC_Init_ex(), HMAC_Update(), HMAC_Final()
and HMAC_CTX_cleanup() do not return values.
HISTORY
HMAC(), HMAC_Init(), HMAC_Update(), HMAC_Final() and HMAC_cleanup()
are available since SSLeay 0.9.0.
HMAC_CTX_init(), HMAC_Init_ex() and HMAC_CTX_cleanup() are
available since OpenSSL 0.9.7.
