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);
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);
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.
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 or NULL if an
error occurred.
HMAC_Init_ex(),
HMAC_Update() and
HMAC_Final() return 1 for
success or 0 if an error occurred.
HMAC_CTX_init() and
HMAC_CTX_cleanup() do not return values.
RFC 2104
SEE ALSO¶
sha(3),
evp(3)
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.
HMAC_Init_ex(),
HMAC_Update() and
HMAC_Final() did not
return values in versions of OpenSSL before 1.0.0.