NAME
HMAC,
HMAC_CTX_new,
HMAC_CTX_reset,
HMAC_CTX_free, HMAC_Init_ex,
HMAC_Update, HMAC_Final,
HMAC_CTX_copy,
HMAC_CTX_set_flags,
HMAC_CTX_get_md, HMAC_size
— 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, size_t
n, unsigned char *md, unsigned
int *md_len);
HMAC_CTX *
HMAC_CTX_new(void);
int
HMAC_CTX_reset(HMAC_CTX
*ctx);
void
HMAC_CTX_free(HMAC_CTX
*ctx);
int
HMAC_Init_ex(HMAC_CTX *ctx,
const void *key, int key_len,
const EVP_MD *md, ENGINE
*engine);
int
HMAC_Update(HMAC_CTX *ctx,
const unsigned char *data, size_t
len);
int
HMAC_Final(HMAC_CTX *ctx,
unsigned char *md, unsigned int
*len);
int
HMAC_CTX_copy(HMAC_CTX *dctx,
HMAC_CTX *sctx);
void
HMAC_CTX_set_flags(HMAC_CTX
*ctx, unsigned long flags);
const EVP_MD *
HMAC_CTX_get_md(const HMAC_CTX
*ctx);
size_t
HMAC_size(const HMAC_CTX
*e);
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. The size of the output is
placed in md_len, unless it is
NULL.
evp_md can be EVP_sha1(3), EVP_ripemd160(3), etc.
HMAC_CTX_new()
allocates and initializes a new HMAC_CTX object.
HMAC_CTX_reset()
zeroes and re-initializes ctx and associated
resources, making it suitable for new computations as if it was deleted with
HMAC_CTX_free() and newly created with
HMAC_CTX_new().
HMAC_CTX_free()
erases the key and other data from ctx, releases any
associated resources, and finally frees ctx
itself.
The following functions may be used if the message is not completely stored in memory:
HMAC_Init_ex()
sets up or reuses ctx to use the hash function
evp_md and the key key. Either
can be NULL, in which case the existing one is
reused. The ctx must have been created with
HMAC_CTX_new() before the first use in this
function. If HMAC_Init_ex() is called with a
NULL key but
evp_md is neither NULL nor the
same as the previous digest used by ctx, then an error
is returned because reuse of an existing key with a different digest is not
supported. The ENGINE *engine argument is always
ignored and passing NULL is recommended.
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.
HMAC_CTX_copy()
copies all of the internal state from sctx into
dctx.
HMAC_CTX_set_flags()
applies the specified flags to the internal EVP_MD_CTX
objects. Possible flag values EVP_MD_CTX_FLAG_* are
defined in
<openssl/evp.h>.
HMAC_size()
returns the length in bytes of the underlying hash function output. It is
implemented as a macro.
RETURN VALUES
HMAC() returns a pointer to the message
authentication code or NULL if an error
occurred.
HMAC_CTX_new() returns a pointer to the
new HMAC_CTX object or NULL if
an error occurred.
HMAC_CTX_reset(),
HMAC_Init_ex(),
HMAC_Update(), HMAC_Final(),
and HMAC_CTX_copy() return 1 for success or 0 if an
error occurred.
HMAC_CTX_get_md() returns the message
digest that was previously set for ctx with
HMAC_Init_ex(), or NULL if
none was set.
HMAC_size() returns the length in bytes of
the underlying hash function output or 0 on error.
SEE ALSO
STANDARDS
RFC 2104
HISTORY
HMAC(),
HMAC_Update(), HMAC_Final(),
and HMAC_size() first appeared in SSLeay 0.9.0 and
have been available since OpenBSD 2.4.
HMAC_Init_ex() first appeared in OpenSSL
0.9.7 and have been available since OpenBSD 3.2.
HMAC_CTX_set_flags() first appeared in
OpenSSL 0.9.7f and have been available since OpenBSD
3.8.
HMAC_CTX_copy() first appeared in OpenSSL
1.0.0 and has been available since OpenBSD 4.9.
HMAC_CTX_new(),
HMAC_CTX_reset(),
HMAC_CTX_free(), and
HMAC_CTX_get_md() first appeared in OpenSSL 1.1.0
and have been available since OpenBSD 6.3.
CAVEATS
Other implementations allow md in
HMAC() to be NULL and return
a static array, which is not thread safe.