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_reset(HMAC_CTX *ctx);
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_free(HMAC_CTX *ctx);
#if OPENSSL_API_COMPAT < 0x10100000L
int HMAC_Init(HMAC_CTX *ctx, const void *key, int key_len,
const EVP_MD *md);
HMAC is a MAC (message authentication code), i.e. a keyed hash
function used for message authentication, which is based on a hash
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_new() creates a new HMAC_CTX in heap memory.
HMAC_CTX_reset() zeroes an existing HMAC_CTX and associated
resources, making it suitable for new computations as if it was newly
created with HMAC_CTX_new().
HMAC_CTX_free() erases the key and other data from the HMAC_CTX,
releases any associated resources and finally frees the HMAC_CTX
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. ctx must have been created
with HMAC_CTX_new() 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.
HMAC() returns a pointer to the message authentication code or NULL if
an error occurred.
HMAC_CTX_new() returns a pointer to a new HMAC_CTX on success or
NULL if an error occurred.
HMAC_CTX_reset(), HMAC_Init_ex(), HMAC_Update() and HMAC_Final() return 1
for success or 0 if an error occurred.
HMAC_CTX_free() do not return values.
HMAC_CTX_init() was replaced with HMAC_CTX_reset() in OpenSSL versions 1.1.
HMAC_CTX_cleanup() existed in OpenSSL versions before 1.1.
HMAC_CTX_new() and HMAC_CTX_free() are new in OpenSSL version 1.1.
HMAC_Init_ex(), HMAC_Update() and HMAC_Final() did not return values in
versions of OpenSSL before 1.0.0.