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hmac(3) OpenSSL hmac(3)
NAME
HMAC, HMAC_CTX_init, HMAC_Init, HMAC_Init_ex, HMAC_Update, HMAC_Final, HMAC_CTX_cleanup,
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 hash function evp_md
and key key. If both are NULL (or evp_md is the same as the previous digest used by ctx
and key is NULL) the existing key is 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.
NB: if HMAC_Init_ex() is called with key NULL and evp_md is not 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.
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.
CONFORMING TO
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.
1.0.2k 2017-01-26 hmac(3)
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