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HMAC, HMAC_Init, HMAC_Update, HMAC_Final, HMAC_cleanup - HMAC message authentication code


#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);


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.


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


sha(3), evp(3)


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.
2014-04-16 OpenBSD 6.0