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.

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.