hmac(3)                      OpenSSL                      hmac(3)


     HMAC, HMAC_CTX_init, HMAC_Init, HMAC_Init_ex, HMAC_Update,
     HMAC_Final, HMAC_CTX_cleanup, 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. Note: passing a NULL value for md
     to use the static array is not thread safe.

     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.

1.0.2t               Last change: 2019-09-10                    1

hmac(3)                      OpenSSL                      hmac(3)

     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. HHMMAACC_IInniitt(()) had this
     undocumented behaviour in previous versions of OpenSSL -
     failure to switch to HHMMAACC_IInniitt_eexx(()) in programs that expect
     it will cause them to stop working.

     NB: if HHMMAACC_IInniitt_eexx(()) 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.


     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.

1.0.2t               Last change: 2019-09-10                    2

hmac(3)                      OpenSSL                      hmac(3)

     HMAC_Init_ex(), HMAC_Update() and HMAC_Final() did not
     return values in versions of OpenSSL before 1.0.0.

1.0.2t               Last change: 2019-09-10                    3

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