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);

        HMAC_CTX *HMAC_CTX_new(void);
        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);

        int HMAC_CTX_copy(HMAC_CTX *dctx, HMAC_CTX *sctx);
        void HMAC_CTX_set_flags(HMAC_CTX *ctx, unsigned long flags);
        const EVP_MD *HMAC_CTX_get_md(const HMAC_CTX *ctx);

        size_t HMAC_size(const HMAC_CTX *e);


        #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 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

       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 is a message digest such as EVP_sha1(), EVP_ripemd160() etc.
       HMAC does not support variable output length digests such as
       EVP_shake128() and EVP_shake256().

       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
       reused. ctx must have been created with HMAC_CTX_new() before the first
       use of an HMAC_CTX in this function.

       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_Init() initializes a HMAC_CTX structure to use the hash function
       evp_md and the key key which is key_len bytes long.

       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_CTX_copy() copies all of the internal state from sctx into dctx.

       HMAC_CTX_set_flags() applies the specified flags to the internal
       EVP_MD_CTXs.  These flags have the same meaning as for

       HMAC_CTX_get_md() returns the EVP_MD that has previously been set for
       the supplied HMAC_CTX.

       HMAC_size() returns the length in bytes of the underlying hash function

       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(), HMAC_Final() and
       HMAC_CTX_copy() return 1 for success or 0 if an error occurred.

       HMAC_CTX_get_md() return the EVP_MD previously set for the supplied
       HMAC_CTX or NULL if no EVP_MD has been set.

       HMAC_size() returns the length in bytes of the underlying hash function
       output or zero on error.

       RFC 2104

       SHA1(3), evp(7)

       HMAC_CTX_init() was replaced with HMAC_CTX_reset() in OpenSSL 1.1.0.

       HMAC_CTX_cleanup() existed in OpenSSL before version 1.1.0.
       in the file LICENSE in the source distribution or at

1.1.1c                            2019-05-31                        HMAC(3SSL)
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