• 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);
  
  void HMAC_Init(HMAC_CTX *ctx, const void *key, int key_len,
                 const EVP_MD *md);
  void HMAC_Init_ex(HMAC_CTX *ctx, const void *key, int key_len,
                     const EVP_MD *md, ENGINE *impl);
  void HMAC_Update(HMAC_CTX *ctx, const unsigned char *data, int len);
  void 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. key and evp_md may be NULL if a key and hash function have been set in a previous call to HMAC_Init() for that HMAC_CTX.
  
  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. <(>BIHMAC_Init() had this undocumented behaviour in previous versions of OpenSSL - failure to switch to <(>BIHMAC_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.
  
  HMAC_CTX_init(), HMAC_Init_ex(), HMAC_Update(), HMAC_Final() 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.

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