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hmac(3) OpenSSL hmac(3) NAME HMAC, HMAC_CTX_init, HMAC_Init, HMAC_Init_ex, HMAC_Update, HMAC_Final, HMAC_CTX_cleanup, HMAC_cleanup - HMAC message authentication code LIBRARY libcrypto, -lcrypto 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 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. 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 openssl_sha(3), openssl_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. 1.0.2k 2016-10-14 hmac(3)