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Title "hmac 3" C \s-1HMAC\s0, HMAC_Init, HMAC_Update, HMAC_Final, HMAC_cleanup - \s-1HMAC\s0 message authentication code Header "SYNOPSIS" .Vb 1 #include <openssl/hmac.h> .Ve .Vb 3 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); .Ve .Vb 4 void HMAC_Init(HMAC_CTX *ctx, const void *key, int key_len, const EVP_MD *md); void HMAC_Update(HMAC_CTX *ctx, const unsigned char *data, int len); void HMAC_Final(HMAC_CTX *ctx, unsigned char *md, unsigned int *len); .Ve .Vb 1 void HMAC_cleanup(HMAC_CTX *ctx); .Ve Header "DESCRIPTION" \s-1HMAC\s0 is a \s-1MAC\s0 (message authentication code), i.e. a keyed hash function used for message authentication, which is based on a hash function.

\fIHMAC() computes the message authentication code of the n bytes at \fBd using the hash function evp_md and the key key which is \fBkey_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 \s-1EVP_MAX_MD_SIZE\s0 bytes). If md is \s-1NULL\s0, the digest is placed in a static array. The size of the output is placed in md_len, unless it is \s-1NULL\s0.

\fBevp_md can be EVP_sha1(), EVP_ripemd160() etc. \fBkey and evp_md may be \s-1NULL\s0 if a key and hash function have been set in a previous call to HMAC_Init() for that \s-1HMAC_CTX\s0.

\fIHMAC_cleanup() erases the key and other data from the \s-1HMAC_CTX\s0.

The following functions may be used if the message is not completely stored in memory:

\fIHMAC_Init() initializes a \s-1HMAC_CTX\s0 structure to use the hash function evp_md and the key key which is key_len bytes long.

\fIHMAC_Update() can be called repeatedly with chunks of the message to be authenticated (len bytes at data).

\fIHMAC_Final() places the message authentication code in md, which must have space for the hash function output.

Header "RETURN VALUES" \fIHMAC() returns a pointer to the message authentication code.

\fIHMAC_Init(), HMAC_Update(), HMAC_Final() and HMAC_cleanup() do not return values.

Header "CONFORMING TO" \s-1RFC\s0 2104 Header "SEE ALSO" sha(3), evp(3) Header "HISTORY" \fIHMAC(), HMAC_Init(), HMAC_Update(), HMAC_Final() and HMAC_cleanup() are available since SSLeay 0.9.0.