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hmac.3
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Always turn off hyphenation; it makes .\" way too many mistakes in technical documents. .if n .ad l .nh .SH "NAME" HMAC, HMAC_CTX_init, HMAC_Init, HMAC_Init_ex, HMAC_Update, HMAC_Final, HMAC_CTX_cleanup, HMAC_cleanup \- HMAC message authentication code .SH "SYNOPSIS" .IX Header "SYNOPSIS" .Vb 1 \& #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); .Ve .SH "DESCRIPTION" .IX 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. .PP \&\s-1\fBHMAC\s0()\fR computes the message authentication code of the \fBn\fR bytes at \&\fBd\fR using the hash function \fBevp_md\fR and the key \fBkey\fR which is \&\fBkey_len\fR bytes long. .PP It places the result in \fBmd\fR (which must have space for the output of the hash function, which is no more than \fB\s-1EVP_MAX_MD_SIZE\s0\fR bytes). If \fBmd\fR is \s-1NULL,\s0 the digest is placed in a static array. The size of the output is placed in \fBmd_len\fR, unless it is \fB\s-1NULL\s0\fR. Note: passing a \s-1NULL\s0 value for \fBmd\fR to use the static array is not thread safe. .PP \&\fBevp_md\fR can be \fBEVP_sha1()\fR, \fBEVP_ripemd160()\fR etc. .PP \&\fBHMAC_CTX_init()\fR initialises a \fB\s-1HMAC_CTX\s0\fR before first use. It must be called. .PP \&\fBHMAC_CTX_cleanup()\fR erases the key and other data from the \fB\s-1HMAC_CTX\s0\fR and releases any associated resources. It must be called when an \&\fB\s-1HMAC_CTX\s0\fR is no longer required. .PP \&\fBHMAC_cleanup()\fR is an alias for \fBHMAC_CTX_cleanup()\fR included for back compatibility with 0.9.6b, it is deprecated. .PP The following functions may be used if the message is not completely stored in memory: .PP \&\fBHMAC_Init()\fR initializes a \fB\s-1HMAC_CTX\s0\fR structure to use the hash function \fBevp_md\fR and the key \fBkey\fR which is \fBkey_len\fR bytes long. It is deprecated and only included for backward compatibility with OpenSSL 0.9.6b. .PP \&\fBHMAC_Init_ex()\fR initializes or reuses a \fB\s-1HMAC_CTX\s0\fR structure to use the hash function \fBevp_md\fR and key \fBkey\fR. If both are \s-1NULL\s0 (or \fBevp_md\fR is the same as the previous digest used by \fBctx\fR and \fBkey\fR is \s-1NULL\s0) the existing key is reused. \fBctx\fR must have been created with \fBHMAC_CTX_new()\fR before the first use of an \fB\s-1HMAC_CTX\s0\fR in this function. \fBN.B. \fBHMAC_Init()\fB had this undocumented behaviour in previous versions of OpenSSL \- failure to switch to \fBHMAC_Init_ex()\fB in programs that expect it will cause them to stop working\fR. .PP \&\fB\s-1NB:\s0 if \fBHMAC_Init_ex()\fB is called with \fBkey\fB \s-1NULL\s0 and \fBevp_md\fB is not the same as the previous digest used by \fBctx\fB then an error is returned because reuse of an existing key with a different digest is not supported.\fR .PP \&\fBHMAC_Update()\fR can be called repeatedly with chunks of the message to be authenticated (\fBlen\fR bytes at \fBdata\fR). .PP \&\fBHMAC_Final()\fR places the message authentication code in \fBmd\fR, which must have space for the hash function output. .SH "RETURN VALUES" .IX Header "RETURN VALUES" \&\s-1\fBHMAC\s0()\fR returns a pointer to the message authentication code or \s-1NULL\s0 if an error occurred. .PP \&\fBHMAC_Init_ex()\fR, \fBHMAC_Update()\fR and \fBHMAC_Final()\fR return 1 for success or 0 if an error occurred. .PP \&\fBHMAC_CTX_init()\fR and \fBHMAC_CTX_cleanup()\fR do not return values. .SH "CONFORMING TO" .IX Header "CONFORMING TO" \&\s-1RFC 2104\s0 .SH "SEE ALSO" .IX Header "SEE ALSO" \&\fBsha\fR\|(3), \fBevp\fR\|(3) .SH "HISTORY" .IX Header "HISTORY" \&\s-1\fBHMAC\s0()\fR, \fBHMAC_Init()\fR, \fBHMAC_Update()\fR, \fBHMAC_Final()\fR and \fBHMAC_cleanup()\fR are available since SSLeay 0.9.0. .PP \&\fBHMAC_CTX_init()\fR, \fBHMAC_Init_ex()\fR and \fBHMAC_CTX_cleanup()\fR are available since OpenSSL 0.9.7. .PP \&\fBHMAC_Init_ex()\fR, \fBHMAC_Update()\fR and \fBHMAC_Final()\fR did not return values in versions of OpenSSL before 1.0.0.
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