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.\" ========================================================================
.\"
.IX Title "hmac 3"
.TH hmac 3 "2014-10-15" "1.0.1j" "OpenSSL"
.\" For nroff, turn off justification. Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "NAME"
HMAC, HMAC_Init, HMAC_Update, HMAC_Final, 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\fIHMAC\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.
.PP
\&\fBevp_md\fR can be \fIEVP_sha1()\fR, \fIEVP_ripemd160()\fR etc.
.PP
\&\fIHMAC_CTX_init()\fR initialises a \fB\s-1HMAC_CTX\s0\fR before first use. It must be
called.
.PP
\&\fIHMAC_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
\&\fIHMAC_cleanup()\fR is an alias for \fIHMAC_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
\&\fIHMAC_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
\&\fIHMAC_Init_ex()\fR initializes or reuses a \fB\s-1HMAC_CTX\s0\fR structure to use
the function \fBevp_md\fR and key \fBkey\fR. Either can be \s-1NULL\s0, in which
case the existing one will be reused. \fIHMAC_CTX_init()\fR must have been
called before the first use of an \fB\s-1HMAC_CTX\s0\fR in this
function. \fBN.B. \f(BIHMAC_Init()\fB had this undocumented behaviour in
previous versions of OpenSSL \- failure to switch to \f(BIHMAC_Init_ex()\fB in
programs that expect it will cause them to stop working\fR.
.PP
\&\fIHMAC_Update()\fR can be called repeatedly with chunks of the message to
be authenticated (\fBlen\fR bytes at \fBdata\fR).
.PP
\&\fIHMAC_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\fIHMAC\s0()\fR returns a pointer to the message authentication code or \s-1NULL\s0 if
an error occurred.
.PP
\&\fIHMAC_Init_ex()\fR, \fIHMAC_Update()\fR and \fIHMAC_Final()\fR return 1 for success or 0 if
an error occurred.
.PP
\&\fIHMAC_CTX_init()\fR and \fIHMAC_CTX_cleanup()\fR do not return values.
.SH "CONFORMING TO"
.IX Header "CONFORMING TO"
\&\s-1RFC\s0 2104
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fIsha\fR\|(3), \fIevp\fR\|(3)
.SH "HISTORY"
.IX Header "HISTORY"
\&\s-1\fIHMAC\s0()\fR, \fIHMAC_Init()\fR, \fIHMAC_Update()\fR, \fIHMAC_Final()\fR and \fIHMAC_cleanup()\fR
are available since SSLeay 0.9.0.
.PP
\&\fIHMAC_CTX_init()\fR, \fIHMAC_Init_ex()\fR and \fIHMAC_CTX_cleanup()\fR are available
since OpenSSL 0.9.7.
.PP
\&\fIHMAC_Init_ex()\fR, \fIHMAC_Update()\fR and \fIHMAC_Final()\fR did not return values in
versions of OpenSSL before 1.0.0.
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