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.\" ========================================================================
.\"
.IX Title "hmac 3"
.TH hmac 3 "2019-12-20" "1.0.2u" "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_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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✏️ HMAC.3ssl