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.\" ========================================================================
.\"
.IX Title "HMAC 3"
.TH HMAC 3 "2023-09-11" "1.1.1w" "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_new, HMAC_CTX_reset, HMAC_CTX_free, HMAC_Init, HMAC_Init_ex, HMAC_Update, HMAC_Final, HMAC_CTX_copy, HMAC_CTX_set_flags, HMAC_CTX_get_md, HMAC_size \&\- 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, size_t n,
\&                     unsigned char *md, unsigned int *md_len);
\&
\& HMAC_CTX *HMAC_CTX_new(void);
\& int HMAC_CTX_reset(HMAC_CTX *ctx);
\&
\& 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, size_t len);
\& int HMAC_Final(HMAC_CTX *ctx, unsigned char *md, unsigned int *len);
\&
\& void HMAC_CTX_free(HMAC_CTX *ctx);
\&
\& int HMAC_CTX_copy(HMAC_CTX *dctx, HMAC_CTX *sctx);
\& void HMAC_CTX_set_flags(HMAC_CTX *ctx, unsigned long flags);
\& const EVP_MD *HMAC_CTX_get_md(const HMAC_CTX *ctx);
\&
\& size_t HMAC_size(const HMAC_CTX *e);
.Ve
.PP
Deprecated:
.PP
.Vb 4
\& #if OPENSSL_API_COMPAT < 0x10100000L
\& int HMAC_Init(HMAC_CTX *ctx, const void *key, int key_len,
\&               const EVP_MD *md);
\& #endif
.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. Note: passing a \s-1NULL\s0
value for \fBmd\fR  to use the static array is not thread safe.
.PP
\&\fBevp_md\fR is a message digest such as \fIEVP_sha1()\fR, \fIEVP_ripemd160()\fR etc. \s-1HMAC\s0 does
not support variable output length digests such as \fIEVP_shake128()\fR and
\&\fIEVP_shake256()\fR.
.PP
\&\fIHMAC_CTX_new()\fR creates a new \s-1HMAC_CTX\s0 in heap memory.
.PP
\&\fIHMAC_CTX_reset()\fR zeros an existing \fB\s-1HMAC_CTX\s0\fR and associated
resources, making it suitable for new computations as if it was newly
created with \fIHMAC_CTX_new()\fR.
.PP
\&\fIHMAC_CTX_free()\fR erases the key and other data from the \fB\s-1HMAC_CTX\s0\fR,
releases any associated resources and finally frees the \fB\s-1HMAC_CTX\s0\fR
itself.
.PP
The following functions may be used if the message is not completely
stored in memory:
.PP
\&\fIHMAC_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 if \fBkey\fR is \s-1NULL\s0
and \fBevp_md\fR is the same as the previous call, then the
existing key is
reused. \fBctx\fR must have been created with \fIHMAC_CTX_new()\fR before the first use
of an \fB\s-1HMAC_CTX\s0\fR in this function.
.PP
If \fIHMAC_Init_ex()\fR is called with \fBkey\fR \s-1NULL\s0 and \fBevp_md\fR is not the
same as the previous digest used by \fBctx\fR then an error is returned
because reuse of an existing key with a different digest is not supported.
.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.
.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.
.PP
\&\fIHMAC_CTX_copy()\fR copies all of the internal state from \fBsctx\fR into \fBdctx\fR.
.PP
\&\fIHMAC_CTX_set_flags()\fR applies the specified flags to the internal EVP_MD_CTXs.
These flags have the same meaning as for \fIEVP_MD_CTX_set_flags\fR\|(3).
.PP
\&\fIHMAC_CTX_get_md()\fR returns the \s-1EVP_MD\s0 that has previously been set for the
supplied \s-1HMAC_CTX.\s0
.PP
\&\fIHMAC_size()\fR returns the length in bytes of the underlying 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_CTX_new()\fR returns a pointer to a new \fB\s-1HMAC_CTX\s0\fR on success or
\&\fB\s-1NULL\s0\fR if an error occurred.
.PP
\&\fIHMAC_CTX_reset()\fR, \fIHMAC_Init_ex()\fR, \fIHMAC_Update()\fR, \fIHMAC_Final()\fR and
\&\fIHMAC_CTX_copy()\fR return 1 for success or 0 if an error occurred.
.PP
\&\fIHMAC_CTX_get_md()\fR return the \s-1EVP_MD\s0 previously set for the supplied \s-1HMAC_CTX\s0 or
\&\s-1NULL\s0 if no \s-1EVP_MD\s0 has been set.
.PP
\&\fIHMAC_size()\fR returns the length in bytes of the underlying hash function output
or zero on error.
.SH "CONFORMING TO"
.IX Header "CONFORMING TO"
\&\s-1RFC 2104\s0
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\s-1\fISHA1\s0\fR\|(3), \fIevp\fR\|(7)
.SH "HISTORY"
.IX Header "HISTORY"
\&\fIHMAC_CTX_init()\fR was replaced with \fIHMAC_CTX_reset()\fR in OpenSSL 1.1.0.
.PP
\&\fIHMAC_CTX_cleanup()\fR existed in OpenSSL before version 1.1.0.
.PP
\&\fIHMAC_CTX_new()\fR, \fIHMAC_CTX_free()\fR and \fIHMAC_CTX_get_md()\fR are new in OpenSSL 1.1.0.
.PP
\&\fIHMAC_Init_ex()\fR, \fIHMAC_Update()\fR and \fIHMAC_Final()\fR did not return values in
OpenSSL before version 1.0.0.
.SH "COPYRIGHT"
.IX Header "COPYRIGHT"
Copyright 2000\-2020 The OpenSSL Project Authors. All Rights Reserved.
.PP
Licensed under the OpenSSL license (the \*(L"License\*(R").  You may not use
this file except in compliance with the License.  You can obtain a copy
in the file \s-1LICENSE\s0 in the source distribution or at
<https://www.openssl.org/source/license.html>.

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