.\" Automatically generated by Pod::Man 4.11 (Pod::Simple 3.35)
.\"
.\" Standard preamble:
.\" ========================================================================
.de Sp \" Vertical space (when we can't use .PP)
.if t .sp .5v
.if n .sp
..
.de Vb \" Begin verbatim text
.ft CW
.nf
.ne \\$1
..
.de Ve \" End verbatim text
.ft R
.fi
..
.\" Set up some character translations and predefined strings.  \*(-- will
.\" give an unbreakable dash, \*(PI will give pi, \*(L" will give a left
.\" double quote, and \*(R" will give a right double quote.  \*(C+ will
.\" give a nicer C++.  Capital omega is used to do unbreakable dashes and
.\" therefore won't be available.  \*(C` and \*(C' expand to `' in nroff,
.\" nothing in troff, for use with C<>.
.tr \(*W-
.ds C+ C\v'-.1v'\h'-1p'\s-2+\h'-1p'+\s0\v'.1v'\h'-1p'
.ie n \{\
.    ds -- \(*W-
.    ds PI pi
.    if (\n(.H=4u)&(1m=24u) .ds -- \(*W\h'-12u'\(*W\h'-12u'-\" diablo 10 pitch
.    if (\n(.H=4u)&(1m=20u) .ds -- \(*W\h'-12u'\(*W\h'-8u'-\"  diablo 12 pitch
.    ds L" ""
.    ds R" ""
.    ds C` ""
.    ds C' ""
'br\}
.el\{\
.    ds -- \|\(em\|
.    ds PI \(*p
.    ds L" ``
.    ds R" ''
.    ds C`
.    ds C'
'br\}
.\"
.\" Escape single quotes in literal strings from groff's Unicode transform.
.ie \n(.g .ds Aq \(aq
.el       .ds Aq '
.\"
.\" If the F register is >0, we'll generate index entries on stderr for
.\" titles (.TH), headers (.SH), subsections (.SS), items (.Ip), and index
.\" entries marked with X<> in POD.  Of course, you'll have to process the
.\" output yourself in some meaningful fashion.
.\"
.\" Avoid warning from groff about undefined register 'F'.
.de IX
..
.nr rF 0
.if \n(.g .if rF .nr rF 1
.if (\n(rF:(\n(.g==0)) \{\
.    if \nF \{\
.        de IX
.        tm Index:\\$1\t\\n%\t"\\$2"
..
.        if !\nF==2 \{\
.            nr % 0
.            nr F 2
.        \}
.    \}
.\}
.rr rF
.\"
.\" Accent mark definitions (@(#)ms.acc 1.5 88/02/08 SMI; from UCB 4.2).
.\" Fear.  Run.  Save yourself.  No user-serviceable parts.
.    \" fudge factors for nroff and troff
.if n \{\
.    ds #H 0
.    ds #V .8m
.    ds #F .3m
.    ds #[ \f1
.    ds #] \fP
.\}
.if t \{\
.    ds #H ((1u-(\\\\n(.fu%2u))*.13m)
.    ds #V .6m
.    ds #F 0
.    ds #[ \&
.    ds #] \&
.\}
.    \" simple accents for nroff and troff
.if n \{\
.    ds ' \&
.    ds ` \&
.    ds ^ \&
.    ds , \&
.    ds ~ ~
.    ds /
.\}
.if t \{\
.    ds ' \\k:\h'-(\\n(.wu*8/10-\*(#H)'\'\h"|\\n:u"
.    ds ` \\k:\h'-(\\n(.wu*8/10-\*(#H)'\`\h'|\\n:u'
.    ds ^ \\k:\h'-(\\n(.wu*10/11-\*(#H)'^\h'|\\n:u'
.    ds , \\k:\h'-(\\n(.wu*8/10)',\h'|\\n:u'
.    ds ~ \\k:\h'-(\\n(.wu-\*(#H-.1m)'~\h'|\\n:u'
.    ds / \\k:\h'-(\\n(.wu*8/10-\*(#H)'\z\(sl\h'|\\n:u'
.\}
.    \" troff and (daisy-wheel) nroff accents
.ds : \\k:\h'-(\\n(.wu*8/10-\*(#H+.1m+\*(#F)'\v'-\*(#V'\z.\h'.2m+\*(#F'.\h'|\\n:u'\v'\*(#V'
.ds 8 \h'\*(#H'\(*b\h'-\*(#H'
.ds o \\k:\h'-(\\n(.wu+\w'\(de'u-\*(#H)/2u'\v'-.3n'\*(#[\z\(de\v'.3n'\h'|\\n:u'\*(#]
.ds d- \h'\*(#H'\(pd\h'-\w'~'u'\v'-.25m'\f2\(hy\fP\v'.25m'\h'-\*(#H'
.ds D- D\\k:\h'-\w'D'u'\v'-.11m'\z\(hy\v'.11m'\h'|\\n:u'
.ds th \*(#[\v'.3m'\s+1I\s-1\v'-.3m'\h'-(\w'I'u*2/3)'\s-1o\s+1\*(#]
.ds Th \*(#[\s+2I\s-2\h'-\w'I'u*3/5'\v'-.3m'o\v'.3m'\*(#]
.ds ae a\h'-(\w'a'u*4/10)'e
.ds Ae A\h'-(\w'A'u*4/10)'E
.    \" corrections for vroff
.if v .ds ~ \\k:\h'-(\\n(.wu*9/10-\*(#H)'\s-2\u~\d\s+2\h'|\\n:u'
.if v .ds ^ \\k:\h'-(\\n(.wu*10/11-\*(#H)'\v'-.4m'^\v'.4m'\h'|\\n:u'
.    \" for low resolution devices (crt and lpr)
.if \n(.H>23 .if \n(.V>19 \
\{\
.    ds : e
.    ds 8 ss
.    ds o a
.    ds d- d\h'-1'\(ga
.    ds D- D\h'-1'\(hy
.    ds th \o'bp'
.    ds Th \o'LP'
.    ds ae ae
.    ds Ae AE
.\}
.rm #[ #] #H #V #F C
.\" ========================================================================
.\"
.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.