NetBSD/lib/libcrypto/man/openssl_hmac.3

.\"	$NetBSD: openssl_hmac.3,v 1.2 2002/02/07 07:00:44 ross Exp $
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.\" ======================================================================
.\"
.IX Title "hmac 3"
.TH hmac 3 "0.9.6a" "2001-04-12" "OpenSSL"
.UC
.SH "NAME"
\&\s-1HMAC\s0, HMAC_Init, HMAC_Update, HMAC_Final, HMAC_cleanup \- \s-1HMAC\s0 message
authentication code
.SH "LIBRARY"
libcrypto, -lcrypto
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\& #include \*[Lt]openssl/hmac.h\*[Gt]
.Ve
.Vb 3
\& 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);
.Ve
.Vb 4
\& void HMAC_Init(HMAC_CTX *ctx, const void *key, int key_len,
\&               const EVP_MD *md);
\& void HMAC_Update(HMAC_CTX *ctx, const unsigned char *data, int len);
\& void HMAC_Final(HMAC_CTX *ctx, unsigned char *md, unsigned int *len);
.Ve
.Vb 1
\& 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
\&\fIHMAC()\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.
\&\fBkey\fR and \fBevp_md\fR may be \fB\s-1NULL\s0\fR if a key and hash function have
been set in a previous call to \fIHMAC_Init()\fR for that \fB\s-1HMAC_CTX\s0\fR.
.PP
\&\fIHMAC_cleanup()\fR erases the key and other data from the \fB\s-1HMAC_CTX\s0\fR.
.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.
.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"
\&\fIHMAC()\fR returns a pointer to the message authentication code.
.PP
\&\fIHMAC_Init()\fR, \fIHMAC_Update()\fR, \fIHMAC_Final()\fR and \fIHMAC_cleanup()\fR do not
return values.
.SH "CONFORMING TO"
.IX Header "CONFORMING TO"
\&\s-1RFC\s0 2104
.SH "SEE ALSO"
.IX Header "SEE ALSO"
openssl_sha(3), openssl_evp(3)
.SH "HISTORY"
.IX Header "HISTORY"
\&\fIHMAC()\fR, \fIHMAC_Init()\fR, \fIHMAC_Update()\fR, \fIHMAC_Final()\fR and \fIHMAC_cleanup()\fR
are available since SSLeay 0.9.0.