path: root/lib/libcrypto/man/HMAC.3

.\" $OpenBSD: HMAC.3,v 1.20 2022/01/25 17:55:39 tb Exp $
.\" full merge up to: OpenSSL crypto/hmac a528d4f0 Oct 27 13:40:11 2015 -0400
.\" selective merge up to: OpenSSL man3/HMAC b3696a55 Sep 2 09:35:50 2017 -0400
.\"
.\" This file was written by Ulf Moeller <ulf@openssl.org>,
.\" Richard Levitte <levitte@openssl.org>, and
.\" Matt Caswell <matt@openssl.org>.
.\" Copyright (c) 2000-2002, 2006, 2008, 2009, 2013, 2015, 2016
.\" The OpenSSL Project.  All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\"
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\"
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in
.\"    the documentation and/or other materials provided with the
.\"    distribution.
.\"
.\" 3. All advertising materials mentioning features or use of this
.\"    software must display the following acknowledgment:
.\"    "This product includes software developed by the OpenSSL Project
.\"    for use in the OpenSSL Toolkit. (http://www.openssl.org/)"
.\"
.\" 4. The names "OpenSSL Toolkit" and "OpenSSL Project" must not be used to
.\"    endorse or promote products derived from this software without
.\"    prior written permission. For written permission, please contact
.\"    openssl-core@openssl.org.
.\"
.\" 5. Products derived from this software may not be called "OpenSSL"
.\"    nor may "OpenSSL" appear in their names without prior written
.\"    permission of the OpenSSL Project.
.\"
.\" 6. Redistributions of any form whatsoever must retain the following
.\"    acknowledgment:
.\"    "This product includes software developed by the OpenSSL Project
.\"    for use in the OpenSSL Toolkit (http://www.openssl.org/)"
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE OpenSSL PROJECT ``AS IS'' AND ANY
.\" EXPRESSED OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
.\" PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE OpenSSL PROJECT OR
.\" ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
.\" SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
.\" LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
.\" STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
.\" OF THE POSSIBILITY OF SUCH DAMAGE.
.\"
.Dd $Mdocdate: January 25 2022 $
.Dt HMAC 3
.Os
.Sh NAME
.Nm HMAC ,
.Nm HMAC_CTX_new ,
.Nm HMAC_CTX_reset ,
.Nm HMAC_CTX_free ,
.Nm HMAC_Init_ex ,
.Nm HMAC_Init ,
.Nm HMAC_Update ,
.Nm HMAC_Final ,
.Nm HMAC_CTX_copy ,
.Nm HMAC_CTX_set_flags ,
.Nm HMAC_CTX_get_md ,
.Nm HMAC_size
.Nd HMAC message authentication code
.Sh SYNOPSIS
.In openssl/hmac.h
.Ft unsigned char *
.Fo HMAC
.Fa "const EVP_MD *evp_md"
.Fa "const void *key"
.Fa "int key_len"
.Fa "const unsigned char *d"
.Fa "size_t n"
.Fa "unsigned char *md"
.Fa "unsigned int *md_len"
.Fc
.Ft HMAC_CTX *
.Fn HMAC_CTX_new void
.Ft int
.Fo HMAC_CTX_reset
.Fa "HMAC_CTX *ctx"
.Fc
.Ft void
.Fo HMAC_CTX_free
.Fa "HMAC_CTX *ctx"
.Fc
.Ft int
.Fo HMAC_Init_ex
.Fa "HMAC_CTX *ctx"
.Fa "const void *key"
.Fa "int key_len"
.Fa "const EVP_MD *md"
.Fa "ENGINE *impl"
.Fc
.Ft int
.Fo HMAC_Init
.Fa "HMAC_CTX *ctx"
.Fa "const void *key"
.Fa "int key_len"
.Fa "const EVP_MD *md"
.Fc
.Ft int
.Fo HMAC_Update
.Fa "HMAC_CTX *ctx"
.Fa "const unsigned char *data"
.Fa "size_t len"
.Fc
.Ft int
.Fo HMAC_Final
.Fa "HMAC_CTX *ctx"
.Fa "unsigned char *md"
.Fa "unsigned int *len"
.Fc
.Ft int
.Fo HMAC_CTX_copy
.Fa "HMAC_CTX *dctx"
.Fa "HMAC_CTX *sctx"
.Fc
.Ft void
.Fo HMAC_CTX_set_flags
.Fa "HMAC_CTX *ctx"
.Fa "unsigned long flags"
.Fc
.Ft const EVP_MD *
.Fo HMAC_CTX_get_md
.Fa "const HMAC_CTX *ctx"
.Fc
.Ft size_t
.Fo HMAC_size
.Fa "const HMAC_CTX *e"
.Fc
.Sh DESCRIPTION
HMAC is a MAC (message authentication code), i.e. a keyed hash
function used for message authentication, which is based on a hash
function.
.Pp
.Fn HMAC
computes the message authentication code of the
.Fa n
bytes at
.Fa d
using the hash function
.Fa evp_md
and the key
.Fa key
which is
.Fa key_len
bytes long.
.Pp
It places the result in
.Fa md ,
which must have space for the output of the hash function, which is no
more than
.Dv EVP_MAX_MD_SIZE
bytes.
If
.Fa md
is
.Dv NULL ,
the digest is placed in a static array, which is not thread safe.
The size of the output is placed in
.Fa md_len ,
unless it is
.Dv NULL .
.Pp
.Fa evp_md
can be
.Xr EVP_sha1 3 ,
.Xr EVP_ripemd160 3 ,
etc.
.Pp
.Fn HMAC_CTX_new
allocates and initializes a new
.Vt HMAC_CTX
object.
.Pp
.Fn HMAC_CTX_reset
zeroes and re-initializes
.Fa ctx
and associated resources, making it suitable for new computations
as if it was deleted with
.Fn HMAC_CTX_free
and newly created with
.Fn HMAC_CTX_new .
.Pp
.Fn HMAC_CTX_free
erases the key and other data from
.Fa ctx ,
releases any associated resources, and finally frees
.Fa ctx
itself.
.Pp
The following functions may be used if the message is not completely
stored in memory:
.Pp
.Fn HMAC_Init_ex
sets up or reuses
.Fa ctx
to use the hash function
.Fa evp_md
and the key
.Fa key .
Either can be
.Dv NULL ,
in which case the existing one is reused.
The
.Fa ctx
must have been created with
.Fn HMAC_CTX_new
before the first use in this function.
If
.Fn HMAC_Init_ex
is called with a
.Dv NULL
.Fa key
but
.Fa evp_md
is neither
.Dv NULL
nor the same as the previous digest used by
.Fa ctx ,
then an error is returned because reuse of an existing key with a
different digest is not supported.
.Pp
.Fn HMAC_Init
is a deprecated wrapper around
.Fn HMAC_Init_ex
which performs no longer useful extra initialization in
some circumstances.
.Pp
.Fn HMAC_Update
can be called repeatedly with chunks of the message to be authenticated
.Pq Fa len No bytes at Fa data .
.Pp
.Fn HMAC_Final
places the message authentication code in
.Fa md ,
which must have space for the hash function output.
.Pp
.Fn HMAC_CTX_copy
copies all of the internal state from
.Fa sctx
into
.Fa dctx .
.Pp
.Fn HMAC_CTX_set_flags
applies the specified flags to the internal
.Vt EVP_MD_CTX
objects.
Possible flag values
.Dv EVP_MD_CTX_FLAG_*
are defined in
.In openssl/evp.h .
.Pp
.Fn HMAC_size
returns the length in bytes of the underlying hash function output.
It is implemented as a macro.
.Sh RETURN VALUES
.Fn HMAC
returns a pointer to the message authentication code or
.Dv NULL
if an error occurred.
.Pp
.Fn HMAC_CTX_new
returns a pointer to the new
.Vt HMAC_CTX
object or
.Dv NULL
if an error occurred.
.Pp
.Fn HMAC_CTX_reset ,
.Fn HMAC_Init_ex ,
.Fn HMAC_Update ,
.Fn HMAC_Final ,
and
.Fn HMAC_CTX_copy
return 1 for success or 0 if an error occurred.
.Pp
.Fn HMAC_CTX_get_md
returns the message digest that was previously set for
.Fa ctx
with
.Fn HMAC_Init_ex ,
or
.Dv NULL
if none was set.
.Pp
.Fn HMAC_size
returns the length in bytes of the underlying hash function output
or 0 on error.
.Sh SEE ALSO
.Xr CMAC_Init 3 ,
.Xr EVP_DigestInit 3
.Sh STANDARDS
RFC 2104
.Sh HISTORY
.Fn HMAC ,
.Fn HMAC_Init ,
.Fn HMAC_Update ,
.Fn HMAC_Final ,
and
.Fn HMAC_size
first appeared in SSLeay 0.9.0 and have been available since
.Ox 2.4 .
.Pp
.Fn HMAC_Init_ex
first appeared in OpenSSL 0.9.7 and have been available since
.Ox 3.2 .
.Pp
.Fn HMAC_CTX_set_flags
first appeared in OpenSSL 0.9.7f and have been available since
.Ox 3.8 .
.Pp
.Fn HMAC_CTX_copy
first appeared in OpenSSL 1.0.0 and has been available since
.Ox 4.9 .
.Pp
.Fn HMAC_CTX_new ,
.Fn HMAC_CTX_reset ,
.Fn HMAC_CTX_free ,
and
.Fn HMAC_CTX_get_md
first appeared in OpenSSL 1.1.0 and have been available since
.Ox 6.3 .