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.\" Copyright (c) 1990, 1991 The Regents of the University of California.
.\" All rights reserved.
.\"
.\" This code is derived from software contributed to Berkeley by
.\" Chris Torek.
.\" 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. Neither the name of the University nor the names of its contributors
.\" may be used to endorse or promote products derived from this software
.\" without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS 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 REGENTS OR 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.
.\"
.\" $OpenBSD: bcmp.3,v 1.9 2011/07/25 00:38:52 schwarze Exp $
.\"
.Dd $Mdocdate: July 25 2011 $
.Dt BCMP 3
.Os
.Sh NAME
.Nm bcmp ,
.Nm timingsafe_bcmp
.Nd compare byte string
.Sh SYNOPSIS
.Fd #include <string.h>
.Ft int
.Fn bcmp "const void *b1" "const void *b2" "size_t len"
.Ft int
.Fn timingsafe_bcmp "const void *b1" "const void *b2" "size_t len"
.Sh DESCRIPTION
The
.Fn bcmp
function compares byte string
.Fa b1
against byte string
.Fa b2 ,
returning zero if they are identical, non-zero otherwise.
Both strings are assumed to be
.Fa len
bytes long.
Zero-length strings are always identical.
.Pp
The strings may overlap.
.Pp
The
.Fn timingsafe_bcmp
function has the same semantics as
.Fn bcmp ,
but its running time is independent of the contents of
.Fa b1
and
.Fa b2 ,
making it safe to use for comparing secret values such as cryptographic MACs.
In contrast,
.Fn bcmp
returns after finding the first differing byte,
making it vulnerable to timing attacks.
.Sh SEE ALSO
.Xr memcmp 3 ,
.Xr strcasecmp 3 ,
.Xr strcmp 3 ,
.Xr strcoll 3 ,
.Xr strxfrm 3
.Sh HISTORY
The
.Fn bcmp
function first appeared in
.Bx 4.2 .
.Pp
The
.Fn timingsafe_bcmp
function first appeared in
.Ox 4.9 .
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