1.\" Copyright (c) 1990, 1991 The Regents of the University of California. 2.\" All rights reserved. 3.\" 4.\" This code is derived from software contributed to Berkeley by 5.\" Chris Torek. 6.\" Redistribution and use in source and binary forms, with or without 7.\" modification, are permitted provided that the following conditions 8.\" are met: 9.\" 1. Redistributions of source code must retain the above copyright 10.\" notice, this list of conditions and the following disclaimer. 11.\" 2. Redistributions in binary form must reproduce the above copyright 12.\" notice, this list of conditions and the following disclaimer in the 13.\" documentation and/or other materials provided with the distribution. 14.\" 3. Neither the name of the University nor the names of its contributors 15.\" may be used to endorse or promote products derived from this software 16.\" without specific prior written permission. 17.\" 18.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 19.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 20.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 21.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 22.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 23.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 24.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 25.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 26.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 27.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 28.\" SUCH DAMAGE. 29.\" 30.\" $OpenBSD: bcmp.3,v 1.9 2011/07/25 00:38:52 schwarze Exp $ 31.\" 32.Dd $Mdocdate: July 25 2011 $ 33.Dt BCMP 3 34.Os 35.Sh NAME 36.Nm bcmp , 37.Nm timingsafe_bcmp 38.Nd compare byte string 39.Sh SYNOPSIS 40.Fd #include <string.h> 41.Ft int 42.Fn bcmp "const void *b1" "const void *b2" "size_t len" 43.Ft int 44.Fn timingsafe_bcmp "const void *b1" "const void *b2" "size_t len" 45.Sh DESCRIPTION 46The 47.Fn bcmp 48function compares byte string 49.Fa b1 50against byte string 51.Fa b2 , 52returning zero if they are identical, non-zero otherwise. 53Both strings are assumed to be 54.Fa len 55bytes long. 56Zero-length strings are always identical. 57.Pp 58The strings may overlap. 59.Pp 60The 61.Fn timingsafe_bcmp 62function has the same semantics as 63.Fn bcmp , 64but its running time is independent of the contents of 65.Fa b1 66and 67.Fa b2 , 68making it safe to use for comparing secret values such as cryptographic MACs. 69In contrast, 70.Fn bcmp 71returns after finding the first differing byte, 72making it vulnerable to timing attacks. 73.Sh SEE ALSO 74.Xr memcmp 3 , 75.Xr strcasecmp 3 , 76.Xr strcmp 3 , 77.Xr strcoll 3 , 78.Xr strxfrm 3 79.Sh HISTORY 80The 81.Fn bcmp 82function first appeared in 83.Bx 4.2 . 84.Pp 85The 86.Fn timingsafe_bcmp 87function first appeared in 88.Ox 4.9 . 89