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.\"	$OpenBSD: bcmp.3,v 1.8 2010/09/24 13:33:00 matthew Exp $
.\"
.Dd $Mdocdate: September 24 2010 $
.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
A
.Fn bcmp
function first appeared in
.Bx 4.2 .
.Pp
The
.Fn timingsafe_bcmp
function first appeared in
.Ox 4.9 .