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+.\" $NetBSD: consttime_memequal.3,v 1.1 2013/06/24 04:21:20 riastradh Exp $
+.\"
+.\" Copyright (c) 2013 The NetBSD Foundation, Inc.
+.\" All rights reserved.
+.\"
+.\" This documentation is derived from text contributed to The NetBSD
+.\" Foundation by Taylor R. Campbell.
+.\"
+.\" 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.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. 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 FOUNDATION 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.
+.\"
+.Dd June 23, 2013
+.Dt CONSTTIME_MEMEQUAL 3
+.Os
+.Sh NAME
+.Nm consttime_memequal
+.Nd compare byte strings for equality without timing leaks
+.Sh LIBRARY
+.Lb libc
+.Sh SYNOPSIS
+.In string.h
+.Ft int
+.Fn consttime_memequal "void *b1" "void *b2" "size_t len"
+.Sh DESCRIPTION
+The
+.Fn consttime_memequal
+function compares
+.Fa len
+bytes of memory at
+.Fa b1
+and
+.Fa b2
+for equality, returning zero if they are identical and nonzero
+otherwise.
+.Pp
+The time taken by
+.Fn consttime_memequal
+depends on
+.Fa len ,
+but not on the data at
+.Fa b1
+or
+.Fa b2 .
+Thus,
+.Fn consttime_memequal
+is appropriate for comparing cryptographic secrets, hashes, message
+authentication codes, etc., without leaking information about them
+through a timing side channel.
+In crypto literature,
+.Fn consttime_memequal
+is said to take
+.Sq constant time ,
+meaning time that does not vary depending on the data it processes.
+.Pp
+Note that unlike
+.Xr memcmp 3 ,
+.Fn consttime_memequal
+does not return a lexicographic ordering on the data at
+.Fa b1
+and
+.Fa b2 ;
+it tells only whether they are equal.
+.Sh SEE ALSO
+.Xr explicit_memset 3 ,
+.Xr memcmp 3
+.Sh HISTORY
+The
+.Fn consttime_memequal
+function appeared in
+.Nx 7.0 .