.\"	$NetBSD: consttime_memequal.3,v 1.5 2015/03/23 07:41:16 apb Exp $
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.Dd March 23, 2015
.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 0 if they are distinct and 1 if they are
identical.
.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.
.Nx
does not provide a
.Fn consttime_memcmp
function, because all known use cases that require
.Sq constant time
memory comparison also require only comparison for equality,
not lexicographic ordering.
.Sh SEE ALSO
.Xr explicit_memset 3 ,
.Xr memcmp 3
.Sh HISTORY
The
.Fn consttime_memequal
function appeared in
.Nx 7.0 .