minix/lib/libc/string/consttime_memequal.3

.\"	$NetBSD: consttime_memequal.3,v 1.2 2013/08/28 15:24:41 riastradh Exp $
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.Dd August 28, 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 distinct and nonzero 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.
.Sh SEE ALSO
.Xr explicit_memset 3 ,
.Xr memcmp 3
.Sh HISTORY
The
.Fn consttime_memequal
function appeared in
.Nx 7.0 .