1.\" $NetBSD: consttime_memequal.3,v 1.2 2013/08/28 15:24:41 riastradh Exp $ 2.\" 3.\" Copyright (c) 2013 The NetBSD Foundation, Inc. 4.\" All rights reserved. 5.\" 6.\" This documentation is derived from text contributed to The NetBSD 7.\" Foundation by Taylor R. Campbell. 8.\" 9.\" Redistribution and use in source and binary forms, with or without 10.\" modification, are permitted provided that the following conditions 11.\" are met: 12.\" 1. Redistributions of source code must retain the above copyright 13.\" notice, this list of conditions and the following disclaimer. 14.\" 2. Redistributions in binary form must reproduce the above copyright 15.\" notice, this list of conditions and the following disclaimer in the 16.\" documentation and/or other materials provided with the distribution. 17.\" 18.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS 19.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED 20.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR 21.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS 22.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR 23.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF 24.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS 25.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN 26.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 27.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE 28.\" POSSIBILITY OF SUCH DAMAGE. 29.\" 30.Dd August 28, 2013 31.Dt CONSTTIME_MEMEQUAL 3 32.Os 33.Sh NAME 34.Nm consttime_memequal 35.Nd compare byte strings for equality without timing leaks 36.Sh LIBRARY 37.Lb libc 38.Sh SYNOPSIS 39.In string.h 40.Ft int 41.Fn consttime_memequal "void *b1" "void *b2" "size_t len" 42.Sh DESCRIPTION 43The 44.Fn consttime_memequal 45function compares 46.Fa len 47bytes of memory at 48.Fa b1 49and 50.Fa b2 51for equality, returning zero if they are distinct and nonzero if they 52are identical. 53.Pp 54The time taken by 55.Fn consttime_memequal 56depends on 57.Fa len , 58but not on the data at 59.Fa b1 60or 61.Fa b2 . 62Thus, 63.Fn consttime_memequal 64is appropriate for comparing cryptographic secrets, hashes, message 65authentication codes, etc., without leaking information about them 66through a timing side channel. 67In crypto literature, 68.Fn consttime_memequal 69is said to take 70.Sq constant time , 71meaning time that does not vary depending on the data it processes. 72.Pp 73Note that unlike 74.Xr memcmp 3 , 75.Fn consttime_memequal 76does not return a lexicographic ordering on the data at 77.Fa b1 78and 79.Fa b2 ; 80it tells only whether they are equal. 81.Sh SEE ALSO 82.Xr explicit_memset 3 , 83.Xr memcmp 3 84.Sh HISTORY 85The 86.Fn consttime_memequal 87function appeared in 88.Nx 7.0 . 89