1.\" $OpenBSD: CRYPTO_memcmp.3,v 1.1 2019/08/25 06:20:22 schwarze Exp $ 2.\" full merge up to: OpenSSL 1075139c Jun 24 09:18:48 2019 +1000 3.\" 4.\" This file was written by Pauli <paul.dale@oracle.com>. 5.\" Copyright (c) 2019 The OpenSSL Project. All rights reserved. 6.\" 7.\" Redistribution and use in source and binary forms, with or without 8.\" modification, are permitted provided that the following conditions 9.\" are met: 10.\" 11.\" 1. Redistributions of source code must retain the above copyright 12.\" notice, this list of conditions and the following disclaimer. 13.\" 14.\" 2. Redistributions in binary form must reproduce the above copyright 15.\" notice, this list of conditions and the following disclaimer in 16.\" the documentation and/or other materials provided with the 17.\" distribution. 18.\" 19.\" 3. All advertising materials mentioning features or use of this 20.\" software must display the following acknowledgment: 21.\" "This product includes software developed by the OpenSSL Project 22.\" for use in the OpenSSL Toolkit. (http://www.openssl.org/)" 23.\" 24.\" 4. The names "OpenSSL Toolkit" and "OpenSSL Project" must not be used to 25.\" endorse or promote products derived from this software without 26.\" prior written permission. For written permission, please contact 27.\" openssl-core@openssl.org. 28.\" 29.\" 5. Products derived from this software may not be called "OpenSSL" 30.\" nor may "OpenSSL" appear in their names without prior written 31.\" permission of the OpenSSL Project. 32.\" 33.\" 6. Redistributions of any form whatsoever must retain the following 34.\" acknowledgment: 35.\" "This product includes software developed by the OpenSSL Project 36.\" for use in the OpenSSL Toolkit (http://www.openssl.org/)" 37.\" 38.\" THIS SOFTWARE IS PROVIDED BY THE OpenSSL PROJECT ``AS IS'' AND ANY 39.\" EXPRESSED OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 40.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR 41.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE OpenSSL PROJECT OR 42.\" ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, 43.\" SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 44.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; 45.\" LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 46.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, 47.\" STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 48.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED 49.\" OF THE POSSIBILITY OF SUCH DAMAGE. 50.\" 51.Dd $Mdocdate: August 25 2019 $ 52.Dt CRYPTO_MEMCMP 3 53.Os 54.Sh NAME 55.Nm CRYPTO_memcmp 56.Nd constant time memory comparison 57.Sh SYNOPSIS 58.In openssl/crypto.h 59.Ft int 60.Fo CRYPTO_memcmp 61.Fa "const void *a" 62.Fa "const void *b" 63.Fa "size_t len" 64.Fc 65.Sh DESCRIPTION 66.Fn CRYPTO_memcmp 67compares the 68.Fa len 69bytes pointed to by 70.Fa a 71and 72.Fa b 73for equality. 74It takes an amount of time dependent on 75.Fa len , 76but independent of the contents of the memory regions pointed to by 77.Fa a 78and 79.Fa b . 80.Sh RETURN VALUES 81.Fn CRYPTO_memcmp 82returns 0 if the content of the memory regions is equal 83or non-zero otherwise. 84.Sh HISTORY 85.Fn CRYPTO_memcmp 86first appeared in OpenSSL 1.0.1d and has been available since 87.Ox 5.6 . 88.Sh BUGS 89Unlike 90.Xr memcmp 3 91and 92.Xr timingsafe_memcmp 3 , 93this function cannot be used to order the two memory regions. 94In the current implementation, the return value is always greater 95than or equal to 0. 96