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.\" $OpenBSD: CRYPTO_memcmp.3,v 1.1 2019/08/25 06:20:22 schwarze Exp $
.\" full merge up to: OpenSSL 1075139c Jun 24 09:18:48 2019 +1000
.\"
.\" This file was written by Pauli <paul.dale@oracle.com>.
.\" Copyright (c) 2019 The OpenSSL Project. All rights reserved.
.\"
.\" 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.
.\"
.\" 3. All advertising materials mentioning features or use of this
.\" software must display the following acknowledgment:
.\" "This product includes software developed by the OpenSSL Project
.\" for use in the OpenSSL Toolkit. (http://www.openssl.org/)"
.\"
.\" 4. The names "OpenSSL Toolkit" and "OpenSSL Project" must not be used to
.\" endorse or promote products derived from this software without
.\" prior written permission. For written permission, please contact
.\" openssl-core@openssl.org.
.\"
.\" 5. Products derived from this software may not be called "OpenSSL"
.\" nor may "OpenSSL" appear in their names without prior written
.\" permission of the OpenSSL Project.
.\"
.\" 6. Redistributions of any form whatsoever must retain the following
.\" acknowledgment:
.\" "This product includes software developed by the OpenSSL Project
.\" for use in the OpenSSL Toolkit (http://www.openssl.org/)"
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE OpenSSL PROJECT ``AS IS'' AND ANY
.\" EXPRESSED 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 OpenSSL PROJECT OR
.\" ITS 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)
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.\"
.Dd $Mdocdate: August 25 2019 $
.Dt CRYPTO_MEMCMP 3
.Os
.Sh NAME
.Nm CRYPTO_memcmp
.Nd constant time memory comparison
.Sh SYNOPSIS
.In openssl/crypto.h
.Ft int
.Fo CRYPTO_memcmp
.Fa "const void *a"
.Fa "const void *b"
.Fa "size_t len"
.Fc
.Sh DESCRIPTION
.Fn CRYPTO_memcmp
compares the
.Fa len
bytes pointed to by
.Fa a
and
.Fa b
for equality.
It takes an amount of time dependent on
.Fa len ,
but independent of the contents of the memory regions pointed to by
.Fa a
and
.Fa b .
.Sh RETURN VALUES
.Fn CRYPTO_memcmp
returns 0 if the content of the memory regions is equal
or non-zero otherwise.
.Sh HISTORY
.Fn CRYPTO_memcmp
first appeared in OpenSSL 1.0.1d and has been available since
.Ox 5.6 .
.Sh BUGS
Unlike
.Xr memcmp 3
and
.Xr timingsafe_memcmp 3 ,
this function cannot be used to order the two memory regions.
In the current implementation, the return value is always greater
than or equal to 0.
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