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.\" $OpenBSD: BN_swap.3,v 1.6 2021/12/19 22:06:35 schwarze Exp $
.\" full merge up to: OpenSSL 61f805c1 Jan 16 01:01:46 2018 +0800
.\"
.\" This file is a derived work.
.\" The changes are covered by the following Copyright and license:
.\"
.\" Copyright (c) 2021 Ingo Schwarze <schwarze@openbsd.org>
.\"
.\" Permission to use, copy, modify, and distribute this software for any
.\" purpose with or without fee is hereby granted, provided that the above
.\" copyright notice and this permission notice appear in all copies.
.\"
.\" THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
.\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
.\" MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
.\" ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
.\" WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
.\" ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
.\" OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
.\"
.\" The original file was written by Bodo Moeller <bodo@openssl.org>.
.\" Copyright (c) 2000 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
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.\" openssl-core@openssl.org.
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.\"
.Dd $Mdocdate: December 19 2021 $
.Dt BN_SWAP 3
.Os
.Sh NAME
.Nm BN_swap ,
.Nm BN_consttime_swap
.Nd exchange BIGNUMs
.Sh SYNOPSIS
.In openssl/bn.h
.Ft void
.Fo BN_swap
.Fa "BIGNUM *a"
.Fa "BIGNUM *b"
.Fc
.Ft void
.Fo BN_consttime_swap
.Fa "BN_ULONG condition"
.Fa "BIGNUM *a"
.Fa "BIGNUM *b"
.Fa "int nwords"
.Fc
.Sh DESCRIPTION
.Fn BN_swap
and
.Fn BN_consttime_swap
exchange the values of
.Fa a
and
.Fa b .
.Pp
.Fn BN_swap
implements this by exchanging the pointers to the data buffers of
.Fa a
and
.Fa b
and also exchanging the values of the
.Dv BN_FLG_STATIC_DATA
bits.
Consequently, the operation is fast and execution time does not depend
on any properties of the two numbers.
However, execution time obviously differs between swapping (by calling
this function) and not swapping (by not calling this function).
.Pp
.Fn BN_consttime_swap
only performs the exchange if the
.Fa condition
is non-zero; otherwise, it has no effect.
It implements the exchange by exchanging the contents of the data
buffers rather than the pointers to the data buffers.
This is slower, but implemented in such a way that the execution time
is not only independent of the properties of the two numbers, but also
independent of the
.Fa condition
argument, i.e. the same for swapping or not swapping.
Execution time does however grow in an approximately linear manner with the
.Fa nwords
argument.
.Pp
.Fn BN_consttime_swap
calls
.Xr abort 3
if at least one of
.Fa a
or
.Fa b
has fewer than
.Fa nwords
data words allocated or more than
.Fa nwords
data words are currently in use in at least one of them.
.Sh SEE ALSO
.Xr BN_new 3 ,
.Xr BN_set_flags 3
.Sh HISTORY
.Fn BN_swap
first appeared in OpenSSL 0.9.7 and has been available since
.Ox 3.2 .
.Pp
.Fn BN_consttime_swap
first appeared in OpenSSL 1.0.1g and has been available since
.Ox 5.6 .
|
