.\" $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 .\" .\" 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. 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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) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, .\" STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) .\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED .\" OF THE POSSIBILITY OF SUCH DAMAGE. .\" .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 .