document BN_consttime_swap(3); this will probably require more work,

but what i have so far is already better than nothing

diff --git a/lib/libcrypto/man/BN_swap.3 b/lib/libcrypto/man/BN_swap.3
index db9082d..218ca1c 100644 (file)
--- a/lib/libcrypto/man/BN_swap.3
+++ b/lib/libcrypto/man/BN_swap.3
@@ -1,7 +1,24 @@
-.\"    $OpenBSD: BN_swap.3,v 1.5 2018/03/22 21:08:22 schwarze Exp $
-.\"    OpenSSL b97fdb57 Nov 11 09:33:09 2016 +0100
+.\" $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 was written by Bodo Moeller <bodo@openssl.org>.
+.\" 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
@@ -48,11 +65,12 @@
 .\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
 .\" OF THE POSSIBILITY OF SUCH DAMAGE.
 .\"
-.Dd $Mdocdate: March 22 2018 $
+.Dd $Mdocdate: December 19 2021 $
 .Dt BN_SWAP 3
 .Os
 .Sh NAME
-.Nm BN_swap
+.Nm BN_swap ,
+.Nm BN_consttime_swap
 .Nd exchange BIGNUMs
 .Sh SYNOPSIS
 .In openssl/bn.h
@@ -61,15 +79,70 @@
 .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
-exchanges the values of
+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_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 .