1.\" $OpenBSD: BN_mod_inverse.3,v 1.10 2018/04/29 15:58:21 schwarze Exp $ 2.\" OpenSSL b97fdb57 Nov 11 09:33:09 2016 +0100 3.\" 4.\" This file was written by Ulf Moeller <ulf@openssl.org>. 5.\" Copyright (c) 2000 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: April 29 2018 $ 52.Dt BN_MOD_INVERSE 3 53.Os 54.Sh NAME 55.Nm BN_mod_inverse 56.Nd compute inverse modulo n 57.Sh SYNOPSIS 58.In openssl/bn.h 59.Ft BIGNUM * 60.Fo BN_mod_inverse 61.Fa "BIGNUM *r" 62.Fa "BIGNUM *a" 63.Fa "const BIGNUM *n" 64.Fa "BN_CTX *ctx" 65.Fc 66.Sh DESCRIPTION 67.Fn BN_mod_inverse 68computes the inverse of 69.Fa a 70modulo 71.Fa n 72and places the result in 73.Fa r 74.Pq Li (a*r)%n==1 . 75If 76.Fa r 77is 78.Dv NULL , 79a new 80.Vt BIGNUM 81is created. 82.Pp 83If the flag 84.Dv BN_FLG_CONSTTIME 85is set on 86.Fa a 87or 88.Fa n , 89it operates in constant time. 90.Pp 91.Fa ctx 92is a previously allocated 93.Vt BN_CTX 94used for temporary variables. 95.Fa r 96may be the same 97.Vt BIGNUM 98as 99.Fa a 100or 101.Fa n . 102.Sh RETURN VALUES 103.Fn BN_mod_inverse 104returns the 105.Vt BIGNUM 106containing the inverse, or 107.Dv NULL 108on error. 109The error codes can be obtained by 110.Xr ERR_get_error 3 . 111.Sh SEE ALSO 112.Xr BN_add 3 , 113.Xr BN_new 3 , 114.Xr BN_set_flags 3 115.Sh HISTORY 116.Fn BN_mod_inverse 117first appeared in SSLeay 0.5.1 and has been available since 118.Ox 2.4 . 119.Pp 120The 121.Fa r 122argument was added in SSLeay 0.9.1 and 123.Ox 2.6 . 124