1.\" $OpenBSD: BN_mod_inverse.3,v 1.13 2023/10/21 13:53:43 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: October 21 2023 $ 52.Dt BN_MOD_INVERSE 3 53.Os 54.Sh NAME 55.Nm BN_mod_inverse 56.Nd compute inverse modulo m 57.Sh SYNOPSIS 58.In openssl/bn.h 59.Ft BIGNUM * 60.Fo BN_mod_inverse 61.Fa "BIGNUM *r" 62.Fa "const BIGNUM *a" 63.Fa "const BIGNUM *m" 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 m 72and places the result in 73.Fa r , 74so that 75.Fa r 76satisfies 77.Li a * r == 1 (mod m) . 78If 79.Fa r 80is 81.Dv NULL , 82a new 83.Vt BIGNUM 84is allocated. 85.Pp 86If the flag 87.Dv BN_FLG_CONSTTIME 88is set on 89.Fa a 90or 91.Fa m , 92it operates in constant time. 93.Pp 94.Fa ctx 95is a previously allocated 96.Vt BN_CTX 97used for temporary variables. 98.Fa r 99may be the same 100.Vt BIGNUM 101as 102.Fa a 103or 104.Fa m . 105.Sh RETURN VALUES 106.Fn BN_mod_inverse 107returns the 108.Vt BIGNUM 109containing the inverse, or 110.Dv NULL 111on error. 112The error codes can be obtained by 113.Xr ERR_get_error 3 . 114.Sh SEE ALSO 115.Xr BN_add 3 , 116.Xr BN_new 3 , 117.Xr BN_set_flags 3 118.Sh HISTORY 119.Fn BN_mod_inverse 120first appeared in SSLeay 0.5.1 and has been available since 121.Ox 2.4 . 122.Pp 123The 124.Fa r 125argument was added in SSLeay 0.9.1 and 126.Ox 2.6 . 127