BN_MOD_MUL_RECIPROCAL(3ossl)

📦
version 3.5.9 · 8 versions
, in fedora 43, fedora 44

💖 BN_MOD_MUL_RECIPROCAL(3ossl) 💖

okay bestie, so this is the OpenSSL manpage for modular multiplication using reciprocal, and honestly? it's giving "small but mighty" ✨ it's from the 3ossl manual, and we're about to make reciprocal-based modular math make sense. no cap.

🌸 NAME 🌸

BN_mod_mul_reciprocal, BN_div_recp, BN_RECP_CTX_new, BN_RECP_CTX_free, BN_RECP_CTX_set — modular multiplication using reciprocal 🎀

✨ SYNOPSIS ✨

#include <openssl/bn.h>

BN_RECP_CTX *BN_RECP_CTX_new(void);
void BN_RECP_CTX_free(BN_RECP_CTX *recp);

int BN_RECP_CTX_set(BN_RECP_CTX *recp, const BIGNUM *m, BN_CTX *ctx);

int BN_div_recp(BIGNUM *dv, BIGNUM *rem, const BIGNUM *a, BN_RECP_CTX *recp,
                BN_CTX *ctx);

int BN_mod_mul_reciprocal(BIGNUM *r, const BIGNUM *a, const BIGNUM *b,
                          BN_RECP_CTX *recp, BN_CTX *ctx);

🦋 DESCRIPTION 🦋

so here's the tea 🍓 BN_mod_mul_reciprocal() is your go-to when you gotta do a BN_mod_mul(3) operation efficiently — but only when you're gonna do it over and over and over again with the same modulus. it computes r=(a*b)%m using recp=1/m, which gets set up the way we're about to describe below, iykyk. and ctx? that's a previously allocated BN_CTX you use for temporary variables, periodt.

BN_RECP_CTX_new() allocates and initializes a BN_RECP structure. iconic, love that for us 💅

BN_RECP_CTX_free() frees the components of the BN_RECP, and — if it was created by BN_RECP_CTX_new() — it also frees the structure itself. oh and if recp is NULL? nothing happens. it just chills. 🙈

BN_RECP_CTX_set() stores m in recp and sets it up for computing 1/m, then shifting it left by BN_num_bits(m)+1 to make it an integer. the result and the number of bits it was shifted left get stored in recp later. slay. ✨

BN_div_recp() divides a by m using recp. quotient goes in dv, remainder goes in rem. very demure, very mindful. 🎀

⚠️ okay important bestie: the BN_RECP_CTX structure cannot be shared between threads. like, at all. no sharing. it's not that kind of structure. 💀

👑 RETURN VALUES 👑

BN_RECP_CTX_new() returns the newly allocated BN_RECP_CTX, and NULL on error. 😭

BN_RECP_CTX_free() has no return value. it just does its thing and leaves. main character exit. 🚪✨

for the other functions: 1 is returned for success, 0 on error. the error codes can be obtained by ERR_get_error(3). so if it returns 0, go check those codes, fr fr. 🔍

🌈 SEE ALSO 🌈

💜 HISTORY 💜

BN_RECP_CTX_init() was removed in OpenSSL 1.1.0. gone. not invited to the function party anymore. rip bozo 🪦 (affectionately)

Copyright 2000-2017 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (the "License"). You may not use this file except in compliance with the License. You can obtain a copy in the file LICENSE in the source distribution or at <https://www.openssl.org/source/license.html>.

and that's the whole page bestie!! 🫶 this was version 3.5.9, dated 2026-09-29, straight from the OpenSSL crew — the BN_MOD_MUL_RECIPROCAL(3ossl) girlies ate and left no crumbs 👑💖 mwah, go do some modular multiplication with your reciprocal, you got this 🌸🧚

📦 where this page comes from