BN_MOD_MUL_RECIPROCAL(3ossl)

📦
version 15.1 · 2 versions
, in freebsd 15.1

💅 BN_MOD_MUL_RECIPROCAL — modular multiplication using reciprocal ✨

okay so bestie, this is the 3ossl page for the OpenSSL BIGNUM reciprocal squad 🎀 — yes, it's a whole family of functions, and yes, they're all here:

BN_mod_mul_reciprocal, BN_div_recp, BN_RECP_CTX_new, BN_RECP_CTX_free, BN_RECP_CTX_set — all doing modular multiplication using reciprocal. no gatekeeping, we're covering every single one 💖

🌸 SYNOPSIS 🌸

like, here's the whole cast, literally:

#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);

copy it exactly, don't be creative with the headers, i'm begging 🫶

✨ DESCRIPTION ✨

🧮 BN_mod_mul_reciprocal()

bestie this is the main character 👑. Use it when you're doing an efficient BN_mod_mul(3) operation and you're gonna be doing it over and over with the same modulus — that's the whole vibe check it passes. It computes r=(a*b)%m using recp=1/m, which you set up the way described below. And ctx? That's a previously allocated BN_CTX used for temporary variables — it's giving backstage crew, lowkey essential 🎀

🦋 BN_RECP_CTX_new()

Allocates and initializes a BN_RECP structure. brand new, fresh out the box ✨

🎀 BN_RECP_CTX_free()

Frees the components of the BN_RECP, and — if it was created by BN_RECP_CTX_new() — also the structure itself. and hey, if recp is NULL, nothing is done. no cap, it just minds its business 🙈

💜 BN_RECP_CTX_set()

Stores m in recp and sets it up for computing 1/m and 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 will later be stored in recp. it's doing the math so future-you doesn't have to 💅

🍓 BN_div_recp()

Divides a by m using recp. It places the quotient in dv and the remainder in rem. two outputs, one call, we love efficiency 🌈

⚠️ ok important thing, listen up: the BN_RECP_CTX structure cannot be shared between threads. like, do not pass it around the group chat. fr fr, it's a one-thread-at-a-time icon 💀

🔥 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's giving "no thoughts, just vibes".
  • For the other functions, 1 is returned for success, 0 on error. The error codes can be obtained by ERR_get_error(3). 🧚

🌟 SEE ALSO 🌟

go read these too bestie, they're all linked, iykyk:

🧚 HISTORY 🧚

BN_RECP_CTX_init() was removed in OpenSSL 1.1.0. rip to a real one 🕯️ — don't go looking for it, it's literally not here anymore.

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>. 🎀

okay that's the whole page bestie, you ate that 💅✨ — this was version 3.5.6, dated 2026-04-07, the BN_MOD_MUL_RECIPROCAL(3ossl) of it all. mwah, go multiply something reciprocally 🫶💜

📦 where this page comes from