BN_MOD_MUL_RECIPROCAL(3ossl)

📦 version 3.5.0, in opensuse-leap 16.0

💅 BN_MOD_MUL_RECIPROCAL(3ossl)

okay bestie, so this is page BN_MOD_MUL_RECIPROCAL(3ossl) straight outta the OpenSSL manual, and it's lowkey iconic — it's all about doing modular multiplication the fast way when you're gonna do it over and over with the same modulus. 🎀

✨ NAME ✨

BN_mod_mul_reciprocal, BN_div_recp, BN_RECP_CTX_new, BN_RECP_CTX_free, BN_RECP_CTX_set — modular multiplication using reciprocal. periodt. 💅

🌸 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 🦋

okay so the whole vibe here: precompute once, slay forever. 💖

  • BN_mod_mul_reciprocal() — this one is for when you're doing an efficient BN_mod_mul(3) operation, but like, repeatedly, with the same modulus. It computes r=(a*b)%m using recp=1/m, which you set up as described below. ctx is a previously allocated BN_CTX that it uses for temporary variables. no cap, that's the whole trick.
  • BN_RECP_CTX_new() — allocates and initializes a BN_RECP structure. freshly allocated, main character energy. ✨
  • 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 if recp is NULL, nothing is done — it literally just minds its own 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. math girlboss behaviour fr fr. 📐
  • 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. 💅
⚠️ bestie, important: the BN_RECP_CTX structure cannot be shared between threads. do not pass it around the group chat. 🙈

👑 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. 💀
  • 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 🌈

ERR_get_error(3), BN_add(3), BN_CTX_new(3) — go read these next, iykyk. 🧚

⭐ HISTORY ⭐

BN_RECP_CTX_init() was removed in OpenSSL 1.1.0. gone but not forgotten, rip bozo. 🕯️

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 babe, you ate. 🍓 this was version 3.5.0, dated 2026-09-25, and it's the BN_MOD_MUL_RECIPROCAL(3ossl) page — mwah, go multiply something modularly 💋✨

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