BN_ZERO(3ossl)

✨ BN_ZERO — bestie we're doing BIGNUM assignments 💖

okay so this is the BN_ZERO(3ossl) page from the OpenSSL manual, and it's literally all about the glow-up of setting a BIGNUM to a value. no cap, it's shorter than you'd think but every line eats. 🫶

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🌸 NAME 🌸

BN_zero, BN_one, BN_value_one, BN_set_word, BN_get_word — the BIGNUM assignment operations 💅

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🎀 SYNOPSIS 🎀

#include <openssl/bn.h>

void BN_zero(BIGNUM *a);
int BN_one(BIGNUM *a);

const BIGNUM *BN_value_one(void);

int BN_set_word(BIGNUM *a, BN_ULONG w);
unsigned BN_ULONG BN_get_word(BIGNUM *a);

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🦋 DESCRIPTION 🦋

first up, the lore: BN_ULONG is a macro that'll be an unsigned integral type, and it's optimized for whatever is the most efficient implementation on the local platform. it's giving ✨ bespoke ✨, it's giving built for your machine specifically. 💖

BN_zero(), BN_one() and BN_set_word() set a to the values 0, 1 and w respectively. and like, tiny detail but important: BN_zero() and BN_one() are macros. 🫶

BN_value_one() returns a BIGNUM constant of value 1. this constant is useful for use in comparisons and assignment — it's basically the main character of constants, lowkey iconic. 👑

BN_get_word() returns a, if it can be represented as a BN_ULONG. that "if" is doing a lot of work bestie, keep an eye on it. 👀

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⭐ RETURN VALUES ⭐

• BN_get_word() returns the value a, or all-bits-set if a cannot be represented as a single integer. 😭
• BN_one() and BN_set_word() return 1 on success, 0 otherwise.
• BN_value_one() returns the constant. 💜
• BN_zero() never fails and returns no value. it simply cannot flop, periodt.

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💀 BUGS 💀

okay so this is the messy part, and it's fr fr important:

if a BIGNUM is equal to the value of all-bits-set, it will collide with the error condition returned by BN_get_word(), which uses that as an error value. so like, the "error" signal and a totally legit number are wearing the same outfit to the party. 🙈

also BN_ULONG should probably be a typedef. that's just the tea. 🍓

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🌈 SEE ALSO 🌈

• BN_bn2bin(3)

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🧚 HISTORY 🧚

back in OpenSSL 0.9.8, BN_zero() was changed to not return a value; previous versions returned an int. character development honestly. 💅

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👑 COPYRIGHT 👑

Copyright 2000-2018 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>.

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and that's the whole page bestie, you understood it, you ate, no crumbs. 🫶✨

this was OpenSSL 3.5.9, dated 2026-09-29, and it's the BN_ZERO(3ossl) manual — ily, go set some bignums 💖🎀🦋