BN_NUM_BYTES(3ossl)

💖 BN_NUM_BYTES 💖

okay bestie so this is the BN_NUM_BYTES(3ossl) page from the OpenSSL manual, and it's literally all about asking your BIGNUM "girl how big are you actually" 💅 three different flavours of that question, to be exact.

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✨ NAME ✨

BN_num_bits, BN_num_bytes, BN_num_bits_word — get BIGNUM size 🎀

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✨ SYNOPSIS ✨

#include <openssl/bn.h>

int BN_num_bytes(const BIGNUM *a);

int BN_num_bits(const BIGNUM *a);

int BN_num_bits_word(BN_ULONG w);

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✨ DESCRIPTION ✨

so here's the tea, one function at a time 🫖

• BN_num_bytes() — hands you the size of a BIGNUM in bytes. that's it, that's the whole job, and she does it flawlessly. 💖
• BN_num_bits_word() — returns the number of significant bits in a word. ok let's do the classic example: take 0x00000432. she says 11. not 16. not 32. eleven. ✨ basically — except for a zero — she returns floor(log2(w))+1. math queen behavior, no cap. 🧮👑
• BN_num_bits() — same energy as BN_num_bits_word() but for a whole BIGNUM: she gives you the number of significant bits, following that exact same principle. 🌸

oh and important lil detail: BN_num_bytes() is a macro. not a real function, a macro. just so you know what you're dealing with, bestie 🦋

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✨ RETURN VALUES ✨

The size. that's it. that's the answer. iconic, unbothered, brief. 💅

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✨ NOTES ✨

ok so listen, this is the part where people get confused and honestly I get it 😭

some people have tried calling BN_num_bits() on individual numbers inside RSA keys, DH keys, and DSA keys, and found that it doesn't always give back the number of bits they expected — like, they were expecting something like 512, 1024, 2048, ... and got something else. 🙈

and the reason is: generating a number with some specific number of bits doesn't always set the highest bits, which makes the number of significant bits come out a little lower. the vibe check fails, basically. 💀

so if you actually want to know the "key size" of a key like that, don't do the naive thing! instead either use functions like RSA_size(), DH_size() and DSA_size(), or use BN_num_bytes() and multiply with 8. and ngl, even then there's no real guarantee that it'll match the "key size" — it's just a lot more probability. so like. set your expectations accordingly, bestie. 🫶

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✨ SEE ALSO ✨

DH_size(3), DSA_size(3), RSA_size(3) — go read these, they're the moment 🔥

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✨ COPYRIGHT ✨

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>.

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and that's the whole page, bestie!! 🎀 this was OpenSSL 3.5.9, dated 2026-09-29 — so if you're reading this in the future, hiii, hope your BIGNUMs are all behaving 💖✨🦋 mwah, go count some bits, ily 🫶💅