BN_RAND(3SSL)

📦
version 4.0.3 · 14 versions
, in ubuntu stonking

💖 BN_RAND(3SSL) — the girlypop guide to making big random numbers 💖

okay bestie, this is the BN_RAND page from the OpenSSL manual, section 3SSL. it's all about generating pseudo-random numbers into BIGNUMs. cryptographically strong ones. the good stuff. let's get into it 🎀

✨ NAME ✨

BN_rand_ex, BN_rand, BN_priv_rand_ex, BN_priv_rand, BN_pseudo_rand, BN_rand_range_ex, BN_rand_range, BN_priv_rand_range_ex, BN_priv_rand_range, BN_pseudo_rand_range — generate pseudo-random number ✨

like... that's the whole job. make a random number. slay.

🌸 SYNOPSIS 🌸

#include <openssl/bn.h>

int BN_rand_ex(BIGNUM *rnd, int bits, int top, int bottom,
               unsigned int strength, BN_CTX *ctx);
int BN_rand(BIGNUM *rnd, int bits, int top, int bottom);

int BN_priv_rand_ex(BIGNUM *rnd, int bits, int top, int bottom,
                    unsigned int strength, BN_CTX *ctx);
int BN_priv_rand(BIGNUM *rnd, int bits, int top, int bottom);

int BN_rand_range_ex(BIGNUM *rnd, const BIGNUM *range, unsigned int strength,
                     BN_CTX *ctx);
int BN_rand_range(BIGNUM *rnd, const BIGNUM *range);

int BN_priv_rand_range_ex(BIGNUM *rnd, const BIGNUM *range, unsigned int strength,
                          BN_CTX *ctx);
int BN_priv_rand_range(BIGNUM *rnd, const BIGNUM *range);

okay and these next two? deprecated since OpenSSL 3.0 💀 you can make them vanish entirely by defining OPENSSL_API_COMPAT with a version value that fits, see openssl_user_macros(7):

int BN_pseudo_rand(BIGNUM *rnd, int bits, int top, int bottom);
int BN_pseudo_rand_range(BIGNUM *rnd, const BIGNUM *range);

🦋 DESCRIPTION 🦋

BN_rand_ex() generates a cryptographically strong pseudo-random number that's bits in length, with security strength at least strength bits, using the random number generator for the library context associated with ctx. it drops the generated data into rnd. ctx can be NULL and that's totally fine — then the default library context gets used. if bits is less than zero, or too small to fit whatever the top and bottom params are asking for, you get an error.

so the top param is the vibe check on the most significant bit of the number:

  • BN_RAND_TOP_ANY — no constraint at all, free spirit 🌈
  • BN_RAND_TOP_ONE — the top bit must be one
  • BN_RAND_TOP_TWO — the two most significant bits get set to 1, so that the product of two such random numbers will always have 2*bits length (iconic behaviour, honestly)

and bottom? if it's BN_RAND_BOTTOM_ODD the number will be odd. if it's BN_RAND_BOTTOM_ANY it can be odd or even, whatever it feels like.

⚠️ heads up: if bits is 1 then top cannot also be BN_RAND_TOP_TWO. no exceptions, bestie.

BN_rand() is the same as BN_rand_ex() except the default library context is always used. periodt.

BN_rand_range_ex() generates a cryptographically strong pseudo-random number rnd, with security strength at least strength bits, in the range 0 <= rnd < range, using the random number generator for the library context associated with ctx. and again, ctx may be NULL, in which case the default library context is used. 🫶

BN_rand_range() is the same as BN_rand_range_ex() except that the default library context is always used.

BN_priv_rand_ex(), BN_priv_rand(), BN_priv_rand_rand_ex() and BN_priv_rand_range() have the same semantics as BN_rand_ex(), BN_rand(), BN_rand_range_ex() and BN_rand_range() respectively. they're meant for generating values that should stay private 🔒 and they mirror the same difference between RAND_bytes(3) and RAND_priv_bytes(3).

💅 NOTES 💅

always check the error return value of these functions, and do not take randomness for granted. an error occurs if the CSPRNG has not been seeded with enough randomness to ensure an unpredictable byte sequence. like, that's the whole point, don't skip the vibe check just because it's ✨ random ✨.

⭐ RETURN VALUES ⭐

the functions return 1 on success, 0 on error. you can get the error codes with ERR_get_error(3).

👑 SEE ALSO 👑

ERR_get_error(3), RAND_add(3), RAND_bytes(3), RAND_priv_bytes(3), RAND(7), EVP_RAND(7)

🌈 HISTORY 🌈

  • starting with OpenSSL release 1.1.0, BN_pseudo_rand() has been identical to BN_rand() and BN_pseudo_rand_range() has been identical to BN_rand_range(). the BN_pseudo_rand() and BN_pseudo_rand_range() functions were deprecated in OpenSSL 3.0. 💀
  • the BN_priv_rand() and BN_priv_rand_range() functions were added in OpenSSL 1.1.1. 🎀
  • the BN_rand_ex(), BN_priv_rand_ex(), BN_rand_range_ex() and BN_priv_rand_range_ex() functions were added in OpenSSL 3.0. ✨

Copyright 2000-2022 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 OpenSSL version 4.0.3, dated 2026-09-29, and it lives in section 3SSL as BN_RAND(3SSL). go forth and generate responsibly, ily 💖

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