BN_BN2BIN(3SSL)

📦
version 4.0.3 · 6 versions
, in ubuntu stonking

🌸 NAME 🌸

okay so bestie, welcome to BN_BN2BIN(3SSL) — it's the OpenSSL page that's literally all about format conversions. like, numbers to bytes, bytes to numbers, hex, decimal, MPI, the whole glow-up pipeline. 💅

the squad of functions here is:

BN_bn2binpad, BN_signed_bn2bin, BN_bn2bin, BN_bin2bn, BN_signed_bin2bn, BN_bn2lebinpad, BN_signed_bn2lebin, BN_lebin2bn, BN_signed_lebin2bn, BN_bn2nativepad, BN_signed_bn2native, BN_native2bn, BN_signed_native2bn, BN_bn2hex, BN_bn2dec, BN_hex2bn, BN_dec2bn, BN_print, BN_print_fp, BN_bn2mpi, BN_mpi2bn

and their whole deal? format conversions. that's it, that's the vibe. ✨

🦋 SYNOPSIS 🦋

#include <openssl/bn.h>

int BN_bn2bin(const BIGNUM *a, unsigned char *to);
int BN_bn2binpad(const BIGNUM *a, unsigned char *to, int tolen);
int BN_signed_bn2bin(const BIGNUM *a, unsigned char *to, int tolen);
BIGNUM *BN_bin2bn(const unsigned char *s, int len, BIGNUM *ret);
BIGNUM *BN_signed_bin2bn(const unsigned char *s, int len, BIGNUM *ret);

int BN_bn2lebinpad(const BIGNUM *a, unsigned char *to, int tolen);
int BN_signed_bn2lebin(const BIGNUM *a, unsigned char *to, int tolen);
BIGNUM *BN_lebin2bn(const unsigned char *s, int len, BIGNUM *ret);
BIGNUM *BN_signed_lebin2bn(const unsigned char *s, int len, BIGNUM *ret);

int BN_bn2nativepad(const BIGNUM *a, unsigned char *to, int tolen);
int BN_signed_bn2native(const BIGNUM *a, unsigned char *to, int tolen);
BIGNUM *BN_native2bn(const unsigned char *s, int len, BIGNUM *ret);
BIGNUM *BN_signed_native2bn(const unsigned char *s, int len, BIGNUM *ret);

char *BN_bn2hex(const BIGNUM *a);
char *BN_bn2dec(const BIGNUM *a);
int BN_hex2bn(BIGNUM **a, const char *str);
int BN_dec2bn(BIGNUM **a, const char *str);

int BN_print(BIO *fp, const BIGNUM *a);
int BN_print_fp(FILE *fp, const BIGNUM *a);

int BN_bn2mpi(const BIGNUM *a, unsigned char *to);
BIGNUM *BN_mpi2bn(unsigned char *s, int len, BIGNUM *ret);

👑 DESCRIPTION 👑

🎀 big-endian byte stuff

  • BN_bn2bin() — takes the absolute value of a, glows it up into big-endian form, and drops it at to. to needs to point at BN_num_bytes(a) bytes of memory, no shortcuts. and listen: a and to MUST NOT be NULL. like at all. 🚫
  • BN_bn2binpad() — same cute big-endian absolute-value energy, also written to to. tolen is the length of the output buffer to. it pads with zeros if it needs to. but if tolen is less than BN_num_bytes(a)? error, bestie, sorry. 😭
  • BN_signed_bn2bin() — converts the value of a (not just abs, we're doing signs now) into big-endian signed 2's complement form, stored at to. tolen = length of the output buffer to. if needed it signs-extends: padded with 0x00 for positive numbers, or 0xff for negative numbers. and if tolen is smaller than the necessary size (which may be <BN_num_bytes(a) + 1>), you get an error. 🙈

💜 bytes → BIGNUM

  • BN_bin2bn() — takes the positive integer in big-endian form, length len, sitting at s, and turns it into a BIGNUM, placing it in ret. if ret is NULL, a brand-new BIGNUM gets born ✨. and s MUST NOT be NULL.
  • BN_signed_bin2bn() — takes the integer in big-endian signed 2's complement form, length len, at s, and makes it a BIGNUM in ret. ret is NULL? new BIGNUM, same deal.

🌈 little-endian remixes (they're literally the same but endian-swapped)

BN_bn2lebinpad(), BN_signed_bn2lebin() and BN_lebin2bn() are identical to BN_bn2binpad(), BN_signed_bn2bin() and BN_bin2bn()... except the buffer is in little-endian format. that's the whole plot twist. 🦋

🧚 native format (aka "it depends on your platform, iykyk")

BN_bn2nativepad(), BN_signed_bn2native() and BN_native2bn() are identical to BN_bn2binpad(), BN_signed_bn2bin() and BN_bin2bn()... except the buffer is in native format. meaning: most significant byte first on big-endian platforms, and least significant byte first on little-endian platforms. your machine decides. main character energy is platform-dependent fr fr. 💀

🍓 hex + dec, the printable girlies

  • BN_bn2hex() and BN_bn2dec() — return printable strings holding the hexadecimal and decimal encoding of a, respectively. negative numbers get a leading '-' prefaced on there. and ⚠️ you must free the string later using OPENSSL_free() — don't leave it hanging, that's not the vibe.
  • BN_hex2bn() — grabs as many characters as it possibly can from the string str, including a leading '-' character (which means negative, obvi), to form a valid hexadecimal number representation, converts them to a BIGNUM, and stores it in **a. if *a is NULL, a new BIGNUM is created. if a is NULL, it only computes the length of the valid representation. also a "negative zero" gets converted to zero — iconic, honestly. BN_dec2bn() is the exact same thing but in the decimal system. 💅

🔥 printing

  • BN_print() and BN_print_fp() — write the hexadecimal encoding of a to the BIO or FILE fp, with a leading '-' for negative numbers. simple, clean, slay.

⭐ MPI format

BN_bn2mpi() and BN_mpi2bn() convert BIGNUMs from and to a format that's made of two parts: the number's length in bytes represented as a 4-byte big-endian number, plus the number itself in big-endian format, where the most significant bit signals a negative number (and the representation of numbers with the MSB set is prefixed with a null byte). ✨

  • BN_bn2mpi() — stores the representation of a at to, where to has to be large enough to hold the result. wanna know the size first? call BN_bn2mpi(a, NULL). cute lil trick. 🎀
  • BN_mpi2bn() — converts the len bytes long representation at s to a BIGNUM and stores it at ret, or in a newly allocated BIGNUM if ret is NULL.

💖 RETURN VALUES 💖

  • BN_bn2bin() returns the length of the big-endian number placed at to. BN_bin2bn() returns the BIGNUM, or NULL on error.
  • BN_bn2binpad(), BN_signed_bn2bin(), BN_bn2lebinpad(), BN_signed_bn2lebin(), BN_bn2nativepad(), and_signed BN_bn2native() return the number of bytes written, or -1 if the supplied buffer is too small. (yes that "and_signed" reads wild in the original manpage — keeping it exactly as written, no editing bestie 🫶)
  • BN_bn2hex() and BN_bn2dec() return a NUL-terminated string, or NULL on error. BN_hex2bn() and BN_dec2bn() return the number of characters used in parsing, or 0 on error — and in that error case, no new BIGNUM will be created. 😭
  • BN_print_fp() and BN_print() return 1 on success, 0 on write errors.
  • BN_bn2mpi() returns the length of the representation. BN_mpi2bn() returns the BIGNUM, and NULL on error.
  • the error codes can be obtained by ERR_get_error(3). 🔍

🌸 SEE ALSO 🌸

ERR_get_error(3), BN_zero(3), ASN1_INTEGER_to_BN(3), BN_num_bytes(3)

✨ HISTORY ✨

the functions BN_signed_bin2bn(), BN_signed_bn2bin(), BN_signed_lebin2bn(), BN_signed_bn2lebin(), BN_signed_native2bn(), BN_signed_bn2native() were added in OpenSSL 3.2. the signed girlies are the newbies of the group. 🎀

Copyright 2000-2024 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 BN_BN2BIN, from OpenSSL 4.0.3, dated 2026-09-29. go forth and convert formats responsibly, mwah 💖✨🌸

📦 where this page comes from