BN_BN2BIN(3ossl)

📦 version 15.1, in freebsd 15.1

💅 BN_BN2BIN — format conversions (the 3ossl manual, bestie)

okay so this is the OpenSSL page all about turning BIGNUMs into bytes and strings and back again. it's giving "glow-up for numbers" ✨

🌸 NAME

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 — format conversions 🦋

✨ 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 bytes, going out

BN_bn2bin() takes the absolute value of a, converts it into big-endian form, and drops it at to. to must point to BN_num_bytes(a) bytes of memory. and like — a and to MUST NOT be NULL. no cap, that's a hard no. 🫶

BN_bn2binpad() does the same absolute-value-to-big-endian thing and stores it at to, but now tolen tells it how long your output buffer to actually is. the result gets padded with zeros if it needs to be. if tolen is less than BN_num_bytes(a), it's an error — the buffer said "not today" 💀

BN_signed_bn2bin() converts the value of a into big-endian signed 2's complement form and stores it at to. tolen is the length of your output buffer to. the result gets signed-extended if necessary (padded with 0x00 for positive numbers or with 0xff for negative numbers). if tolen is smaller than the necessary size (which may be <BN_num_bytes(a) + 1>), it's an error. 🎀

🌈 bytes, coming in

BN_bin2bn() converts the positive integer in big-endian form, of length len at s, into a BIGNUM and places it in ret. if ret is NULL, a brand new BIGNUM gets created. s MUST NOT be NULL. ✨

BN_signed_bin2bn() converts the integer in big-endian signed 2's complement form, of length len at s, into a BIGNUM and places it in ret. if ret is NULL, a new BIGNUM is created. 🦋

🍓 little-endian besties

BN_bn2lebinpad(), BN_signed_bn2lebin() and BN_lebin2bn() are literally identical to BN_bn2binpad(), BN_signed_bn2bin() and BN_bin2bn() — the only difference is the buffer is in little-endian format. same energy, different byte order 💅

👑 native format

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, i.e. most significant byte first on big-endian platforms, and least significant byte first on little-endian platforms. it just matches whatever your machine vibes with 🧚

🎀 strings!

BN_bn2hex() and BN_bn2dec() return printable strings containing the hexadecimal and decimal encoding of a respectively. for negative numbers, the string is prefaced with a leading '-'. and listen — the string must be freed later using OPENSSL_free(). don't ghost it, free it 🙈

BN_hex2bn() takes as many characters as possible from the string str — including the leading character '-' which means negative — 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 valid representation. a "negative zero" is converted to zero. 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, with a leading '-' for negative numbers, to the BIO or FILE fp. 🖨️

💜 MPI format

BN_bn2mpi() and BN_mpi2bn() convert BIGNUMs from and to a format that consists of the number's length in bytes represented as a 4-byte big-endian number, and the number itself in big-endian format, where the most significant bit signals a negative number (the representation of numbers with the MSB set is prefixed with null byte). iconic little container, honestly 📦

BN_bn2mpi() stores the representation of a at to, where to must be large enough to hold the result. the size can be determined by calling BN_bn2mpi(a, NULL). that's the trick, iykyk 💡

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, NULL on error. 😭
  • BN_bn2binpad(), BN_signed_bn2bin(), BN_bn2lebinpad(), BN_signed_bn2lebin(), BN_bn2nativepad(), and and_signed BN_bn2native() return the number of bytes written or -1 if the supplied buffer is too small. 💀
  • 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, in which 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. new drop, ate 💅

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(3ossl) from 3.5.6, dated 2026-04-07 — go forth and convert formats responsibly ✨💖

📦 where this page comes from