---
name: "libressl-BN_bn2bin"
section: "3"
platform: "linux"
package: "libressl"
version: "4.4.0-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/libressl-BN_bn2bin"
---

# libressl-BN\_bn2bin(3)

format conversions

## NAME

**BN\_bn2bin**, **BN\_bn2binpad**, **BN\_bin2bn**, **BN\_bn2lebinpad**, **BN\_lebin2bn**, **BN\_bn2hex**, **BN\_bn2dec**, **BN\_hex2bn**, **BN\_dec2bn**, **BN\_asc2bn**, **BN\_print**, **BN\_print\_fp**, **BN\_bn2mpi**, **BN\_mpi2bn** – format conversions

## SYNOPSIS

Crypto Library (libcrypto, -lcrypto)

**#include \<openssl/bn.h\>**

*int*\

**BN\_bn2bin**(*const BIGNUM \*a*, *unsigned char \*to*);

*int*\

**BN\_bn2binpad**(*const BIGNUM \*a*, *unsigned char \*to*, *int tolen*);

*BIGNUM* *\**\

**BN\_bin2bn**(*const unsigned char \*s*, *int len*, *BIGNUM \*ret*);

*int*\

**BN\_bn2lebinpad**(*const BIGNUM \*a*, *unsigned char \*to*, *int tolen*);

*BIGNUM* *\**\

**BN\_lebin2bn**(*const unsigned char \*s*, *int len*, *BIGNUM \*ret*);

*char* *\**\

**BN\_bn2hex**(*const BIGNUM \*a*);

*char* *\**\

**BN\_bn2dec**(*const BIGNUM \*a*);

*int*\

**BN\_hex2bn**(*BIGNUM \*\*ap*, *const char \*str*);

*int*\

**BN\_dec2bn**(*BIGNUM \*\*ap*, *const char \*str*);

*int*\

**BN\_asc2bn**(*BIGNUM \*\*ap*, *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

**BN\_bn2bin**() converts the absolute value of *a* into big-endian form and stores it at *to*. *to* must point to **BN\_num\_bytes**(*a*) bytes of memory.

**BN\_bn2binpad**() also converts the absolute value of *a* into big-endian form and stores it at *to*. *tolen* indicates the length of the output buffer \**to*. The result is padded with zeros if necessary. If *tolen* is less than **BN\_num\_bytes**(*a*), an error is returned.

**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 new *BIGNUM* is created.

**BN\_bn2lebinpad**() and **BN\_lebin2bn**() are identical to **BN\_bn2binpad**() and **BN\_bin2bn**() except the buffer \**to* is in little-endian format.

**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 minus sign. The string must be freed later using [free(3)](/free/3.md).

**BN\_hex2bn**() interprets *str* as a hexadecimal number. The string may start with a minus sign (‘-’). Conversion stops at the first byte that is not a hexadecimal digit. The number is converted to a *BIGNUM* and stored in \*\**ap*. If \**ap* is NULL, a new *BIGNUM* is created. If *ap* is NULL, it only computes the number's length in hexadecimal digits, also counting the leading minus sign if there is one. A "negative zero" is converted to zero. **BN\_dec2bn**() is the same using the decimal system. **BN\_asc2bn**() infers the number base from an optional prefix. If *str* starts with "0x" or "0X", it calls **BN\_hex2bn**(), otherwise **BN\_dec2bn**(). If the number is negative, the minus sign can be given before or after the prefix.

**BN\_print**() and **BN\_print\_fp**() write the hexadecimal encoding of *a*, with a leading minus sign for negative numbers, to the *BIO* or *FILE* *fp*.

**BN\_bn2mpi**() and **BN\_mpi2bn**() convert *BIGNUM*s 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 a NUL byte).

**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*).

**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\_bn2binpad**() and **BN\_bn2lebinpad**() return the number of bytes written or -1 if the supplied buffer is too small.

**BN\_bin2bn**() and **BN\_lebin2bn**() return the *BIGNUM*, or NULL on error.

**BN\_bn2hex**() and **BN\_bn2dec**() return a NUL-terminated string, or NULL on error. **BN\_hex2bn**() and **BN\_dec2bn**() return the number's length in hexadecimal or decimal digits, also counting the leading minus sign if there is one, or 0 on error, in which case no new *BIGNUM* is created. **BN\_asc2bn**() returns 1 on success or 0 on error, in which case no new *BIGNUM* is 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*, or NULL on error.

The error codes can be obtained by ERR\_get\_error(3).

## SEE ALSO

ASN1\_INTEGER\_to\_BN(3), BN\_new(3), BN\_num\_bytes(3), BN\_zero(3)

## HISTORY

**BN\_bn2bin**(), **BN\_bin2bn**(), and **BN\_print**() first appeared in SSLeay 0.5.1. **BN\_print\_fp**() first appeared in SSLeay 0.6.0. **BN\_bn2hex**(), **BN\_bn2dec**(), **BN\_hex2bn**(), **BN\_dec2bn**(), **BN\_bn2mpi**(), and **BN\_mpi2bn**() first appeared in SSLeay 0.9.0. All these functions have been available since OpenBSD 2.4.

**BN\_asc2bin**() first appeared in OpenSSL 1.0.0 and has been available since OpenBSD 4.9.

**BN\_bn2binpad**(), **BN\_bn2lebinpad**(), and **BN\_lebin2bn**() first appeared in OpenSSL 1.1.0 and have been available since OpenBSD 7.0.