BN_BN2BIN(3) | Library Functions Manual | BN_BN2BIN(3)

# 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

library “libcrypto”
`#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).

`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 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 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.

June 8, 2025 | Debian