# BN_bn2bin.3ssl man page

BN_bn2bin, BN_bin2bn, 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);
BIGNUM *BN_bin2bn(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

*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_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_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 '-'. The string must be freed later using *OPENSSL_free()*.

*BN_hex2bn()* converts the string **str** containing a hexadecimal number to a **BIGNUM** and stores it in ****bn**. If ***bn** is NULL, a new **BIGNUM** is created. If **bn** is NULL, it only computes the number's length in hexadecimal digits. If the string starts with '-', the number is negative. A "negative zero" is converted to zero. *BN_dec2bn()* is the same using the decimal system.

*BN_print()* and *BN_print_fp()* write the hexadecimal encoding of **a**, with a leading '-' 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 null 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_bin2bn()* returns the **BIGNUM**, NULL on error.

*BN_bn2hex()* and *BN_bn2dec()* return a null-terminated string, or NULL on error. *BN_hex2bn()* and *BN_dec2bn()* return the number's length in hexadecimal or decimal digits, and 0 on error.

*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

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

## History

*BN_bn2bin()*, *BN_bin2bn()*, *BN_print_fp()* and *BN_print()* are available in all versions of SSLeay and OpenSSL.

*BN_bn2hex()*, *BN_bn2dec()*, *BN_hex2bn()*, *BN_dec2bn()*, *BN_bn2mpi()* and *BN_mpi2bn()* were added in SSLeay 0.9.0.