### NAME

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 ****a**. If ***a** is NULL, a new **BIGNUM** is created. If **a** 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 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

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.