Updated: 2016-05-03

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#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);

*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. *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.

*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).

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