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. 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 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 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.