allocate and free BIGNUMs
BIGNUM structure. BN_init() initializes an existing uninitialized BIGNUM.BN_clear() is used to destroy sensitive data such as keys when they are no longer needed. It erases the memory used by a and sets it to the value 0.BN_free() frees the components of the BIGNUM, and if it was created by BN_new(), also the structure itself. BN_clear_free() additionally overwrites the data before the memory is returned to the system.
BIGNUM. If the allocation fails, it returns NULL and sets an error code that can be obtained by ERR_get_error(3).BN_init(), BN_clear(), BN_free(), and BN_clear_free() return no values.