BN_new() allocates and initializes a 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.
RETURN VALUES
BN_new() returns a pointer to the 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() have no return
values.