B<a> is composite; if B<a> passes all these tests, it is considered
prime.
-=over 4
-
-=item *
-
BN_is_prime_fasttest(), when called with B<do_trial_division == 1>,
first attempts trial division by a number of small primes;
if no divisors are found by this test and B<callback> is not B<NULL>,
B<callback(1, -1, cb_arg)> is called.
If B<do_trial_division == 0>, this test is skipped.
-=item *
-
Both BN_is_prime() and BN_is_prime_fasttest() perform a Miller-Rabin
probabilistic primality test with B<checks> iterations. If
B<checks == BN_prime_check>, a number of iterations is used that
B<NULL> (lacking this parameter, BN_is_prime() always has to allocate
an additional B<CN_CTX>).
-
=head1 RETURN VALUES
BN_generate_prime() returns the prime number on success, B<NULL> otherwise.