

o  callback(0, i, cb_arg) is called after generating the ith potential prime number. 
o  While the number is being tested for primality, callback(1, j, cb_arg) is called as described below. 
o  When a prime has been found, callback(2, i, cb_arg) is called. 
If add is not NULL, the prime will fulfill the condition p % add == rem (p % add == 1 if rem == NULL) in order to suit a given generator.
If safe is true, it will be a safe prime (i.e. a prime p so that (p1)/2 is also prime).
The PRNG must be seeded prior to calling BN_generate_prime(). The prime number generation has a negligible error probability.
BN_is_prime() and BN_is_prime_fasttest() test if the number a is prime. The following tests are performed until one of them shows that a is composite; if a passes all these tests, it is considered prime.
BN_is_prime_fasttest(), when called with do_trial_division == 1, first attempts trial division by a number of small primes; if no divisors are found by this test and callback is not NULL, callback(1, 1, cb_arg) is called. If do_trial_division == 0, this test is skipped.
Both BN_is_prime() and BN_is_prime_fasttest() perform a MillerRabin probabilistic primality test with checks iterations. If checks == BN_prime_checks, a number of iterations is used that yields a false positive rate of at most 2^80 for random input.
If callback is not NULL, callback(1, j, cb_arg) is called after the jth iteration (j = 0, 1, ...). ctx is a preallocated BN_CTX (to save the overhead of allocating and freeing the structure in a loop), or NULL.
BN_generate_prime() returns the prime number on success, NULL otherwise.BN_is_prime() returns 0 if the number is composite, 1 if it is prime with an error probability of less than 0.25^checks, and 1 on error.
The error codes can be obtained by ERR_get_error(3).
bn(3), ERR_get_error(3), rand(3)
The cb_arg arguments to BN_generate_prime() and to BN_is_prime() were added in SSLeay 0.9.0. The ret argument to BN_generate_prime() was added in SSLeay 0.9.1. BN_is_prime_fasttest() was added in OpenSSL 0.9.5.
1.0.1s  BN_GENERATE_PRIME (3)  20160301 
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