openssl/doc/crypto/BN_generate_prime.pod

=pod

=head1 NAME

BN_generate_prime_ex, BN_is_prime_ex, BN_is_prime_fasttest_ex, BN_GENCB_call,
BN_GENCB_new, BN_GENCB_free, BN_GENCB_set_old, BN_GENCB_set, BN_GENCB_get_arg,
BN_generate_prime, BN_is_prime, BN_is_prime_fasttest - generate primes and test
for primality

=head1 SYNOPSIS

 #include <openssl/bn.h>

 int BN_generate_prime_ex(BIGNUM *ret,int bits,int safe, const BIGNUM *add,
     const BIGNUM *rem, BN_GENCB *cb);

 int BN_is_prime_ex(const BIGNUM *p,int nchecks, BN_CTX *ctx, BN_GENCB *cb);

 int BN_is_prime_fasttest_ex(const BIGNUM *p,int nchecks, BN_CTX *ctx,
     int do_trial_division, BN_GENCB *cb);

 int BN_GENCB_call(BN_GENCB *cb, int a, int b);

 BN_GENCB *BN_GENCB_new(void);

 void BN_GENCB_free(BN_GENCB *cb);

 void BN_GENCB_set_old(BN_GENCB *gencb,
     void (*callback)(int, int, void *), void *cb_arg);

 void BN_GENCB_set(BN_GENCB *gencb,
     int (*callback)(int, int, BN_GENCB *), void *cb_arg);

 void *BN_GENCB_get_arg(BN_GENCB *cb);

Deprecated:

 #if OPENSSL_API_COMPAT < 0x00908000L
 BIGNUM *BN_generate_prime(BIGNUM *ret, int num, int safe, BIGNUM *add,
     BIGNUM *rem, void (*callback)(int, int, void *), void *cb_arg);

 int BN_is_prime(const BIGNUM *a, int checks, void (*callback)(int, int, 
     void *), BN_CTX *ctx, void *cb_arg);

 int BN_is_prime_fasttest(const BIGNUM *a, int checks,
     void (*callback)(int, int, void *), BN_CTX *ctx, void *cb_arg,
     int do_trial_division);
 #endif

=head1 DESCRIPTION

BN_generate_prime_ex() generates a pseudo-random prime number of
at least bit length B<bits>.
If B<ret> is not B<NULL>, it will be used to store the number.

If B<cb> is not B<NULL>, it is used as follows:

=over 4

=item *

B<BN_GENCB_call(cb, 0, i)> is called after generating the i-th
potential prime number.

=item *

While the number is being tested for primality,
B<BN_GENCB_call(cb, 1, j)> is called as described below.

=item *

When a prime has been found, B<BN_GENCB_call(cb, 2, i)> is called.

=back

The prime may have to fulfill additional requirements for use in
Diffie-Hellman key exchange:

If B<add> is not B<NULL>, the prime will fulfill the condition p % B<add>
== B<rem> (p % B<add> == 1 if B<rem> == B<NULL>) in order to suit a given
generator.

If B<safe> is true, it will be a safe prime (i.e. a prime p so
that (p-1)/2 is also prime).

The PRNG must be seeded prior to calling BN_generate_prime_ex().
The prime number generation has a negligible error probability.

BN_is_prime_ex() and BN_is_prime_fasttest_ex() test if the number B<p> is
prime.  The following tests are performed until one of them shows that
B<p> is composite; if B<p> passes all these tests, it is considered
prime.

BN_is_prime_fasttest_ex(), 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<cb> is not B<NULL>,
B<BN_GENCB_call(cb, 1, -1)> is called.
If B<do_trial_division == 0>, this test is skipped.

Both BN_is_prime_ex() and BN_is_prime_fasttest_ex() perform a Miller-Rabin
probabilistic primality test with B<nchecks> iterations. If
B<nchecks == BN_prime_checks>, a number of iterations is used that
yields a false positive rate of at most 2^-80 for random input.

If B<cb> is not B<NULL>, B<BN_GENCB_call(cb, 1, j)> is called
after the j-th iteration (j = 0, 1, ...). B<ctx> is a
pre-allocated B<BN_CTX> (to save the overhead of allocating and
freeing the structure in a loop), or B<NULL>.

BN_GENCB_call calls the callback function held in the B<BN_GENCB> structure
and passes the ints B<a> and B<b> as arguments. There are two types of
B<BN_GENCB> structure that are supported: "new" style and "old" style. New
programs should prefer the "new" style, whilst the "old" style is provided
for backwards compatibility purposes.

A BN_GENCB structure should be created through a call to BN_GENCB_new(),
and freed through a call to BN_GENCB_free().

For "new" style callbacks a BN_GENCB structure should be initialised with a
call to BN_GENCB_set(), where B<gencb> is a B<BN_GENCB *>, B<callback> is of
type B<int (*callback)(int, int, BN_GENCB *)> and B<cb_arg> is a B<void *>.
"Old" style callbacks are the same except they are initialised with a call
to BN_GENCB_set_old() and B<callback> is of type
B<void (*callback)(int, int, void *)>.

A callback is invoked through a call to B<BN_GENCB_call>. This will check
the type of the callback and will invoke B<callback(a, b, gencb)> for new
style callbacks or B<callback(a, b, cb_arg)> for old style.

It is possible to obtained the argument associated with a BN_GENCB structure
(set via a call to BN_GENCB_set or BN_GENCB_set_old) using BN_GENCB_get_arg.

BN_generate_prime (deprecated) works in the same way as
BN_generate_prime_ex but expects an old style callback function
directly in the B<callback> parameter, and an argument to pass to it in
the B<cb_arg>. Similarly BN_is_prime and BN_is_prime_fasttest are
deprecated and can be compared to BN_is_prime_ex and
BN_is_prime_fasttest_ex respectively.

=head1 RETURN VALUES

BN_generate_prime_ex() return 1 on success or 0 on error.

BN_is_prime_ex(), BN_is_prime_fasttest_ex(), BN_is_prime() and
BN_is_prime_fasttest() return 0 if the number is composite, 1 if it is
prime with an error probability of less than 0.25^B<nchecks>, and
-1 on error.

BN_generate_prime() returns the prime number on success, B<NULL> otherwise.

BN_GENCB_new returns a pointer to a BN_GENCB structure on success, or B<NULL>
otherwise.

BN_GENCB_get_arg returns the argument previously associated with a BN_GENCB
structure.

Callback functions should return 1 on success or 0 on error.

The error codes can be obtained by L<ERR_get_error(3)>.

=head1 REMOVED FUNCTIONALITY

As of OpenSSL 1.1.0 it is no longer possible to create a BN_GENCB structure
directly, as in:

 BN_GENCB callback;

Instead applications should create a BN_GENCB structure using BN_GENCB_new:

 BN_GENCB *callback;
 callback = BN_GENCB_new();
 if(!callback) /* handle error */
 ...
 BN_GENCB_free(callback);

=head1 SEE ALSO

L<bn(3)>, L<ERR_get_error(3)>, L<rand(3)>

=head1 HISTORY

BN_GENCB_new(), BN_GENCB_free(),
and BN_GENCB_get_arg() were added in OpenSSL 1.1.0

=cut