openssl/doc/crypto/DSA_generate_parameters.pod

=pod

=head1 NAME

DSA_generate_parameters - Generate DSA parameters

=head1 SYNOPSIS

 #include <openssl/dsa.h>

 DSA *DSA_generate_parameters(int bits, unsigned char *seed,
                int seed_len, int *counter_ret, unsigned long *h_ret,
		void (*callback)(), void *cb_arg);

=head1 DESCRIPTION

DSA_generate_parameters() generates primes p and q and a generator g
for use in the DSA.

B<bits> is the length of the prime to be generated; the DSS allows a
maximum of 1024 bits.

If B<seed> is B<NULL> or B<seed_len> E<lt> 20, the primes will be
generated at random. Otherwise, the seed is used to generate
them. If the given seed does not yield a prime q, a new random
seed is chosen and placed at B<seed>.

DSA_generate_parameters() places the iteration count in
*B<counter_ret> and a counter used for finding a generator in
*B<h_ret>, unless these are B<NULL>.

A callback function may be used to provide feedback about the progress
of the key generation. If B<callback> is not B<NULL>, it will be
called as follows:

=over 4

=item *

When a candidate for q is generated, B<callback(0, m++, cb_arg)> is called
(m is 0 for the first candidate).

=item *

When a candidate for q has passed a test by trial division,
B<callback(1, -1, cb_arg)> is called.
While a candidate for q is tested by Miller-Rabin primality tests,
B<callback(1, i, cb_arg)> is called in the outer loop
(once for each witness that confirms that the candidate may be prime);
i is the loop counter (starting at 0).

=item *

When a prime q has been found, B<callback(2, 0, cb_arg)> and
B<callback(3, 0, cb_arg)> are called.

=item *

Before a candidate for p (other than the first) is generated and tested,
B<callback(0, counter, cb_arg)> is called.

=item *

When a candidate for p has passed the test by trial division,
B<callback(1, -1, cb_arg)> is called.
While it is tested by the Miller-Rabin primality test,
B<callback(1, i, cb_arg)> is called in the outer loop
(once for each witness that confirms that the candidate may be prime).
i is the loop counter (starting at 0).

=item *

When p has been found, B<callback(2, 1, cb_arg)> is called.

=item *

When the generator has been found, B<callback(3, 1, cb_arg)> is called.

=back

=head1 RETURN VALUE

DSA_generate_parameters() returns a pointer to the DSA structure, or
B<NULL> if the parameter generation fails. The error codes can be
obtained by L<ERR_get_error(3)|ERR_get_error(3)>.

=head1 BUGS

Seed lengths E<gt> 20 are not supported.

=head1 SEE ALSO

L<dsa(3)|dsa(3)>, L<err(3)|err(3)>, L<rand(3)|rand(3)>,
L<DSA_free(3)|DSA_free(3)>

=head1 HISTORY

DSA_generate_parameters() appeared in SSLeay 0.8. The B<cb_arg>
argument was added in SSLeay 0.9.0.
In versions up to OpenSSL 0.9.4, B<callback(1, ...)> was called
in the inner loop of the Miller-Rabin test whenever it reached the
squaring step (the parameters to B<callback> did not reveal how many
witnesses had been tested); since OpenSSL 0.9.5, B<callback(1, ...)>
is called as in BN_is_prime(3), i.e. once for each witness.
=cut