=pod

=head1 NAME

DH_generate_parameters_ex, DH_generate_parameters,
DH_check - generate and check Diffie-Hellman parameters

=head1 SYNOPSIS

 #include <openssl/dh.h>

 int DH_generate_parameters_ex(DH *dh, int prime_len,int generator, BN_GENCB *cb);

 int DH_check(DH *dh, int *codes);

Deprecated:

 DH *DH_generate_parameters(int prime_len, int generator,
     void (*callback)(int, int, void *), void *cb_arg);

=head1 DESCRIPTION

DH_generate_parameters_ex() generates Diffie-Hellman parameters that can
be shared among a group of users, and stores them in the provided B<DH>
structure.

B<prime_len> is the length in bits of the safe prime to be generated.
B<generator> is a small number E<gt> 1, typically 2 or 5.

A callback function may be used to provide feedback about the progress
of the key generation. If B<cb> is not B<NULL>, it will be
called as described in L<BN_generate_prime(3)|BN_generate_prime(3)> while a random prime number is
generated, and when a prime has been found, B<BN_GENCB_call(cb, 3, 0)> is
called. See L<BN_generate_prime(3)|BN_generate_prime(3)> for information on
the BN_GENCB_call() function.

DH_check() validates Diffie-Hellman parameters. It checks that B<p> is
a safe prime, and that B<g> is a suitable generator. In the case of an
error, the bit flags DH_CHECK_P_NOT_SAFE_PRIME or
DH_NOT_SUITABLE_GENERATOR are set in B<*codes>.
DH_UNABLE_TO_CHECK_GENERATOR is set if the generator cannot be
checked, i.e. it does not equal 2 or 5.

=head1 RETURN VALUES

DH_generate_parameters_ex() and DH_check() return 1 if the check could be
performed, 0 otherwise.

DH_generate_parameters() (deprecated) returns a pointer to the DH structure, or
NULL if the parameter generation fails.

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

=head1 NOTES

DH_generate_parameters_ex() and DH_generate_parameters() may run for several
hours before finding a suitable prime.

The parameters generated by DH_generate_parameters_ex() and DH_generate_parameters()
are not to be used in signature schemes.

=head1 BUGS

If B<generator> is not 2 or 5, B<dh-E<gt>g>=B<generator> is not
a usable generator.

=head1 SEE ALSO

L<dh(3)|dh(3)>, L<ERR_get_error(3)|ERR_get_error(3)>, L<rand(3)|rand(3)>,
L<DH_free(3)|DH_free(3)>

=head1 HISTORY

DH_check() is available in all versions of SSLeay and OpenSSL.
The B<cb_arg> argument to DH_generate_parameters() was added in SSLeay 0.9.0.

In versions before OpenSSL 0.9.5, DH_CHECK_P_NOT_STRONG_PRIME is used
instead of DH_CHECK_P_NOT_SAFE_PRIME.

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