=pod=head1 NAMEDH_generate_parameters, DH_check - generate and check Diffie-Hellman parameters=head1 SYNOPSIS #include <openssl/dh.h> DH *DH_generate_parameters(int prime_len, int generator,     void (*callback)(int, int, void *), void *cb_arg); int DH_check(DH *dh, int *codes);=head1 DESCRIPTIONDH_generate_parameters() generates Diffie-Hellman parameters that canbe shared among a group of users, and returns them in a newlyallocated B<DH> structure. The pseudo-random number generator must beseeded prior to calling DH_generate_parameters().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 progressof the key generation. If B<callback> is not B<NULL>, it will becalled as described in L<BN_generate_prime(3)|BN_generate_prime(3)> while a random primenumber is generated, and when a prime has been found, B<callback(3,0, cb_arg)> is called.DH_check() validates Diffie-Hellman parameters. It checks that B<p> isa safe prime, and that B<g> is a suitable generator. In the case of anerror, the bit flags DH_CHECK_P_NOT_SAFE_PRIME orDH_NOT_SUITABLE_GENERATOR are set in B<*codes>.DH_UNABLE_TO_CHECK_GENERATOR is set if the generator cannot bechecked, i.e. it does not equal 2 or 5.=head1 RETURN VALUESDH_generate_parameters() returns a pointer to the DH structure, orNULL if the parameter generation fails. The error codes can beobtained by L<ERR_get_error(3)|ERR_get_error(3)>.DH_check() returns 1 if the check could be performed, 0 otherwise.=head1 NOTESDH_generate_parameters() may run for several hours before finding asuitable prime.The parameters generated by DH_generate_parameters() are not to beused in signature schemes.=head1 BUGSIf B<generator> is not 2 or 5, B<dh-E<gt>g>=B<generator> is nota usable generator.=head1 SEE ALSOL<dh(3)|dh(3)>, L<err(3)|err(3)>, L<rand(3)|rand(3)>, L<DH_free(3)|DH_free(3)>=head1 HISTORYDH_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 usedinstead of DH_CHECK_P_NOT_SAFE_PRIME.=cut