code: public static ssl_key[] get_filtered(nitro_service service, String filter) throws Exception
  	{
  		ssl_key obj = new ssl_key();
  		options option = new options();
  		option.set_filter(filter);
  		ssl_key[] response = (ssl_key[]) obj.getfiltered(service, option);
  		return response;
  	}
docs: Use this API to fetch filtered set of ssl_key resources.
  filter string should be in JSON format.eg: "vm_state:DOWN,name:[a-z]+"