Called by the factory method to set the mode and padding parameters.
Need because Class.newInstance does not take args.
if (mode.equals("ECB") || mode.equals("")) {
cipher = new DES_ECB(true);
} else if (mode.equals("CBC")) {
cipher = new DES_CBC(true);
} else {
throw new IllegalArgumentException();
}
cipher.setChainingModeAndPadding(mode, padding);