public void sign(String file, String encrypted_file) throws Exception {
        FileOutputStream PGP_file = new FileOutputStream(encrypted_file, true);
        SP sp = new SP(file, pub_exp, priv_exp, mod);
        sp.dump(PGP_file);
        //PGP_file.write((byte)0xDE);
        //PGP_file.write((byte)0xAD);
        PGP_file.close();
    }