攻防世界 XCTF 【Mobile】APK逆向

正常 JADX 反编译，后丢到 IDEA 中审计一下

直接看 MainActivity

java 复制代码

public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    setTitle(R.string.unregister);
    this.edit_userName = "Tenshine";
    this.edit_sn = (EditText) findViewById(R.id.edit_sn);
    this.btn_register = (Button) findViewById(R.id.button_register);
    this.btn_register.setOnClickListener(new View.OnClickListener() { // from class: com.example.crackme.MainActivity.1
        @Override // android.view.View.OnClickListener
        public void onClick(View v) {
            if (!MainActivity.this.checkSN(MainActivity.this.edit_userName.trim(), MainActivity.this.edit_sn.getText().toString().trim())) {
                Toast.makeText(MainActivity.this, (int) R.string.unsuccessed, 0).show();
                return;
            }
            Toast.makeText(MainActivity.this, (int) R.string.successed, 0).show();
            MainActivity.this.btn_register.setEnabled(false);
            MainActivity.this.setTitle(R.string.registered);
        }
    });
}

看一下 onCreate 方法，其实关键的也就一句

java 复制代码

if (!MainActivity.this.checkSN(MainActivity.this.edit_userName.trim(), MainActivity.this.edit_sn.getText().toString().trim())) {

调用 checkSN 方法，去判断一下 edit_userName 和 edit_sn 关系。

java 复制代码

public static boolean checkSN(String userName, String sn) {
    if (userName != null) {
        try {
            if (userName.length() == 0 || sn == null || sn.length() != 22) {
                return false;
            }
            MessageDigest digest = MessageDigest.getInstance("MD5");
            digest.reset();
            digest.update(userName.getBytes());
            byte[] bytes = digest.digest();
            String hexstr = toHexString(bytes, "");
            StringBuilder sb = new StringBuilder();
            for (int i = 0; i < hexstr.length(); i += 2) {
                sb.append(hexstr.charAt(i));
            }
            String userSN = sb.toString();
            String tmp = new StringBuilder().append("flag{").append(userSN).append("}").toString();
            return tmp.equalsIgnoreCase(sn);
        } catch (NoSuchAlgorithmException e) {
            e.printStackTrace();
            return false;
        }
    }
    return false;
}

其实很简单，就是把 userName 给 md5 一下而已，简单调试一下就行了

顺便吐槽一下 java 是真的难用，复制过来一段代码，想要运行还要改改改，不像 python ....