BUUCTF--简单注册器

测试文件：https://www.lanzous.com/iann8pe

 

代码分析

用jadx-gui反编译后，直接看看主要的代码

public class MainActivity extends ActionBarActivity {
    /* access modifiers changed from: protected */
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView((int) R.layout.activity_main);
        if (savedInstanceState == null) {
            getSupportFragmentManager().beginTransaction().add((int) R.id.container, new PlaceholderFragment()).commit();
        }
        final TextView textview = (TextView) findViewById(R.id.textView1);
        final EditText editview = (EditText) findViewById(R.id.editText1);
        ((Button) findViewById(R.id.button1)).setOnClickListener(new View.OnClickListener() {
            /* class com.example.flag.MainActivity.AnonymousClass1 */

            public void onClick(View v) {
                int flag = 1;
                String xx = editview.getText().toString();
                if (!(xx.length() == 32 && xx.charAt(31) == 'a' && xx.charAt(1) == 'b' && (xx.charAt(0) + xx.charAt(2)) - 48 == 56)) {
                    flag = 0;
                }
                if (flag == 1) {
                    char[] x = "dd2940c04462b4dd7c450528835cca15".toCharArray();
                    x[2] = (char) ((x[2] + x[3]) - 50);
                    x[4] = (char) ((x[2] + x[5]) - 48);
                    x[30] = (char) ((x[31] + x[9]) - 48);
                    x[14] = (char) ((x[27] + x[28]) - 97);
                    for (int i = 0; i < 16; i++) {
                        char a = x[31 - i];
                        x[31 - i] = x[i];
                        x[i] = a;
                    }
                    textview.setText("flag{" + String.valueOf(x) + "}");
                    return;
                }
                textview.setText("输入注册码错误");
            }
        });
    }

第17行代码是对我们输入字符串的验证，第20行代码之后是生成flag的地方。

两种方法，第一种构造满足判断条件的字符串，输入进程序，flag就会输出。

第二种方法，将Java代码转换为Python即可。

 

脚本

# -*- coding:utf-8 -*-

flagtrue = "dd2940c04462b4dd7c450528835cca15"
x = [i for i in flagtrue]
x[2] = chr(ord(x[2]) + ord(x[3]) - 0x32)
x[4] = chr(ord(x[2]) + ord(x[5]) - 0x30)
x[0x1e] = chr(ord(x[0x1f]) + ord(x[0x9]) - 0x30)
x[0xe] = chr(ord(x[0x1b]) + ord(x[0x1c]) - 0x61)

for i in range(16):
    x[i],x[31-i] = x[31-i],x[i]

print ("flag{"+ ''.join(x) + "}")

 

get flag！

flag{59acc538825054c7de4b26440c0999dd}