攻防世界安卓逆向app1

首先拿到了一个apk，用7ZIP解压后 将classes.dex文件用dex2jar反汇编得到一个jar包，导进我的idea以后

打开MainActivity分析逻辑，关键是这个onClick函数

public void onClick(View param1View) {
    
    
            try {
    
    
              String str1 = MainActivity.this.text.getText().toString();
              PackageInfo packageInfo = MainActivity.this.getPackageManager().getPackageInfo("com.example.yaphetshan.tencentgreat", 16384);
              String str2 = packageInfo.versionName;
              int j = packageInfo.versionCode;
              for (int i = 0;; i++) {
    
    
                if (i < str1.length() && i < str2.length()) {
    
    
                  if (str1.charAt(i) != (str2.charAt(i) ^ j)) {
    
    
                    Toast.makeText((Context)MainActivity.this, ", 1).show();
                    return;
                  } 
                } else {
    
    
                  if (str1.length() == str2.length()) {
    
    
                    Toast.makeText((Context)MainActivity.this, ", 1).show();
                    return;
                  } 
                  Toast.makeText((Context)MainActivity.this, ", 1).show();
                } 
              } 
            } catch (android.content.pm.PackageManager.NameNotFoundException nameNotFoundException) {
    
    }
            Toast.makeText((Context)MainActivity.this, ", 1).show();
          }

根据分析有几个前提条件：

• str1与str2长度相等
• str1每一位的字母的ascii码值等于str2与j异或的值
• str2为versionName号，j为versioncode

因此根据逻辑我们可以写出

public static void main(String[] args) {
    
    
            String str2 = "X<cP[?PHNB<P?aj";
            int j = 15;
            String res = "";
            for (int i =0;i < str2.length(); i++){
    
    
                res += (char)(str2.charAt(i) ^ j);
            }
        System.out.println(res);
    }

最终输出了W3l_T0_GAM3_0ne即为flag