分析

这是一个apk文件，所以我用jadx来解题，把apk拖入到其中。
进入主函数MainActivity中

package com.example.flag;

import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v7.app.ActionBarActivity;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

/* loaded from: classes.dex */
public class MainActivity extends ActionBarActivity {
    /* JADX INFO: Access modifiers changed from: protected */
    @Override // android.support.v7.app.ActionBarActivity, android.support.v4.app.FragmentActivity, android.app.Activity
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        if (savedInstanceState == null) {
            getSupportFragmentManager().beginTransaction().add(R.id.container, new PlaceholderFragment()).commit();
        }
        Button button = (Button) findViewById(R.id.button1);
        final TextView textview = (TextView) findViewById(R.id.textView1);
        final EditText editview = (EditText) findViewById(R.id.editText1);
        button.setOnClickListener(new View.OnClickListener() { // from class: com.example.flag.MainActivity.1
            @Override // android.view.View.OnClickListener
            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;
                    }
                    String bbb = String.valueOf(x);
                    textview.setText("flag{" + bbb + "}");
                    return;
                }
                textview.setText("输入注册码错误");
            }
        });
    }

    @Override // android.app.Activity
    public boolean onCreateOptionsMenu(Menu menu) {
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

    @Override // android.app.Activity
    public boolean onOptionsItemSelected(MenuItem item) {
        int id = item.getItemId();
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }

    /* loaded from: classes.dex */
    public static class PlaceholderFragment extends Fragment {
        @Override // android.support.v4.app.Fragment
        public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
            View rootView = inflater.inflate(R.layout.fragment_main, container, false);
            return rootView;
        }
    }
}
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其中主要的位置是。

 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;
                    }
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之后针对这里进行改写，flag即可出来

#include
#include
int main() {
	char x[]= "dd2940c04462b4dd7c450528835cca15";
	x[2] = ((x[2] + x[3]) - 50);
	x[4] = ((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;
	}
	printf("flag{%s}", x);
	return 0;
}
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运行获得flag
flag{59acc538825054c7de4b26440c0999dd}