学习破解一个Android程序

首先编写一个android测试程序

功能：校验用户名和注册码，成功则弹出注册成功提示

以下仅给出关键部分的代码

res/layout/activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="vertical">

    <TextView
        android:id="@+id/textView1"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:gravity="center"
        android:text="@string/info"
        android:textSize="20dp" />

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginLeft="10dp"
        android:orientation="horizontal">

        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@string/username" />

        <EditText
            android:id="@+id/edit_username"
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:layout_marginLeft="10dp"
            android:layout_marginRight="10dp"
            android:layout_weight="1"
            android:ems="10"
            android:hint="@string/hint_username"></EditText>
    </LinearLayout>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginLeft="10dp"
        android:orientation="horizontal">

        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@string/sn" />

        <EditText
            android:id="@+id/edit_sn"
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:layout_marginLeft="10dp"
            android:layout_marginRight="10dp"
            android:layout_weight="1"
            android:ems="10"
            android:hint="@string/hint_sn"></EditText>
    </LinearLayout>

    <Button
        android:id="@+id/button_register"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="right"
        android:layout_marginRight="10dp"
        android:text="@string/register" />

</LinearLayout>

java/com/example/myapplication/MainActivity.java

package com.example.myapplication;

import android.app.Activity;
import android.os.Bundle;
import android.view.Menu;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;

public class MainActivity extends Activity {
    private EditText edit_userName;
    private EditText edit_sn;
    private Button btn_register;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        setTitle(R.string.unregister);  //模拟程序未注册
        edit_userName = (EditText) findViewById(R.id.edit_username);
        edit_sn = (EditText) findViewById(R.id.edit_sn);
        btn_register = (Button) findViewById(R.id.button_register);
        btn_register.setOnClickListener(new OnClickListener() {

            public void onClick(View v) {
                if (!checkSN(edit_userName.getText().toString().trim(),
                        edit_sn.getText().toString().trim())) {
                    Toast.makeText(MainActivity.this,       //弹出无效用户名或注册码提示
                            R.string.unsuccessed, Toast.LENGTH_SHORT).show();
                } else {
                    Toast.makeText(MainActivity.this,       //弹出注册成功提示
                            R.string.successed, Toast.LENGTH_SHORT).show();
                    btn_register.setEnabled(false);
                    setTitle(R.string.registered);  //模拟程序已注册
                }
            }
        });
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        getMenuInflater().inflate(R.menu.activity_main, menu);
        return true;
    }

    private boolean checkSN(String userName, String sn) {
        try {
            if ((userName == null) || (userName.length() == 0))
                return false;
            if ((sn == null) || (sn.length() != 16))
                return false;
            MessageDigest digest = MessageDigest.getInstance("MD5");
            digest.reset();
            digest.update(userName.getBytes());
            byte[] bytes = digest.digest();     //采用MD5对用户名进行Hash
            String hexstr = toHexString(bytes, ""); //将计算结果转化成字符串
            StringBuilder sb = new StringBuilder();
            for (int i = 0; i < hexstr.length(); i += 2) {
                sb.append(hexstr.charAt(i));
            }
            String userSN = sb.toString(); //计算出的SN
            //Log.d("crackme", hexstr);
            //Log.d("crackme", userSN);
            if (!userSN.equalsIgnoreCase(sn))   //比较注册码是否正确
                return false;
        } catch (NoSuchAlgorithmException e) {
            e.printStackTrace();
            return false;
        }
        return true;
    }

    private static String toHexString(byte[] bytes, String separator) {
        StringBuilder hexString = new StringBuilder();
        for (byte b : bytes) {
            String hex = Integer.toHexString(0xFF & b);
            if (hex.length() == 1) {
                hexString.append('0');
            }
            hexString.append(hex).append(separator);
        }
        return hexString.toString();
    }

}

运行没有问题之后，通过AndroidStudio编译成apk文件

开始破解程序

使用apk-tool反编译apk程序

下载地址：https://down.52pojie.cn/Tools/Android_Tools/apktool_2.9.3.jar

执行

java -jar apktool_2.9.3.jar d -f app-debug.apk -o output

smail目录下存放了程序的所有反汇编代码，res目录下则是程序的所有资源文件

先通过res\values\string.xml定位程序的错误信息

<resources>
...
    <string name="unsuccessed">无效用户名或注册码</string>
...
</resources>

再通过同目录下的public.xml找到name="unsuccessed"的id

<public type="string" name="unsuccessed" id="0x7f1000a5" />

通过该id可以到smail目录搜索，在output\smali_classes3\com\example\myapplication\MainActivity$1.smali搜索到一处结果

   91      iget-object v0, p0, Lcom/example/myapplication/MainActivity$1;->this$0:Lcom/example/myapplication/MainActivity;
   92  
   93:     const v2, 0x7f1000a5
   94  
   95      invoke-static {v0, v2, v1}, Landroid/widget/Toast;->makeText(Landroid/content/Context;II)Landroid/widget/Toast;

onclick方法

# virtual methods
.method public onClick(Landroid/view/View;)V
    .locals 3
    .param p1, "v"    # Landroid/view/View;

    .line 31
    iget-object v0, p0, Lcom/example/myapplication/MainActivity$1;->this$0:Lcom/example/myapplication/MainActivity;

    invoke-static {v0}, Lcom/example/myapplication/MainActivity;->access$000(Lcom/example/myapplication/MainActivity;)Landroid/widget/EditText;

    move-result-object v1

    invoke-virtual {v1}, Landroid/widget/EditText;->getText()Landroid/text/Editable;

    move-result-object v1

    invoke-virtual {v1}, Ljava/lang/Object;->toString()Ljava/lang/String;

    move-result-object v1

    invoke-virtual {v1}, Ljava/lang/String;->trim()Ljava/lang/String;

    move-result-object v1

    iget-object v2, p0, Lcom/example/myapplication/MainActivity$1;->this$0:Lcom/example/myapplication/MainActivity;

    .line 32
    invoke-static {v2}, Lcom/example/myapplication/MainActivity;->access$100(Lcom/example/myapplication/MainActivity;)Landroid/widget/EditText;

    move-result-object v2

    invoke-virtual {v2}, Landroid/widget/EditText;->getText()Landroid/text/Editable;

    move-result-object v2

    invoke-virtual {v2}, Ljava/lang/Object;->toString()Ljava/lang/String;

    move-result-object v2

    invoke-virtual {v2}, Ljava/lang/String;->trim()Ljava/lang/String;

    move-result-object v2

    .line 31
    invoke-static {v0, v1, v2}, Lcom/example/myapplication/MainActivity;->access$200(Lcom/example/myapplication/MainActivity;Ljava/lang/String;Ljava/lang/String;)Z

    move-result v0

    const/4 v1, 0x0

    if-nez v0, :cond_0 # 关键条件判断

    .line 33
    iget-object v0, p0, Lcom/example/myapplication/MainActivity$1;->this$0:Lcom/example/myapplication/MainActivity;

    const v2, 0x7f1000a5

    invoke-static {v0, v2, v1}, Landroid/widget/Toast;->makeText(Landroid/content/Context;II)Landroid/widget/Toast;

    move-result-object v0

    .line 34
    invoke-virtual {v0}, Landroid/widget/Toast;->show()V

    goto :goto_0

    .line 36
    :cond_0
    iget-object v0, p0, Lcom/example/myapplication/MainActivity$1;->this$0:Lcom/example/myapplication/MainActivity;

    const v2, 0x7f1000a2

    invoke-static {v0, v2, v1}, Landroid/widget/Toast;->makeText(Landroid/content/Context;II)Landroid/widget/Toast;

    move-result-object v0

    .line 37
    invoke-virtual {v0}, Landroid/widget/Toast;->show()V

    .line 38
    iget-object v0, p0, Lcom/example/myapplication/MainActivity$1;->this$0:Lcom/example/myapplication/MainActivity;

    invoke-static {v0}, Lcom/example/myapplication/MainActivity;->access$300(Lcom/example/myapplication/MainActivity;)Landroid/widget/Button;

    move-result-object v0

    invoke-virtual {v0, v1}, Landroid/widget/Button;->setEnabled(Z)V

    .line 39
    iget-object v0, p0, Lcom/example/myapplication/MainActivity$1;->this$0:Lcom/example/myapplication/MainActivity;

    const v1, 0x7f100099

    invoke-virtual {v0, v1}, Lcom/example/myapplication/MainActivity;->setTitle(I)V

    .line 41
    :goto_0
    return-void
.end method

经过分析，if-nez v0, :cond_0判断决定了校验是否通过，这句代码的意思是：如果v0不为0则跳转到cond_0，也就是注册失败的分支。

破解方法：将if-nez改为if-eqz也就是等于则为真

修改后保存，执行如下代码重新编译

java -jar apktool_2.9.3.jar b output

编译后需要对apk进行签名，然后可以通过adb命令安装和启动

adb install app-debug_sign.apk
adb shell am start -n com.example.myapplication/.MainActivity

至此破解就算完成了。

使用IDA pro破解

下载地址：https://down.52pojie.cn/Tools/Disassemblers/IDA_Pro_v8.3_Portable.zip

用压缩包工具打开app-debug.apk，提取出classes.dex文件，通过ida pro打开

注意：如果classes.dex没有可能在classes3.dex

按照先找错误信息的思路，按alt+t打开文本搜索功能，搜索：0x7f1000a5

定位到关键代码（按空格可以切换视图）

可以看到分支判断位于CODE:000006BE，将光标放在if-nez处，点击hex-view，修改if-nez的字节码

39 00 0F 00改为38 00 0F 00

然后关闭ida pro，不需要保存到database

用c32asm打开classes3.dex定位到000006BE，将39改为38，保存退出

这里由于修改了dex文件，会导致dex文件在验证计算checksum会失败，从而导致程序安装失败，因此需要重新计算checksum值

用Dexfixer将classes3.dex文件checksum值修复

将修复好的classes3.dex，重新拉入apk

aapt r app-debug.apk classes3.dex
aapt a app-debug.apk classes3.dex

删除META-INT（可使用winrar工具），并重新签名apk即可破解成功。

参考：《Android软件安全与逆向分析》