360CrackMe第一题题解

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这题目有些日子了,之前做Android时没太接触NDK,CM的APP还原出源码(java)后一直放在那里没动,看着pt哥、鬼哥等人都在出干货,我的虽然没什么技术含量,但还是想分享一下“经验”(板砖轻拍),下面看下题目:

题目说明:

  1. 请重打包本qihootest1.apk,将java层实现的解密算法移至jni层重新实现,并在同一控件中展示解密后的内容;
  2. 本题2分,以非重打包的方式达到同等效果的不计分。

题目附件:qihootest1.apk(软件中已经用java层代码实现了解密软件包的文件并将加密的内容显示在启动界面上)

题目不难,没壳也没做混淆,因此很适合新手做练习,但需要一定的编程基础。

源文件只有一个MainActivity.java,通过下面的smali代码结构猜测onCreate()中可能是调用decrypt()和desCrypto()来对文件进行加解密:

来看下还原出的java代码(当时比较闲,所以直接还原了代码):

try {
           assetManager = getAssets(); //对assets中文件的操作
InputStream inputStream = assetManager.open("encryptedData");  //打开流
           len = inputStream.available();
           inputByte = new byte[len];
           if(len == inputStream.read(inputByte)){
               try {
                   decryResult = decrypt(inputByte, "Qihoo123");   //调用decrypt解密
              } catch (IOException e) {
                  e.printStackTrace();
              }
            }else {
               decryResult = getString(2131034122).getBytes(); //因为资源文件是直接从原APK中取得,所以这里直接用R.java中的id,作用是输出错误提示
           }
            String a = new String(decryResult);
            decodeText.setText(a);
       } catch (Exception e) {
           e.printStackTrace();
       }

具体代码请看附件。

有了java代码就可以移植到native层:

#include "jni.h"
#include "com_qihoo_firsttest_Res.h"
  
/**
 * @author 花墨
 * 2015年1月6日
 * http://xss.sx
 */
  
JNIEXPORT jbyteArray JNICALL Java_com_qihoo_firsttest_Res_decrypt
  (JNIEnv *env, jobject obj, jbyteArray src, jstring password){
  
    jclass clazz_random = env->FindClass("java/security/SecureRandom");
    jmethodID jm_random = env->GetMethodID(clazz_random,"","()V");
    jobject jo_random = env->NewObject(clazz_random,jm_random);
  
    jclass clazz_String = env->FindClass("java/lang/String");
    jmethodID jm_string = env->GetMethodID(clazz_String,"getBytes","()[B");
    jbyteArray bytearray = (jbyteArray)env->CallObjectMethod(password,jm_string);
  
  
    jclass jc_DESKeySpec = env->FindClass("javax/crypto/spec/DESKeySpec");
    jmethodID jm_DESKeySpec = env->GetMethodID(jc_DESKeySpec,"","([B)V");
    jobject jo_DESKeySpec = env->NewObject(jc_DESKeySpec,jm_DESKeySpec,bytearray);
  
    jclass jc_SecretKeyFactory = env->FindClass("javax/crypto/SecretKeyFactory");
    jmethodID jm_SecretKeyFactory = env->GetStaticMethodID(jc_SecretKeyFactory,"getInstance","(Ljava/lang/String;)Ljavax/crypto/SecretKeyFactory;");
    jobject jo_getInstance = env->CallStaticObjectMethod(jc_SecretKeyFactory,jm_SecretKeyFactory,env->NewStringUTF("DES"));
  
    jmethodID jm_SecretKey = env->GetMethodID(jc_SecretKeyFactory,"generateSecret","(Ljava/security/spec/KeySpec;)Ljavax/crypto/SecretKey;");
    jobject jo_SecretKey = env->CallObjectMethod(jo_getInstance,jm_SecretKey,jo_DESKeySpec);
  
    jclass jc_Cipher = env->FindClass("javax/crypto/Cipher");
    jmethodID jm_Cipher_getInstance = env->GetStaticMethodID(jc_Cipher,"getInstance","(Ljava/lang/String;)Ljavax/crypto/Cipher;");
    jobject jo_Cipher = env->CallStaticObjectMethod(jc_Cipher,jm_Cipher_getInstance,env->NewStringUTF("DES"));
  
    jmethodID jm_init = env->GetMethodID(jc_Cipher,"init","(ILjava/security/Key;Ljava/security/SecureRandom;)V");
    env->CallVoidMethod(jo_Cipher,jm_init,2,jo_SecretKey,jo_random);
  
    jmethodID jm_doFinal = env->GetMethodID(jc_Cipher,"doFinal","([B)[B");
    jbyteArray a = (jbyteArray)env->CallObjectMethod(jo_Cipher,jm_doFinal,src);
    return a;
}

最后附上java、C++源码(可直接编译运行)和原APK链接:

链接:http://pan.baidu.com/s/1jGgiBNC 密码:bjw7 解压密码:www.pd521.com

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