github

https://github.com/sea-boat/ByteCodeEncrypt

需求

拿到的需求是要对某特定的jar包实现加密保护，jar包需要提供给外部使用，但核心逻辑部分需要保护以免被简单反编译即能看到。

几个思路

大致想到以下几种方式：

混淆器，将jar包混淆后反编译出来的东西看起来就很眼花，但如果耐心一点也是可以看出来的。
对jar包进行加密，然后在Java层重写类加载器对其进行解密，以达到对jar包的加密保护。包括用对称加密算法和非对称加密算法。不管用什么算法，在Java层面的类加载器实现的话，其实也作用不大，因为类加载器本身被反编译出来后就基本暴露无遗了。
可以修改java编译后的class文件的某些属性，以让反编译软件分析不了，但它也不可靠，只要按照class格式深入分析下也能反编译出来。
修改JDK源码，定制JDK就涉及到JVM的整体改动，而且还要求外部使用，不太可行。
利用JDK中JVM的某些类似钩子机制和事件监听机制，监听加载class事件，使用本地方式完成class的解密。C/C++被编译后想要反编译就很麻烦了，另外还能加壳。这里就看看这种方式。

关于JVMTI

JVMTI即JVM Tool Interface，提供了本地编程接口，主要是提供了调试和分析等接口。JVMTI非常强大，通过它能做很多事，比如可以监听某事件、线程分析等等。

那么一般怎么使用JVMTI？一般使用Agent方式来使用，就是通过-agentlib和-agentpath指定Agent的本地库，然后Java启动时就会加载该动态库。这个时刻其实可以看成是JVM启动的时刻，而并非是Java层程序启动时刻，所以此时还不涉及与Java相关的类和对象什么的。

agent动态库被加载后，JVM肯定会指定一个入口函数，该入口函数为：

JNIEXPORT jint JNICALL Agent_OnLoad(JavaVM *vm,char *options,void *reserved)

于是在JVM启动前要做的事都可以放到这个函数中，比如设置jvmtiCapabilities、设置关注的事件、设置事件的回调函数等等。其中关键的jvmtiCapabilities、jvmtiEventCallbacks、jvmtiEvent三个结构体如下，根据实际情况设置。

typedef struct {
    unsigned int can_tag_objects : 1;
    unsigned int can_generate_field_modification_events : 1;
    unsigned int can_generate_field_access_events : 1;
    unsigned int can_get_bytecodes : 1;
    unsigned int can_get_synthetic_attribute : 1;
    unsigned int can_get_owned_monitor_info : 1;
    unsigned int can_get_current_contended_monitor : 1;
    unsigned int can_get_monitor_info : 1;
    unsigned int can_pop_frame : 1;
    unsigned int can_redefine_classes : 1;
    unsigned int can_signal_thread : 1;
    unsigned int can_get_source_file_name : 1;
    unsigned int can_get_line_numbers : 1;
    unsigned int can_get_source_debug_extension : 1;
    unsigned int can_access_local_variables : 1;
    unsigned int can_maintain_original_method_order : 1;
    unsigned int can_generate_single_step_events : 1;
    unsigned int can_generate_exception_events : 1;
    unsigned int can_generate_frame_pop_events : 1;
    unsigned int can_generate_breakpoint_events : 1;
    unsigned int can_suspend : 1;
    unsigned int can_redefine_any_class : 1;
    unsigned int can_get_current_thread_cpu_time : 1;
    unsigned int can_get_thread_cpu_time : 1;
    unsigned int can_generate_method_entry_events : 1;
    unsigned int can_generate_method_exit_events : 1;
    unsigned int can_generate_all_class_hook_events : 1;
    unsigned int can_generate_compiled_method_load_events : 1;
    unsigned int can_generate_monitor_events : 1;
    unsigned int can_generate_vm_object_alloc_events : 1;
    unsigned int can_generate_native_method_bind_events : 1;
    unsigned int can_generate_garbage_collection_events : 1;
    unsigned int can_generate_object_free_events : 1;
    unsigned int can_force_early_return : 1;
    unsigned int can_get_owned_monitor_stack_depth_info : 1;
    unsigned int can_get_constant_pool : 1;
    unsigned int can_set_native_method_prefix : 1;
    unsigned int can_retransform_classes : 1;
    unsigned int can_retransform_any_class : 1;
    unsigned int can_generate_resource_exhaustion_heap_events : 1;
    unsigned int can_generate_resource_exhaustion_threads_events : 1;
    unsigned int : 7;
    unsigned int : 16;
    unsigned int : 16;
    unsigned int : 16;
    unsigned int : 16;
    unsigned int : 16;
} jvmtiCapabilities;

typedef struct {
                              /*   50 : VM Initialization Event */
    jvmtiEventVMInit VMInit;
                              /*   51 : VM Death Event */
    jvmtiEventVMDeath VMDeath;
                              /*   52 : Thread Start */
    jvmtiEventThreadStart ThreadStart;
                              /*   53 : Thread End */
    jvmtiEventThreadEnd ThreadEnd;
                              /*   54 : Class File Load Hook */
    jvmtiEventClassFileLoadHook ClassFileLoadHook;
                              /*   55 : Class Load */
    jvmtiEventClassLoad ClassLoad;
                              /*   56 : Class Prepare */
    jvmtiEventClassPrepare ClassPrepare;
                              /*   57 : VM Start Event */
    jvmtiEventVMStart VMStart;
                              /*   58 : Exception */
    jvmtiEventException Exception;
                              /*   59 : Exception Catch */
    jvmtiEventExceptionCatch ExceptionCatch;
                              /*   60 : Single Step */
    jvmtiEventSingleStep SingleStep;
                              /*   61 : Frame Pop */
    jvmtiEventFramePop FramePop;
                              /*   62 : Breakpoint */
    jvmtiEventBreakpoint Breakpoint;
                              /*   63 : Field Access */
    jvmtiEventFieldAccess FieldAccess;
                              /*   64 : Field Modification */
    jvmtiEventFieldModification FieldModification;
                              /*   65 : Method Entry */
    jvmtiEventMethodEntry MethodEntry;
                              /*   66 : Method Exit */
    jvmtiEventMethodExit MethodExit;
                              /*   67 : Native Method Bind */
    jvmtiEventNativeMethodBind NativeMethodBind;
                              /*   68 : Compiled Method Load */
    jvmtiEventCompiledMethodLoad CompiledMethodLoad;
                              /*   69 : Compiled Method Unload */
    jvmtiEventCompiledMethodUnload CompiledMethodUnload;
                              /*   70 : Dynamic Code Generated */
    jvmtiEventDynamicCodeGenerated DynamicCodeGenerated;
                              /*   71 : Data Dump Request */
    jvmtiEventDataDumpRequest DataDumpRequest;
                              /*   72 */
    jvmtiEventReserved reserved72;
                              /*   73 : Monitor Wait */
    jvmtiEventMonitorWait MonitorWait;
                              /*   74 : Monitor Waited */
    jvmtiEventMonitorWaited MonitorWaited;
                              /*   75 : Monitor Contended Enter */
    jvmtiEventMonitorContendedEnter MonitorContendedEnter;
                              /*   76 : Monitor Contended Entered */
    jvmtiEventMonitorContendedEntered MonitorContendedEntered;
                              /*   77 */
    jvmtiEventReserved reserved77;
                              /*   78 */
    jvmtiEventReserved reserved78;
                              /*   79 */
    jvmtiEventReserved reserved79;
                              /*   80 : Resource Exhausted */
    jvmtiEventResourceExhausted ResourceExhausted;
                              /*   81 : Garbage Collection Start */
    jvmtiEventGarbageCollectionStart GarbageCollectionStart;
                              /*   82 : Garbage Collection Finish */
    jvmtiEventGarbageCollectionFinish GarbageCollectionFinish;
                              /*   83 : Object Free */
    jvmtiEventObjectFree ObjectFree;
                              /*   84 : VM Object Allocation */
    jvmtiEventVMObjectAlloc VMObjectAlloc;
} jvmtiEventCallbacks;

typedef enum {
    JVMTI_MIN_EVENT_TYPE_VAL = 50,
    JVMTI_EVENT_VM_INIT = 50,
    JVMTI_EVENT_VM_DEATH = 51,
    JVMTI_EVENT_THREAD_START = 52,
    JVMTI_EVENT_THREAD_END = 53,
    JVMTI_EVENT_CLASS_FILE_LOAD_HOOK = 54,
    JVMTI_EVENT_CLASS_LOAD = 55,
    JVMTI_EVENT_CLASS_PREPARE = 56,
    JVMTI_EVENT_VM_START = 57,
    JVMTI_EVENT_EXCEPTION = 58,
    JVMTI_EVENT_EXCEPTION_CATCH = 59,
    JVMTI_EVENT_SINGLE_STEP = 60,
    JVMTI_EVENT_FRAME_POP = 61,
    JVMTI_EVENT_BREAKPOINT = 62,
    JVMTI_EVENT_FIELD_ACCESS = 63,
    JVMTI_EVENT_FIELD_MODIFICATION = 64,
    JVMTI_EVENT_METHOD_ENTRY = 65,
    JVMTI_EVENT_METHOD_EXIT = 66,
    JVMTI_EVENT_NATIVE_METHOD_BIND = 67,
    JVMTI_EVENT_COMPILED_METHOD_LOAD = 68,
    JVMTI_EVENT_COMPILED_METHOD_UNLOAD = 69,
    JVMTI_EVENT_DYNAMIC_CODE_GENERATED = 70,
    JVMTI_EVENT_DATA_DUMP_REQUEST = 71,
    JVMTI_EVENT_MONITOR_WAIT = 73,
    JVMTI_EVENT_MONITOR_WAITED = 74,
    JVMTI_EVENT_MONITOR_CONTENDED_ENTER = 75,
    JVMTI_EVENT_MONITOR_CONTENDED_ENTERED = 76,
    JVMTI_EVENT_RESOURCE_EXHAUSTED = 80,
    JVMTI_EVENT_GARBAGE_COLLECTION_START = 81,
    JVMTI_EVENT_GARBAGE_COLLECTION_FINISH = 82,
    JVMTI_EVENT_OBJECT_FREE = 83,
    JVMTI_EVENT_VM_OBJECT_ALLOC = 84,
    JVMTI_MAX_EVENT_TYPE_VAL = 84
} jvmtiEvent;

具体实现

编写我们的agent动态库，使之在JVM加载时完成一些逻辑，从前面也知道，主要就是在Agent_OnLoad函数中编写逻辑，先获取jvmtiEnv，在通过它设置jvmtiCapabilities，完了再设置回调函数及需要监听的事件。这里关注的事class文件加载时事件。

JNIEXPORT jint JNICALL Agent_OnLoad(JavaVM *vm,char *options,void *reserved)
{

 jvmtiEnv *jvmti;
 jint ret = vm->GetEnv((void **)&jvmti, JVMTI_VERSION);
 if (JNI_OK != ret)
 {
     printf("ERROR: Unable to access JVMTI!\n");
     return ret;
 }
 jvmtiCapabilities capabilities;
 (void)memset(&capabilities, 0, sizeof(capabilities));

 capabilities.can_generate_all_class_hook_events = 1;
 capabilities.can_tag_objects = 1;
 capabilities.can_generate_object_free_events = 1;
 capabilities.can_get_source_file_name = 1;
 capabilities.can_get_line_numbers = 1;
 capabilities.can_generate_vm_object_alloc_events = 1;

 jvmtiError error = jvmti->AddCapabilities(&capabilities);
 if (JVMTI_ERROR_NONE != error)
 {
     printf("ERROR: Unable to AddCapabilities JVMTI!\n");
     return error;
 }

 jvmtiEventCallbacks callbacks;
 (void)memset(&callbacks, 0, sizeof(callbacks));

 callbacks.ClassFileLoadHook = &ClassDecryptHook;
 error = jvmti->SetEventCallbacks(&callbacks, sizeof(callbacks));
 if (JVMTI_ERROR_NONE != error) {
     printf("ERROR: Unable to SetEventCallbacks JVMTI!\n");
     return error;
 }

 error = jvmti->SetEventNotificationMode(JVMTI_ENABLE, JVMTI_EVENT_CLASS_FILE_LOAD_HOOK, NULL);
 if (JVMTI_ERROR_NONE != error) {
     printf("ERROR: Unable to SetEventNotificationMode JVMTI!\n");
     return error;
 }

 return JNI_OK;
}

回调函数，即是上面指定的回调函数，它对应监听的事件，会在对应的事件发生事被调用。这里处理逻辑其实就是判断如果是某包下的类就对其进行解密，否则不处理。

void JNICALL ClassDecryptHook(
 jvmtiEnv *jvmti_env,
 JNIEnv* jni_env,
 jclass class_being_redefined,
 jobject loader,
 const char* name,
 jobject protection_domain,
 jint class_data_len,
 const unsigned char* class_data,
 jint* new_class_data_len,
 unsigned char** new_class_data
 )
{
 *new_class_data_len = class_data_len;
 jvmti_env->Allocate(class_data_len, new_class_data);

 unsigned char* _data = *new_class_data;

 if (name&&strncmp(name, "com/seaboat/", 11) == 0) {
     for (int i = 0; i < class_data_len; i++)
     {
         _data[i] = class_data[i] - 4;
     }
 }
 else {
     for (int i = 0; i < class_data_len; ++i)
     {
         _data[i] = class_data[i];
     }
 }
}

额外写一个加密程序对某jar包加密进行加密处理，这里同样用本地库方式，但加密解密动态库不要一起对外发布，还有Java层调用本地库加密的程序也不要对外发布。

Java层

public class ByteCodeEncryptor {
  static{
    System.loadLibrary("ByteCodeEncryptor"); 
  }

  public native static byte[] encrypt(byte[] text);

  public static void(String[] args){
      try {
      ByteArrayOutputStream baos = new ByteArrayOutputStream();
      byte[] buf = new byte[1024];
      File srcFile = new File(fileName);
      File dstFile = new File(fileName.substring(0, fileName.indexOf("."))+"_encrypted.jar");
      FileOutputStream dstFos = new FileOutputStream(dstFile);
      JarOutputStream dstJar = new JarOutputStream(dstFos);
      JarFile srcJar = new JarFile(srcFile);
      for (Enumeration<JarEntry> enumeration = srcJar.entries(); enumeration.hasMoreElements();) {
          JarEntry entry = enumeration.nextElement();
          InputStream is = srcJar.getInputStream(entry);
          int len;
          while ((len = is.read(buf, 0, buf.length)) != -1) {
              baos.write(buf, 0, len);
          }
          byte[] bytes = baos.toByteArray();
          String name = entry.getName();
          if(name.endsWith(".class")){
              try {
                  bytes = ByteCodeEncryptor.encrypt(bytes);
              } catch (Exception e) {
                  e.printStackTrace();
              }
          }
          JarEntry ne = new JarEntry(name);
          dstJar.putNextEntry(ne);
          dstJar.write(bytes);
          baos.reset();
      }
      srcJar.close();
      dstJar.close();
      dstFos.close();
    } catch (Exception e) {
      e.printStackTrace();
    }
  }

}

本地库

void encode(char *str)
{
    unsigned int m = strlen(str);
    for (int i = 0; i < m; i++)
    {
        str[i] = str[i]+4;
    }

}

extern"C" JNIEXPORT jbyteArray JNICALL
Java_com_seaboat_bytecode_ByteCodeEncryptor_encrypt(JNIEnv * env, jclass cla,jbyteArray text)
{
    char* dst = (char*)env->GetByteArrayElements(text, 0);
    encode(dst);
    env->SetByteArrayRegion(text, 0, strlen(dst), (jbyte *)dst);
    return text;
}

通过agentlib参数启动Java，实现字节码解密，从而实现字节码保护。
```
java -agentlib:xxxxx\ByteCodeEncryptor -cp test_encrypted.jar com.seaboat.AA
```

反编译前后效果

可能报错

下面的错误说明编译的是32位的动态库，不能再64位操作系统运行，可以到vs的vc目录下执行
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC>vcvarsall.bat amd64，重新编译64位的动态库即可。

Error occurred during initialization of VM
Could not find agent library D:\kuaipan\workspace\CPP-workspace\Project3\ByteCodeEncryptor on the library path, with error: Can't load IA 32-bit .dll on a AMD 64-bit platform

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