JNI_Onload

发表于 | 分类于 | 作者：ESE

要逆向，必须会正向编写。so文件混淆——jni_load()

JNI_Onload编写

参考
上一篇只是单纯的编写so文件，只要ida打开就可以直接看见方法名。这里对so进行简单的混淆，利用JNI_Onload()函数

check.java

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package com.example.ese.zjgsuctf;
public class check {
    static {
        System.loadLibrary("check");
    }
    public native static String check(String sstr);
}

check.c

在jni目录创建一个check.c

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#include <string.h>
#include <jni.h>
#include <stdio.h>
#include <assert.h>
#include <android/log.h>
#include "com_example_ese_zjgsuctf_check.h"
#define JNIREG_CLASS "com/example/ese/zjgsuctf/check"//指定要注册的类
typedef unsigned longULONG;
int enc_key[]={89,83,94,88,68,75,87,86,76,96,86,76,96,81,80,75,96,89,83,94,88,66};
int enc_flag[]={0, 79, 216, 222, 132, 213, 175, 19, 82, 136, 9, 16, 156, 179, 246, 113, 109, 5, 183, 122};
void rc4_init(unsigned char*s, unsigned char*key, unsigned long Len)
 {
     int i = 0, j = 0;
     char k[256] = { 0 };
     unsigned char tmp = 0;
     for (i = 0; i<256; i++)
     {
         s[i] = i;
         k[i] = key[i%Len];
     }
     for (i = 0; i<256; i++)
     {
         j = (j + s[i] + k[i]) % 256;
         tmp = s[i];
         s[i] = s[j];
         s[j] = tmp;
     }
 }
 void rc4_crypt(unsigned char*s, unsigned char*Data, unsigned long Len)
 {
     int i = 0, j = 0, t = 0;
     unsigned long k = 0;
     unsigned char tmp;
     for (k = 0; k<Len; k++)
     {
         i = (i + 1) % 256;
         j = (j + s[i]) % 256;
         tmp = s[i];
         s[i] = s[j];
         s[j] = tmp;
         t = (s[i] + s[j]) % 256;
         Data[k] ^= s[t];
     }
 }
 void ckeck_key(char *k)
 {
     int i,len = sizeof(enc_key)/sizeof(enc_key[0]);
     for(i=0;i<len;i++)
     {
         k[i]=enc_key[i]^0x3f;
     }
 }
 int crypt(unsigned char *pData)
 {
     char key[24];
    unsigned char s[256] = { 0 };
    int i,len = strlen(pData);
     ckeck_key(key);
    key[22]='\0';
     rc4_init(s, (unsigned char*)key, strlen(key));
     rc4_crypt(s, (unsigned char*)pData, len);
    for(i=0;i<len;i++)
        if(pData[i]!=enc_flag[i])
            return 0;
    return 1;
 }
//JNI
__attribute__((section (".mytext")))jstring esecheck( JNIEnv* env,jobject thiz,jstring str1)
{
    //LOGV("This is esecheck()");
         const char *str = (*env)->GetStringUTFChars(env, str1, NULL);
            if(str == NULL) {
                return NULL;
            }
            int key = crypt(str);
            char* tmpstr = "succeeded";
            if(key)
            {
                    jstring rtstr = (*env)->NewStringUTF(env, tmpstr);
                    return rtstr;
            }else{
                    tmpstr = "failed!!";
                    jstring rtstr = (*env)->NewStringUTF(env, tmpstr);
                    return rtstr;
            }
}
     /**
     * Table of methods associated with a single class.
     */
static JNINativeMethod gMethods[] = {//绑定，注意，V,Z签名的返回值不能有分号“;”
    { "check", "(Ljava/lang/String;)Ljava/lang/String;", (void*)esecheck},
};
     /*
     * Register several native methods for one class.
     */
     static int registerNativeMethods(JNIEnv* env, const char* className,
             JNINativeMethod* gMethods, int numMethods)
     {
        jclass clazz;
        clazz = (*env)->FindClass(env, className);
        if (clazz == NULL) {
            return JNI_FALSE;
        }
        if ((*env)->RegisterNatives(env, clazz, gMethods, numMethods) < 0) {
            return JNI_FALSE;
        }
        return JNI_TRUE;
     }
     /*
     * Register native methods for all classes we know about.
     */
     static int registerNatives(JNIEnv* env)
     {
        if (!registerNativeMethods(env, JNIREG_CLASS, gMethods,
                                      sizeof(gMethods) / sizeof(gMethods[0])))
            return JNI_FALSE;
        return JNI_TRUE;
     }
     /*
     * Set some test stuff up.
     *
     * Returns the JNI version on success, -1 on failure.
     */
jint JNI_OnLoad(JavaVM* vm, void* reserved)
     {
        JNIEnv* env = NULL;
        jint result = -1;
        if ((*vm)->GetEnv(vm, (void**) &env, JNI_VERSION_1_4) != JNI_OK) {
            return -1;
        }
        assert(env != NULL);
        if (!registerNatives(env)) {//注册
            return -1;
        }
        /* success -- return valid version number */
        result = JNI_VERSION_1_4;
        return result;
     }

Android.mk

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LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
LOCAL_MODULE := check
LOCAL_SRC_FILES := check.c
include $(BUILD_SHARED_LIBRARY)
LOCAL_CFLAGS := -fvisibility=hidden    #隐藏符号表

Application.mk

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APP_ABI := all

JNINativeMethod gMethods[]

调用

在app目录下的build.gradle中添加

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android {
       ......
    sourceSets {
        main() {
            jniLibs.srcDirs = ['src/main/libs']
            jni.srcDirs = [] //屏蔽掉默认的jni编译生成过程
        }
    }
}

这时一个简单的输入结构:

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activity_main.xml
{
    .....
    <EditText
        android:id="@+id/input_flag"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="80dp"
        android:layout_marginEnd="8dp"
        android:layout_marginLeft="8dp"
        android:layout_marginRight="8dp"
        android:layout_marginStart="8dp"
        android:layout_marginTop="100dp"
        android:ems="10"
        android:inputType="textPersonName"
        android:hint="input your flag"
        app:layout_constraintBottom_toTopOf="@+id/button"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        tools:ignore="UnknownId" />
    <Button
        android:id="@+id/input_ok"
        android:layout_width="102dp"
        android:layout_height="45dp"
        android:layout_marginBottom="276dp"
        android:layout_marginEnd="8dp"
        android:layout_marginLeft="8dp"
        android:layout_marginRight="8dp"
        android:layout_marginStart="8dp"
        android:text="确认"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.458"
        app:layout_constraintStart_toStartOf="parent"
        tools:ignore="MissingConstraints" />
}

MainActivity结构

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import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;
public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        final EditText sstr = (EditText) findViewById(R.id.input_flag);
        final Button ok = (Button) findViewById(R.id.input_ok);
        ok.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                String st = sstr.getText().toString().trim();
                String res = check.Check(st);
                Toast.makeText(MainActivity.this, res, Toast.LENGTH_SHORT).show();
            }
        });
    }
}

运行就可以得到调用native方法的apk了。

优点

1.源码改动少，只需要添加JNI_Onload函数
2.无需加解密so，就可以实现混淆so中的JNI函数
3.后续可以添加so加解密，使破解难度更大

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