最近幾年Android 變成最熱門的智慧型手機作業系統，與此同時針對Android 平台的攻擊也隨著市占率而上升，安全問題對於Android 平台本身以及開發者來說也越來越重要，一個知名的應用可能被逆向工程分析而插入惡意的程式碼，並且重新打包進而上架發布而造成很多使用者受到潛在的威脅，這已經是Android平台一個主要的攻擊方式，更不用提及逆向工程所帶來的盜版問題一直是開發者所在乎的。目前有非常多的方法可以保護你的應用免於逆向工程，比如說ProGuard，他可以把你的程式碼打亂讓攻擊者更難以分析，但並不能完全免於攻擊，攻擊者只要花更多時間即有可能攻擊你的應用，因此我們提出了一個安全的方法把程式執行碼加密讓攻擊者無法透過逆向工程來分析攻擊我們的應用，在執行程式時再把程式執行碼解密並且動態加載讓程式正常執行，攻擊者完全無從分析除非他把加密後的程式碼解密，這個方法可以有效的讓開發者的心血結晶免於盜版和攻擊。

Nowadays, Android becomes the most popular mobile operating system, at the same time the attacks aimed at Android increases very fast. The security problem is more and more important for Android and the developers need to pay more attention to it . The hacking method of decomposing a well-known apps, reverse engineering, inserting malicious code, repackaging of it, and then redistributing it accounts for the majority of Android apps hacking. Piracy is a big concern of Android Developer. There are many ways to protect our apps from reverse engineering. For example, ProGuard obfuscates the app and makes analysing source code of the app difficult. But that’s not enough. Attackers can still repackage the app if they spend more time to analysis. We propose a secure way that we encrypt the dex file of the Android app. Decrypts the dex code when we launch the app and load it dynamically. Attackers have no way to analysis dex code unless he decrypts it. This method can efficiently protect the developer’s effort from piracy or attacking.

Table of Contents .......................................................................................... i
List of Figures ................................................................................................ iii
List of Tables .................................................................................................. iv
Chapter 1 Introduction.............................................................................. 1
1.1 Motivation........................................................................................... 3
1.2 Our Contribution ................................................................................ 3
1.3 Organization ....................................................................................... 3
Chapter 2 Background ............................................................................... 4
2.1 Android Background ........................................................................... 4
2.1.1 Apk File Structure ............................................................... 4
2.1.2 Dalvik Executables............................................................... 5
2.1.3 Dalvik virtual machine ......................................................... 6
2.1.4 AndroidManifest.xml............................................................ 6
2.1.5 Context ................................................................................ 8
2.2 Other techniques ................................................................................. 8
2.2.1 Android NDK....................................................................... 8
2.2.2 Dynamic Loading ................................................................. 8
Chapter 3 Related Works.......................................................................... 9
3.1 App protection method....................................................................... 9
3.1.1 Google license Library.......................................................... 9
3.1.2 Protection method by verification ........................................ 10
3.1.3 Protection method by Separation......................................... 10
3.2 Advanced Encryption Standard ......................................................... 11
3.3 Partial Encryption ............................................................................. 11
Chapter 4 System Architecture and Design ........................................ 13
4.1 Attack Model ...................................................................................... 13
4.2 Main Idea............................................................................................ 14
4.2.1 Challenge.............................................................................. 14
i
4.3 Main Framework ................................................................................. 15
4.3.1 Decompilation ...................................................................... 15
4.3.2 Main Processing Phase......................................................... 15
4.3.3 Execution phase ................................................................... 24
4.4 Partial Encryption .............................................................................. 24
Chapter 5 Security and Overhead Analysis........................................... 26
5.1 File Size Increment ............................................................................. 27
5.2 Time Analysis ..................................................................................... 28
5.2.1 Encryption............................................................................ 28
5.2.2 Processing Time in Android Mobile Phone.......................... 30
5.3 Security Analysis................................................................................. 32
Chapter 6 Conclusions ............................................................................... 36
6.1 Conclusion........................................................................................... 36
6.2 Future Work........................................................................................ 37

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