Android Reverse Engineering – A Kick Start

December 3, 2010, by


Introduction
Recently, the open source mobile operating system  'Android' has a large community of developers writing application  programs that extend the functionality of supporting devices. It seems a lot of people are getting crazy about the Android platform these days (everyone is trying to buy an Android phone!). I don‘t have an Android cell phone but still, I did manage to learn a few tricks on this Linux + java clean room engineered platform. In this article I shall explain you with an example, the simple technique of reverse engineering a normal android application.

Setting up the Ground
For the purpose of a demonstration, I have chosen an Android crackme application published by Deurus and explained the steps involved to legally reverse engineer this application. And our objective (Don’t get the wrong idea friends!) is to break the licensing scheme of this application. So to begin our journey we first need to get geared up. First we need the Android SDK and then the necessary tools.

You can download the necessary files from these locations:
Android SDK: http://developer.Android.com/sdk/index.html
Smali and baksmali: http://code.google.com/p/smali/
Dex2jar: http://code.google.com/p/dex2jar/
Java decompiler: http://java.decompiler.free.fr/
Deurus Android crackme 03: http://crackmes.de/users/deurus/android_crackme03/

Getting Started with the Game
Download all these files to your hard disk. Unzip Android SDK to C:. Unzip rest of the tools to C: android-sdk-windowsRE. If everything is in order you can start Android SDK manager from C:android-sdk-windows. Now it will give you a list of packages to download and install. We need to select at least one SDK platform to continue our quest. After downloading and installing, we can move on to creating a new Android Virtual Device.

After creating AVD, we can start emulation by selecting AVD and clicking the start button. Within a few minutes we can see the emulator booting up and showing the phone screen. That’s it! We have our emulator up and running. Now we need to install the software (crackme, its legal!) to the emulator. For that you may have to get acquainted with Android debug bridge (adb).  Installing an apk file is pretty simple; all you have to do is to run two commands from the Android SDK directory/tools.

After the installation you can see the crackme icon from application menu.

Now run the crackme by clicking on it. If everything went as expected you will see the crackme application on the screen.

Now let’s try to play with it a bit, pressing the check button with no inputs pops a message 'Min 4 chars', and if the check button is pressed with a proper name it pops up a message 'Bad boy'. Do remember these strings because we will be using them as our search keys when we deassemble the apk (actually the dex) files. Also note that we have two hardware ids and we need to find out what those exactly mean.

Now for real Reverse Engineering
As our crackme is up and running in the emulator, we can now move on to the reversing part. If you have read the apk file format, you can somewhat visualize it as an extended JAR file which essentially is a zip file. Now you can change the crackme file name from ‘Crackme03.apk’ to ‘Crackme03.zip’ and decompress it to any folder.

Now the interesting file for us is the ‘classes.dex’, which contains the compiled virtual machine (vm) codes. We are now going to deassemble the dex file with baksmali. The commands are pretty simple as mentioned in the screen shots below.

If everything worked fine, we will have a folder structure similar to Java packages. Interesting .smali files are located at 'comexamplehelloandroid'. Open all the .smali files into your favorite text editor(I use Notepad++).

If you have never done anything related to reverse engineering/esoteric programming/assembly(IL) programming, don’t start sweating. We have just opened a disassembled dex file. Next, if you are thinking that how on earth can someone find the correct location of a checking function, Well I hope you remember those pop up strings I told earlier. Yeah, 'Min 4 chars' and 'Bad boy'. It is time to use those strings as our search keys. After searching the’ Min 4 chars’ in all the opened .smali files, we will find a hit in the file HelloAndroid$2.smali, quite specifically line 130.If everything worked fine, we will have a folder structure similar to Java packages. Interesting .smali files are located at 'comexamplehelloandroid'. Open all the .smali files into your favorite text editor(I use Notepad++).

If you have never done anything related to reverse engineering/esoteric programming/assembly(IL) programming, don’t start sweating. We have just opened a disassembled dex file. Next, if you are thinking that how on earth can someone find the correct location of a checking function, Well I hope you remember those pop up strings I told earlier. Yeah, 'Min 4 chars' and 'Bad boy'. It is time to use those strings as our search keys. After searching the’ Min 4 chars’ in all the opened .smali files, we will find a hit in the file HelloAndroid$2.smali, quite specifically line 130.

Our aim is now to understand the serial checking function and write a keygen for it. For which we have to know all the dalvik opcodes that are used here. You can visit this page (http://pallergabor.uw.hu/androidblog/dalvik_opcodes.html) to understand the opcodes and after that you can convert the disassembled code to much higher language constructs. Here, I will provide a brief code snippet which actually implements the algorithm. The two hardware ids used are the IMEI and the sim serial number.

01 //Read name from text box
02 const v23, 0x7f050004
03 invoke-virtual/range {v22 .. v23}, Lcom/example/helloandroid/HelloAndroid;->findViewById(I)Landroid/view/View;
04 move-result-object v9
05
06 //Read serial from text box
07 const v23, 0x7f050006
08 invoke-virtual/range {v22 .. v23}, Lcom/example/helloandroid/HelloAndroid;->findViewById(I)Landroid/view/View;
09 move-result-object v21
10
11 //Checking whether the name is of length greater than 4
12 const/16 v22, 0x4
13 move v0, v11
14 move/from16 v1, v22
15 if-ge v0, v1, :cond_51
16
17 //Popup showing Min 4 chars
18 const-string v23, "Min 4 chars"
19 const/16 v24, 0x1
20 .line 8601 //Read name from text box
02 const v23, 0x7f050004
03 invoke-virtual/range {v22 .. v23}, Lcom/example/helloandroid/HelloAndroid;->findViewById(I)Landroid/view/View;
04 move-result-object v9
05
06 //Read serial from text box
07 const v23, 0x7f050006
08 invoke-virtual/range {v22 .. v23}, Lcom/example/helloandroid/HelloAndroid;->findViewById(I)Landroid/view/View;
09 move-result-object v21
10
11 //Checking whether the name is of length greater than 4
12 const/16 v22, 0x4
13 move v0, v11
14 move/from16 v1, v22
15 if-ge v0, v1, :cond_51
16
17 //Popup showing Min 4 chars
18 const-string v23, "Min 4 chars"
19 const/16 v24, 0x1
20 .line 86


21 invoke-static/range {v22 .. v24}, Landroid/widget/Toast;->makeText(Landroid/content/Context;Ljava/lang/CharSequence;I)Landroid/widget/Toast;
22 move-result-object v13
23 .line 88
24 .local v13, notificacionToast:Landroid/widget/Toast;
25 invoke-virtual {v13}, Landroid/widget/Toast;->show()V
26
27 //There is a little exception trick to make integer string from username
28 //It converts aaaa to 97979797 which is ascii equivalent
29 invoke-virtual {v10, v5}, Ljava/lang/String;->charAt(I)C
30 move-result v3
31
32 //Getting first 5 chars from ascii converted name
33 const/16 v22, 0x0
34 const/16 v23, 0x5
35 move-object v0, v12
36 move/from16 v1, v22
37 move/from16 v2, v23
38 invoke-virtual {v0, v1, v2}, Ljava/lang/String;->substring(II)Ljava/lang/String;
39
40 //Converting it into integer and xoring with 0x6B016 – Serial part 1
41 invoke-static {v12}, Ljava/lang/Integer;->parseInt(Ljava/lang/String;)I
42 move-result v22
43 const v23, 0x6b016
44 xor-int v22, v22, v23
45
46 //Getting IMEI from TelephonyManager
47 //http://developer.Android.com/reference/Android/telephony/TelephonyManager.html
48 invoke-virtual {v8}, Landroid/telephony/TelephonyManager;->getDeviceId()Ljava/lang/String;
49 move-result-object v6
50 .line 102
51 .local v6, imei2:Ljava/lang/String;
52
53 //Getting sim serial
54 invoke-virtual {v8}, Landroid/telephony/TelephonyManager;->getSimSerialNumber()Ljava/lang/String;
55 move-result-object v16
56 .line 103
57 .local v16, simsn:Ljava/lang/String;
58
59 //Getting first 6 chars from IMEI, and similarly from sim serial (IMEI.Substring(0,6) will be used as Serial part 3)
60 const/16 v22, 0x0
61 const/16 v23, 0x6
62 move-object v0, v6
63 move/from16 v1, v22
64 move/from16 v2, v23
65 invoke-virtual {v0, v1, v2}, Ljava/lang/String;->substring(II)Ljava/lang/String;
66
67 //Converting them to integer and xoring – Serial part2
68 invoke-static/range {v19 .. v19}, Ljava/lang/Integer;->parseInt(Ljava/lang/String;)I
69 move-result v22
70 invoke-static/range {v20 .. v20}, Ljava/lang/Integer;->parseInt(Ljava/lang/String;)I
71 move-result v23
72 xor-int v22, v22, v23
73
74 //Making a new StringBuilder object and formatting the string to part1-part2-part3
75 new-instance v22, Ljava/lang/StringBuilder;
76 invoke-static {v12}, Ljava/lang/String;->valueOf(Ljava/lang/Object;)Ljava/lang/String;
77 move-result-object v23
78 invoke-direct/range {v22 .. v23}, Ljava/lang/StringBuilder;-><init>(Ljava/lang/String;)V
79 const-string v23, "-"
80 invoke-virtual/range {v22 .. v23}, Ljava/lang/StringBuilder;->append(Ljava/lang/String;)Ljava/lang/StringBuilder;
81 move-result-object v22
82 invoke-static/range {v17 .. v18}, Ljava/lang/String;->valueOf(J)Ljava/lang/String;
83 move-result-object v23
84 invoke-virtual/range {v22 .. v23}, Ljava/lang/StringBuilder;->append(Ljava/lang/String;)Ljava/lang/StringBuilder;
85 move-result-object v22
86 const-string v23, "-"
87 invoke-virtual/range {v22 .. v23}, Ljava/lang/StringBuilder;->append(Ljava/lang/String;)Ljava/lang/StringBuilder;
88 move-result-object v22
89 move-object/from16 v0, v22
90 move-object/from16 v1, v19
91 invoke-virtual {v0, v1}, Ljava/lang/StringBuilder;->append(Ljava/lang/String;)Ljava/lang/StringBuilder;
92 move-result-object v22
93
94 //Checking whether user entered serial and program made serials are equal.
95 invoke-virtual {v14, v15}, Ljava/lang/String;->equals(Ljava/lang/Object;)

As you can see, the algorithm is pretty straight forward. It is using name and two hardware ids as input and doing some operations on them to make a serial.  We can quite easily recode it in any programming language that we prefer to make it as a keygen. Anyways, I am not posting any keygen sources as it will spoil the whole phun!

Decoding the Algorithm
A demonstrative serial calculation routine is given below:

Name: aaaaa
HW ID1: 0000000000000000
HW ID2: 89014103211118510720

Here are stepwise instructions on generating final serial number

  • At first 'aaaaa' will be converted to '9797979797', from which we will take first 5 letters and convert it into integer 97979
  • This will be ‘xor’ed with 0x6B016 resulting 511661 and this will be the first part of serial.
  • For the second part, we will take the first 6 letters from HW ID1 and HW ID2, convert them to integer and xor, resulting 000000^890141 = 890141.
  • For the third part we will use the first 6 characters from HW ID1.
  • Formatting with the specified delimiter the serial will become '511661-890141-000000'.

Final Verification of Reversing
Now let us put the same magic number into our Crackme application.

Bingo! Everything worked as expected. Now, for all those who think it is pretty hard to read all those deassembled instructions and manually converting them to higher language constructs, there are other options. As dalvik is based on the design of Java, it is also susceptible to decompilation. There is no decompiler available at this moment, but there is a hope.

For now we can use another utility which converts dex files to jar files so that we can use Java decompilers to see much more abstracted code. From the starting of this article you may have noticed a tool dex2jar. You can use the dex2jar to convert classes.dex to classes.dex.dex2jar.jar. Open it in a Java decompiler and you can see much better output than dalvik disassembly. Please note that dex2jar is still in a development phase and the output you get could be meaningless in many places. This should be used only to get a quick understanding of all the functions.

Conclusion
In  this introductory article, I explained reversing of an Android application using the emulator and all available tools in sequence with pictorial elaborative steps. It is mainly based to set up your ground for further reversing work on Android Platform. Well, that’s it! We have analyzed an Android program and defeated its protection. Cheerio!

Dhanesh is a software engineer specialized in .NET programming. In his spare time he works on Win32 reversing and malware analysis.