Friday, May 11, 2012

Reverse Engineering - Android APK

There's nothing really complex about this post. In the past I've always maintained that reversing is kind of tough; and that's true if its an EXE,DLL etc. But in the case of an Android application; it really is very easy.

While all this information is already there, here is a very short blog post summarizing how you get from APK to Source. I used a Vuln app available here; but you can use anything really.

If all you want is the source, do the following:

1. Use dex2jar downloaded from here and run ./ . This results in a JAR file getting created in the directory where the APK is already present. The JAR file contains all the JAVA class files; namely the Java byte code - something that you get once you compile your Java code.

2. Convert your Java byte code into actual Java code. You need something called a Java decompiler for this. You can download one called the Jd-Gui from here - the JAR into it and use the 'Save all sources' feature to save all the source (Java) files to your disk. Now you can review it like you would review any Java code.

Some other interesting things about APKs though:

1. An APK can be extracted to a folder. Its just a Zip file so any archiving program should do; I use the inbuilt Ubuntu GUI archiving tool. You can also use unzip, 7z, WinRar or anything you want.

2. Look at the file AndroidManifest.xml. It'll open in a Text editor but its largely binary; hence unreadable. Use Apktool to decode this XML file as well as every other XML file in the APK. You can get it from here. Run apktool against the APK. It'll run and give you all the XML files totally decoded; so you can now read them.

3. All the application code is in classes.dex. This is Android Byte code, reversible to Java Byte Code. So we first 'dedex'. Use 'dedexer' that can be found here. Run it as follows: java -jar ddx1.22.jar -d classes.dex.

Nothing in this post is original. I just wanted a place to refer to instead of Googling a million times for the syntax.

Thursday, May 10, 2012

Market app - Android 2.2

Was doing a test on an Android app (first ever ;)) and was trying to install an app which was available on Android market. As it turns out, the emulator doesn't have the app pre-installed. So I followed instructions here. That largely works except that despite following everything the app vanishes the next time around. So after more digging I came up on this thread which provided the missing piece. I'll sum it up quickly.

--- You need to edit files on the emulator which will prevent the emulator from checking if its a valid phone or not
--- You need to upload the packages for the Market app and the GoogleServicesFramework as those are not shipped
--- You can do both of the above only if you have WRITE access to the emulator's system directory.

So far everything is covered by Varun's blog. The 1 point which he hasn't made clear is that once you've pushed all your changes to build.prop, uploaded the 2 apps mentioned above and deleted SdkSetup.apk you have to sync your changes and save the temp image in which all these changes are made.

Think of it in this way..all the changes you make are temporary and made in a file /tmp/android-. The filename starts with emulator. Once you're done making your changes save this file somewhere. Shut the emulator down. Now rename the saved file to system.img and copy it into ~/.android/avd/.avd. Now boot the emulator. It should work.

I've said nothing new frankly. If you use Varun's post and the post on the XDA forums together, you will be able to do this. However I was stuck for quite a while here, so thought a few guys would benefit by me linking the two together :)

Saturday, May 5, 2012

Blogging elsewhere for a bit

I've been blogging at Infosec Institute (a little :)) and on the TeamMentor development blog (a lot) ... so that's why this isn't as updated with my experiences. Do peek at my work at II or on TeamMentor if you're interested in knowing what I am up to :)