How to inject code into an existing Apk?

Question

I know this is possible to decompile and recompile an apk. And I saw this post : Injecting code into APK before, But what I want is not decompiling or reverse engineerin

Answer 1

For this purpose first of all you decompile this apk into source code for decompiling the apk following purpose is used

Step 1:

Make a new folder and copy over the .apk file that you want to decode.

Now rename the extension of this .apk file to .zip (e.g. rename from filename.apk to filename.zip) and save it. Now you can access the classes.dex files, etc. At this stage you are able to see drawables but not xml and java files, so continue.

Step 2:

Now extract this .zip file in the same folder (or NEW FOLDER).

Download dex2jar (https://code.google.com/p/dex2jar/downloads/detail?name=dex2jar-0.0.9.15.zip&can=2&q=) and extract it to the same folder (or NEW FOLDER).

Move the classes.dex file into the dex2jar folder.

Now open command prompt and change directory to that folder (or NEW FOLDER). Then write d2j-dex2jar classes.dex and press enter. You now have the classes_dex2jar.jar file in the same folder.

Download java decompiler(http://jd.benow.ca/), double click on jd-gui, click on open file, and open classes.dex.dex2jar file from that folder: now you get class files.

Save all of these class files (In jd-gui, click File -> Save All Sources) by src name. At this stage you get the java source but the .xml files are still unreadable, so continue.

Step 3:

Now open another new folder

Put in the .apk file which you want to decode

Download the latest version of apktool (https://code.google.com/p/android-apktool/downloads/detail?name=apktool1.5.2.tar.bz2&can=2&q=) AND apktool install window (https://code.google.com/p/android-apktool/downloads/detail?name=apktool-install-windows-r05-ibot.tar.bz2&can=2&q= ) (both can be downloaded from the same link) and place them in the same folder

Download framework-res.apk and put it in the same folder (Not all apk file need this file, but it doesn't hurt)

Open a command window

Navigate to the root directory of APKtool and type the following command: apktool if framework-res.apk

apktool d myApp.apk (where myApp.apk denotes the filename that you want to decode)

now you get a file folder in that folder and can easily read the apk's xml files.

Step 4:

It's not any step just copy contents of both folder(in this case both new folder)to the single one

and update the source code...

Answer 2

For something simple like this, all you need is baksmali/smali. Just insert a single call into the assembly into your custom class, and you can write the rest in Java.

Decompiling/recompiling rarely works and is overkill for this situation anyway. Using smali will work even for obfuscated applications, though some applications calculate integrity checks on themselves and will refuse to run if modified.