Decompile and Recompile Android APK

2015-08-01 | android | java

I had the need to take an existing Android APK, tweak it, and rebuild. This is not too difficult, but I did have to download the tools from a few different sites, and find a full list of instructions. Thus to make this easier, here is a quick recap of what’s needed.

Download the following:

  • apktool - tool for reverse engineering Android apk files. In this case can extract and rebuild.
  • keytool - Java tool for creating keys/certs. Comes with the JDK.
  • jarsigner Java tool for signing JAR/APK files. Comes with the JDK.
  • zipalign - archive alignment tool, that comes with the Android SDK.

Some extras:

  • JD-GUI - Java Decompiler
  • dex2jar - Converts Android dex files to class/jar files.

Instructions:

We assume you are on a Linux or Mac, but this will work (with some tweaking) on Windows. Install a recent Java JDK, then the Stand-alone Android SDK, and finally apktool.

Optionally setup some alias:

alias apktool='java -jar ~/bin/apktool_2.0.1.jar'
alias dex2jar='~/bin/dex2jar-2.0/d2j-dex2jar.sh'
alias jd-gui='java -jar ~/bin/jd-gui-1.3.0.jar'

First, unpack the application.apk file. This will create a “application” directory with assets, resources, compiled code, etc.

apktool d -r -s application.apk

Now poke around, and edit any of the files in the application directory. If you wish to decompile any java you can do the following:

# Convert the Dex files into standard class files
dex2jar application/classes.dex

# Now use the JD (Java Decompiler) to inspect the source
jd-gui classes-dex2jar.jar

Once you have made your changes, you need to repack the APK. This will create a my_application.apk file:

apktool b -f -d application
mv application/dist/application.apk my_application.apk

The APK must be signed before it will run on a device. Create a key if you don’t have an existing one. If prompted for a password, enter anything (but remember it).

keytool -genkey -v -keystore my-release-key.keystore -alias alias_name \
                   -keyalg RSA -keysize 2048 -validity 10000

Now sign the APK with the key:

# Sign the apk
jarsigner -verbose -sigalg SHA1withRSA -digestalg SHA1 -keystore my-release-key.keystore my_application.apk alias_name

# Verify apk
jarsigner -verify -verbose -certs my_application.apk

Finally, the apk must be aligned for optimal loading:

zipalign -v 4 my_application.apk my_application-aligned.apk

Voila, now you have a my_application-aligned.apk file, which you can side load onto your device.

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