Injecting code into APK

Question

I know you can decompile code using apktool and recompile it again, but my question is how would you be able to inject large amounts of code into an apk and execute it.

Answer 1

I think this post: http://www.bulbsecurity.com/backdooring-apks-programmatically-2/ describes in pretty much details how injecting of classes can be done. I have followed the steps myself an can confirm it works, with the only drawback being that the apk key will change, which apparently is the case with amazon.

As for the injection of code in existing classes, this can also be done with a bit of understanding of smali (i have done that also on limited scale).