How to decompile and repack Android App on ChromeOS (Part 3)

(1) This continues to discuss -> How to decompile android app, previous articles are here (part 1) & (part2)


(2) Part 1 is to demo the use of ChromeOS (devloper mode on) plus the decompile tools and the process of decode and edit some resource files then rebuild the apk using apktool. And Part 2 is to use the jadx tool to decode the apk to java source file and compile it again in Android Studio.


(3) There are possibilities that the jadx tool can not decompile the complete java scources or ending with error. Even if it can be decoded, the project is maybe too complex to rebuild it in Android Studio.


(4) If we can decompile using apktool, we can edit the resource file and smali code, then repack it into apk file. However, this method is only for small modifications in code


(5) For example
cd ~/DecompileProjects # decompile the apk using apktool
apktool.bat d yourapp.apk -o yourapp_out
# edit the smali file, how ? see point(6) below
vim yourapp_out\smali_classes2\com\yourapp\MyService.smali
# build the apk using apktool
apktool b sjbot2 -o yourapp2.apk
# use jadx to decode it to gradle java source files so that you can check for any errors.
jadx --export-gradle yourapp2.apk -d yourapp2_gradle
# check the decompile java scource code before deployment
vim yourapp2_gradle\app\src\main\java\yourapp\MyService.java
# code sign and align the apk
jarsigner -verbose -sigalg SHA256withRSA -keystore my-release-key256.keystore -storepass 123456 yourapp2.apk mykey256
jarsigner -verify -verbose -certs -keystore my-release-key256.keystore yourapp2.apk
zipalign -v 4 yourapp2.apk yourapp2-aligned.apk
# install to emulator or device for testing
adb -s emulator-5556 install yourapp2-aligned.apk


(6) The problem here is how to edit the smali code, the syntax is so complex and like bytecode. So there is no easy path, and either you learn the smali language, or you can use java2smali plug-in for Android Studio to convert java code to smali code and try and error to modify it bit by bit. So using git version control and branching to keep track of your modification is very important in this case. The limitation of java2smali is that it can only be done on a complete project not a single java file, so you have to put a function that is compilable and put it in a project to compile and then convert to smali code.

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How to decompile and repack Android App on ChromeOS (Part 2)

(1) This is the sequel to How to decompile android app (part 1)
And the demo apk can be download from here. TouchCalculator_1.0-debug.apk

(2) Part 1 is to demo the use of ChromeOS (devloper mode on) plus the decompile tools and the process of decode and edit some resource files then rebuild the apk using apktool.
If you have Windows machine, you can also download the required decompile tools as demo here.

(3) This part 2 is to continue to modify the logic in the Android App which involved the source code. I will use the JADX tools and Android Studio to edit the code.

cd ~/DecompileProjects
# Use jadx to decode the apk and export gradle project file and java source code
jadx --export-gradle TouchCalculator_1.0-debug.apk -d TouchCalculator2


(4) Then, launch Android Studio and open the project in ~/DecompileProjects/TouchCalculator2 and wait for the gradle to sync. If you try to Build -> Make Project (Ctrl F9), you will notice it has a lot of errors and these errors are mainly partial packages download from repo and many of them are not linkable. So we have to study the code, to see what are the actual code files needed for this project.

(5) Then create a new Android Studio project file with Empty Activity and with the following parameters.

(6) After creation of the new project, copy the decompiled java source code and resource files to the New Project as below:

shell script    Select all

cp ~/DecompileProjects/TouchCalculator2/app/src/main/java/com/pragmatouch/calculator/*.java ~/DecompileProjects/TouchCalculator3/app/src/main/java/com/pragmatouch/calculator/. cp ~/DecompileProjects/TouchCalculator2/app/src/main/res/drawable/k*.xml ~/DecompileProjects/TouchCalculator3/app/src/main/res/drawable/. cp ~/DecompileProjects/TouchCalculator2/app/src/main/res/drawable/stackview1.xml ~/DecompileProjects/TouchCalculator3/app/src/main/res/drawable/. cp ~/DecompileProjects/TouchCalculator2/app/src/main/res/drawable/inputview1.xml ~/DecompileProjects/TouchCalculator3/app/src/main/res/drawable/. cp ~/DecompileProjects/TouchCalculator2/app/src/main/res/drawable/appvertical1.xml ~/DecompileProjects/TouchCalculator3/app/src/main/res/drawable/. cp ~/DecompileProjects/TouchCalculator2/app/src/main/res/values/colors.xml ~/DecompileProjects/TouchCalculator3/app/src/main/res/values/. cp ~/DecompileProjects/TouchCalculator2/app/src/main/res/values/strings.xml ~/DecompileProjects/TouchCalculator3/app/src/main/res/values/. cp ~/DecompileProjects/TouchCalculator2/app/src/main/res/layout/activity_main.xml ~/DecompileProjects/TouchCalculator3/app/src/main/res/layout/ cp ~/DecompileProjects/TouchCalculator2/app/src/main/res/layout/calutor.xml ~/DecompileProjects/TouchCalculator3/app/src/main/res/layout/

(7) Then, there are several changes to the build.gradle settings. Target Sdk should be set to 30 in order to build as App Bundle format.
# Sdk should not use 32 and change to 30
   compileSdk 30
   targetSdk 30

# dependencies package change to lower version as below
   implementation 'androidx.appcompat:appcompat:1.3.0'
   implementation 'com.google.android.material:material:1.3.0'

For Release build type add
   debuggable false


(8) Then, we try to Build -> Make Project (Ctrl F9) again in the New Project and see if there are any errors to solve. For complex project, it is not possible to have a full project file to build after decompile. So there is a part 3 for this topic.

(9) In fact, the project code was from here, so I can find out the required project files easily. However, there are still some features of this app which have not been implemented yet and with the decompiled java source code, you can modify the decompiled code and rebuild it to apk and test again. ALternatively, you can copy the original source code from here and paste them to the relevant java source code files of your new project.

(10) Change the resource files as in Part 1 of this artcile and add the following lines in MainActivity.java to implement the PERCENT, SQRT and RRECIPROC functions. And then Run the app in Android Studio or rebuild the apk to test on real Android device.

diff results    Select all

# diff ./TouchCalculator2/app/src/main/java/com/pragmatouc h/calculator/MainActivity.java ./TouchCalculator3/app/src/main/java/com/pragmatouch/calculator/MainActivity.java 192a193,236 > case 16: // PERCENT > double userInputValue4 = tryParseUserInput(); > if (!Double.isNaN(userInputValue4)) { > if (Double.isNaN(this.memoryValue)) { > this.memoryValue = 0.0d; > } > this.memoryValue = (userInputValue4/100); > this.userInputText.setText(doubleToString(this.memoryValue)); > this.hasFinalResult = true; > return; > } > return; > case 17: // SQRT > // reject negative num > if (currentInputLen > 0 && currentInput.charAt(0) == '-') { > return; > } > double userInputValue5 = tryParseUserInput(); > if (!Double.isNaN(userInputValue5)) { > if (Double.isNaN(this.memoryValue)) { > this.memoryValue = 0.0d; > } > this.memoryValue = Math.sqrt(userInputValue5); > this.userInputText.setText(doubleToString(this.memoryValue)); > this.hasFinalResult = true; > return; > } > return; > case 18: // RECIPROC > // reject zero > if (currentInputLen == 1 && currentInput.charAt(0) == '0') { > return; > } > double userInputValue6 = tryParseUserInput(); > if (!Double.isNaN(userInputValue6)) { > if (Double.isNaN(this.memoryValue)) { > this.memoryValue = 0.0d; > } > this.memoryValue = (1/userInputValue6); > this.userInputText.setText(doubleToString(this.memoryValue)); > this.hasFinalResult = true; > return; > } > return; 276a321,332 > } > try { > $SwitchMap$com$pragmatouch$calculator$KeypadButton[KeypadButton.PERCENT.ordinal()] = 16; > } catch (NoSuchFieldError e16) { > } > try { > $SwitchMap$com$pragmatouch$calculator$KeypadButton[KeypadButton.SQRT.ordinal()] = 17; > } catch (NoSuchFieldError e17) { > } > try { > $SwitchMap$com$pragmatouch$calculator$KeypadButton[KeypadButton.RECIPROC.ordinal()] = 18; > } catch (NoSuchFieldError e18) {

There are still bugs here and please feel free to correct them plus any new enhancements of this calculator and Enjoy Android Studio Programming.

(11) When decompiling some apks that was compiled with old sdk or converting some old android projects to the at least using SDK 30, some conversion of libraries are needed as below:
In build.gradle, you should use androidx instead of old Android Support Libraries
     compileSdkVersion 30
    implementation 'androidx.appcompat:appcompat:1.3.0'
instead of 'com.android.support:appcompat-v7:28.0.0' or something like 'com.android.support:support-v13:26.0.2'
You also should use kotlin library if the decompiled project was converted from kotlin to Java
    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"

See here for reference. https://developer.android.com/jetpack/androidx

In gradle.properties, you should setup useAndroidX and enableJetifier
android.useAndroidX=true
android.enableJetifier=true

For converted/decompiled Java classes source, don't use old android support libraries, use androidx instead.
Full class-mapping here https://developer.android.com/jetpack/androidx/migrate/class-mappings
//import android.support.v4.app.NotificationCompat;
//import android.support.v7.app.AppCompatActivity;
//import android.support.v4.app.RemoteInput;
//import android.support.v4.app.Fragment;
import androidx.core.app.NotificationCompat;
import androidx.appcompat.app.AppCompatActivity;
import androidx.core.app.RemoteInput;
import androidx.fragment.app.Fragment;
import androidx.annotation.RequiresApi;
import android.annotation.TargetApi;
import android.annotation.SuppressLint;




(12) If you really want to support old devices and not using androidx, you should use SdkVersion 28 and compatible appcompat version as below:
      compileSdkVersion 28

    implementation 'com.android.support:appcompat-v7:28.0.0'



How to decompile and repack Android App on ChromeOS

(1) You need ChromeOS with developer mode turn on for your machine. Either use a Chromebook (after powerwash) or install a recovery image on your old pc, with developer mode on (see here How to install ChromeOS to your USB stick for your old PC)

(2) You have to install Android Studio and decompile tools on your ChromeOS machine (see here How to run Android Emulator on ChromeOS)

(3) Here is the walkthrough on how to decompile apk and change the resource file and repack and sign the apk file again. Assume you don't have the source code project file. The original demo apk can be download from here. TouchCalculator_1.0-debug.apk

(4) The problem of this TouchCalculator app has two problems on the interface: a) The memory text on the left should be smaller in size b) The key (on the right) with a comma is wrong and it should be a dot for input decimal point. Without the source project file, and in order to modify it, you have to decompile it and study the code and repack it to apk file to run.

(5) Assume, you have the ChromeOS machine and installed software and also setup the decompile tools as in Steps (1) and (2). And In Linux Terminal execute these steps:

mkdir ~/DecompileProjects
cd ~/DecompileProjects
# Then copy the download apk file to this Linux folder DecompileProjects
# Use apktool to decode the apk
apktool decode TouchCalculator_1.0-debug.apk -o TouchCalculator

# After studying the xml files in the res/layout folder
# modify the file: TouchCalculator/res/layout/activity_main.xml
# and change from 25.0sp to a smaller textSize of 12.0sp
   <TextView android:textSize="25.0sp" android:textColor="@color/lightyellow" android:gravity="right" android:id="@id/txtMemory"
#to
   <TextView android:textSize="12.0sp" android:textColor="@color/lightyellow" android:gravity="right" android:id="@id/txtMemory"

# modify the file: TouchCalculator/smali_classes3/com/pragmatouch/calculator/KeypadButton.smali

# line 453 and change the button label from "," to "•"
   const-string v3, ","
# to
   const-string v3, "\u2022"

# Use apktool to build the apk after modification of the resource files
apktool build TouchCalculator -o TouchCalculator_1.01.apk
# generate a keystore with SHA256RSA (if not yet generated) and remember the storepassword
keytool -genkey -v -keystore my-release-key256.keystore -alias mykey256 -sigalg SHA256withRSA -keyalg RSA -keysize 2048 -validity 10000 -deststoretype pkcs12
# sign the apk with self-signed certificate and enter the storepassword
jarsigner -verbose -sigalg SHA256withRSA -keystore my-release-key256.keystore TouchCalculator_1.01.apk mykey256
# verify the signature
jarsigner -verify -verbose -certs -keystore my-release-key256.keystore TouchCalculator_1.01.apk
# zipalign the apk
zipalign -v 4 TouchCalculator_1.01.apk TouchCalculator_1.01-aligned.apk

(6) Install the TouchCalculator_1.01-aligned.apk on your ChromeOS (with developer mode on) machine or your Android device (With Unknown Sources on) for testing. For ChromeOS, please uninstall the previous apk version before installation of the new one.

(7) This is just a proof of concept that using apktool can decompile then modify and repack again to an apk file. To further modify the java / smali code in the decompile source, just stay tune for my next article.

How to run Android Emulator on ChromeOS

(1) You need a ChromeOS machine with x86_64 CPU, 8GB RAM or above and for Intel CPU say i5 or above and 128GB storage.

(2) Setup Linux Environment (here) in ChromeOS with at least 20GB linux file size and Download the installation file here and while downloading the file Open the Terminal app then do step 3 below and then follow the Installation Steps of Android Studio in ChromeOS (here).

(3) In Linux Terminal, you have to install libnns3
sudo apt update && sudo apt upgrade
sudo apt install libnss3-dev libgdk-pixbuf2.0-dev libgtk-3-dev libxss-dev


(4) After the acceptance of the license and Finish with the download of SDK platform and tools and this might take some time to download and unzip. Then start Android Studio and with create a new blank project or import a project from github and wait for the update of gradle sync then goto Tools -> SDK Manager to check whether the required SDK are proerly installed and then goto Tools -> Device Manager and click Create device.

(5) Choose a Hardware layout e.g. Phone Pixel 4 XL and then click Next to select a system image e.g. Release Name S, API 31, x86_64 Andoird 12.0 (Google APIs) and then click the Download next to Release Name. This may take a much longer time to download and unzip and would occupy a lot of Linux System storage space if you download more than one image.

(6) After download is done, click Finish and then Next to name the device and fill in Advanced Settings if any and then click Finish to create.

(7) Then start the emulator by Running your project to the emulator and check whether the compileSdkVersion (e.g. 32) in build.gradle for Module match with your download and first click the Build -> Make Project (Ctrl+F9) to see if there is any error. Then, start the project by clicking the Run -> Run App (Shift+F10) or start the Emulator manually in Device Manager. After this step your ChromeOS will experience a significant slow down when the Emulator is running. My testing is that Intel 10th generation i5 CPU with 8GB Ram and 128 GB storage can run reasonably well for the simple project in Android Emulator.

(8) If you want a larger screen or floating window for the Emulator, you can click the Settings button of the elumlator window and set it to View Mode -> Window and then resize the detached window.

(9) You can debug the project source code with the Emulator if you click Run -> Debug App (Shift+F9). If you have enabled Developer Mode in ChromeOS (which is default for Brunch framework). Alternatively, you could build the apk and side load it to the ChromeOS or real device for test run, but debugging on ChromeOS or real device has to first powerwash the ChromeOS machine and then open the ADB port

(10) If you like to have this android studio project (written in Java) for testing, you can download it from here PeamonCalculator.zip

There is another Android Studio Project on Calculator which is written in Kotlin GitHub project source code download here. You might have to change the build SDK version for this project to your Settings in SDK, e.g. compileSdkVersion 32 which is the default SDK version currently for newly installed Android Studio.

(11) If you want to use the JRE from Android Studio in Terminal, add these lines in your ~/.profile
# set JAVA_HOME for Android Studio
export JAVA_HOME=/opt/android-studio/jre
export ANDROID_HOME="$HOME/Android"
export ANDROID_SDK_ROOT="$ANDROID_HOME/Sdk"
PATH="$JAVA_HOME/bin:$ANDROID_SDK_ROOT/build-tools/30.0.3:$PATH"


(12) If you want to install decompile tools on ChromeOS
sudo apt install -y curl unzip
export ANDROID_DECOMPILE_TOOLS=="$ANDROID_HOME/DecompileTools"
# install apktool (d) decompile apk file to smali and resource folders or (b) build them to apk file
mkdir -p $ANDROID_DECOMPILE_TOOLS/apktool && pushd $ANDROID_DECOMPILE_TOOLS/apktool && curl -OL https://bitbucket.org/iBotPeaches/apktool/downloads/apktool_2.6.0.jar && ln -s apktool_2.6.0.jar apktool.jar && curl -OL https://raw.githubusercontent.com/iBotPeaches/Apktool/master/scripts/linux/apktool && chmod +x apktool && popd
# install jadx (to compile smali file to java source code and support whole project or single file conversion)
curl -OL https://github.com/skylot/jadx/releases/download/v1.3.3/jadx-1.3.3.zip && unzip jadx-1.3.3.zip -d $ANDROID_DECOMPILE_TOOLS/jadx && rm -v jadx-1.3.3.zip
PATH="$ANDROID_HOME/DecompileTools/apktool:$ANDROID_HOME/DecompileTools/jadx/bin:$PATH"


# install jadx (to compile java/kotlin project files to smali code, but does not support single file conversion)
For Java2Smali, install the plugin version in Android Studio. File -> Settings (Ctrl-ALt-S ) -> Plugins and search java2smali to install.