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Simple example of reverse .apk (removing Ads)

Date of publication: 10 Mar 2025

Okay, today I gonna do some simple reverse engineering on Android application. As an example I get an amazing application Apk Extractor Fastest & Suppor

Tools

Process

Get .apk file by Apk Extractor Fastest & Suppor itself.

Main screen of APK Extractor

What we see? We see a terrible Ads in so wonderful application.

Discover Java code

  • open our .apk by JADX

And we can see something like this: JADX screen

  • Fist of all we have to see AndroidManifest.xml. Oh, bro, thanks a lot!
    XML
    <activity
android:name="com.iraavanan.apkextractor.ad.NoAdsActivity"
android:screenOrientation="portrait"/>
  • Let's see NoAdsActivity, obviously.

The path in the JADX is

Source code/com.iraavanan.apkextractor.ad.NoAdsActivity.

Java
protected void onCreate(Bundle bundle) {
    super.onCreate(bundle);
    setContentView(R.layout.activity_no_ads);
    ActionBar supportActionBar = getSupportActionBar();
    if (supportActionBar != null) {
        supportActionBar.setTitle(getString(R.string.premium));
    }
    ActionBar supportActionBar2 = getSupportActionBar();
    if (supportActionBar2 != null) {
        supportActionBar2.setDisplayHomeAsUpEnabled(true);
    }
    com.iraavanan.apkextractor.g.f fVar = com.iraavanan.apkextractor.g.f.a;
    if (fVar.b(this).getBoolean(getString(R.string.is_premium), false)) {
        J();
        return;
    }
// some code
OMG, bro, do you really keep a premium flag in SharedPreferences?!

When I saw it I thought to get another example, but it's OK. It's simple and visible.

Extract and change Smali code

  • Execute apktool:

/opt/apktool/apktool d ApkExtractor_com.iraavanan.apkextractor.apk

So we've got a folder ApkExtractor_com.iraavanan.apkextractor with Smali code. You can open this folder as a project in Android Studio. Our activity in on the same path: smali/com.iraavanan.apkextractor.ad.NoAdsActivity.smali

  • look for the condition in the onCreate() function
Smali
:cond_1
sget-object p1, Lcom/iraavanan/apkextractor/g/f;->a:Lcom/iraavanan/apkextractor/g/f;

invoke-virtual {p1, p0}, Lcom/iraavanan/apkextractor/g/f;->b(Landroid/content/Context;)Landroid/content/SharedPreferences;

move-result-object v0

const v1, 0x7f10004a

invoke-virtual {p0, v1}, Landroid/app/Activity;->getString(I)Ljava/lang/String;

move-result-object v1

const/4 v2, 0x0

invoke-interface {v0, v1, v2}, Landroid/content/SharedPreferences;->getBoolean(Ljava/lang/String;Z)Z

move-result v0

if-eqz v0, :cond_2

invoke-direct {p0}, Lcom/iraavanan/apkextractor/ad/NoAdsActivity;->J()V

goto/16 :goto_1

Looks not so nice :) Let's define what happens. Dalvik opcodes

  • sget-object p1 //etc - we are getting some object and put it into p1
  • invoke-virtual {p1, p0} //etc - execute some function on p1 object, p0 is the pointer to our activity... possibly :) It looks like we've got SharedPreferences
  • move-result-object v0 - put SharedPreferences into v0
  • const v1, 0x7f10004a - some const similar like ID of resource
  • invoke-virtual {p0, v1} //etc - activity of p0 getting string resource by v1 ID
  • move-result-object v1 - put that string into v1
  • const/4 v2, 0x0 - define the false constant
  • invoke-interface {v0, v1, v2} //etc - our key function what defines premium
  • move-result v0 - result of function into v0
  • if-eqz v0, :cond_2 - if our result is 0(false) the program go to :cond_2

Obviously, we have to change the result of function.

Text Only
move-result v0

const/4 v0, 0x1

if-eqz v0, :cond_2

Pack to .apk

  • build an .apk file

/opt/apktool/apktool b ApkExtractor_com.iraavanan.apkextractor -o apkextractor.apk - compress .apk

zipalign -v -p 4 apkextractor.apk apkextractor_compressed.apk - create key for signing by keytool

keytool -genkeypair -v -keystore release-key.keystore -alias apkextractor -keyalg RSA -keysize 2048 - sign

apksigner sign --ks release-key.keystore --ks-key-alias apkextractor --out apkextractor_signed.apk apkextractor_compressed.apk

Install and run

  • remove the base app version

adb uninstall com.iraavanan.apkextractor - install changed version

adb install apkextractor_signed.apk - run

Main screen of APK Extractor with Ads

Hmm... something went wrong...

Okay, maybe I should go to premium option in the app.

Premium screen of APK Extractor

Let's recreate activity by Back button or rotate screen.

Main screen of APK Extractor without Ads

So... that's all, no Ads.

What do I wanna say at the end? Don't keep premium flags inside the app.

Thanks for your attention!