Reverse Engineer Android Apps for API Keys

Reverse engineering Android applications to extract API keys is commonly used in security research, penetration testing, and understanding application behavior. Below is a comprehensive tutorial outlining the necessary tools, setup procedures, and steps to identify API keys within Android applications.

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Tools Required

1. Android Studio - For running and testing APKs.
2. ADB - For interacting with Android devices or emulators.
3. APKTool- For decompiling and recompiling APKs.
4. JADX - For decompiling APKs to Java source code.
5. HTTP Toolkit - For intercepting and analyzing HTTP/HTTPS traffic.

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Setup Procedures

1. Configure Android Emulator or Device

• Emulator: Use Android Studio to create and run an emulator.
• Physical Device: Enable Developer Options and USB Debugging.

2. Install ADB

• Extract the Platform Tools and add the directory to your system's PATH.

3. Install APKTool and JADX

• APKTool:

  • Download the APKTool jar and wrapper script.
  • Place them in the same directory and make the script executable.

• JADX:

  • Download and extract the JADX binaries.
  • Run jadx-gui for a graphical interface.

4. Configure HTTP Toolkit

• Install and run the application.
• Follow prompts to set up the Android device for traffic interception.

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Steps to Extract API Keys

Step 1: Obtain the APK

• From Device:

  • Use ADB to pull the APK:

    adb shell pm list packages
    adb shell pm path com.example.app
    adb pull /data/app/com.example.app/base.apk

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• From Play Store:

  • Use third-party services or tools to download the APK like APKPure.

Step 2: Static Analysis with APKTool and JADX

• Decompile APK:

  • Use APKTool:

    apktool d base.apk -o decmpld_app

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• Inspect Code:

  • Open the APK with JADX:

    jadx-gui base.apk

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• Search for API Keys:

  • Look for hardcoded strings, especially in files like BuildConfig.java, Constants.java, or any class that handles network requests.

Step 3: Modify Network Security Configuration

• Edit AndroidManifest.xml:

  • Add or modify the android:networkSecurityConfig attribute:

    <application
        android:networkSecurityConfig="@xml/network_security_config"
        ... >
    </application>

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• Create network_security_config.xml:

  • Define a configuration that trusts user-added CAs:

    <?xml version="1.0" encoding="utf-8"?>
    <network-security-config>
        <base-config>
            <trust-anchors>
                <certificates src="user" />
                <certificates src="system" />
            </trust-anchors>
        </base-config>
    </network-security-config>

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• Rebuild APK:

  • Use APKTool to rebuild:

    apktool b decmpld_app -o modified_app.apk

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• Sign APK:

  • Use jarsigner or apksigner to sign the APK.

Step 5: Intercept Network Traffic

• Configure the Android device to route traffic through HTTP Toolkit.
• Run the application and monitor the intercepted requests for API keys or tokens.

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