How to Encrypt Java Class Files (.dex) in Android Apps

Learn to Encrypt Dex Files in Android apps, in mobile CI/CD with a Data-Driven DevSecOps™ build system.

What is DEX File Encryption?

In Android, compiled Java/Kotlin code resides in classes.dex files (see structure of Android applications). The common tools to reverse engineer DEX files are disassemblers such as baksmali and dex2jar and decompilers such as jadx and jdgui. Appdome’s DEX File Encryption is a security feature uses Appdome’s APPCode Packer, the industry’s first no-code Android App Packer, to hide and encrypt all Java code of an Android app when the app is built by using Appdome. Using code packing with Appdome APPCode Packer makes the disasembling tools ineffective and even unusable. For details see White Hat Android Packer from Appdome.

Why Encrypt DEX Files (Java Classes) in Android apps?

Appdome encrypts all DEX files not needed for app initialization, making it impossible for disassemblers to find the original code. At run-time, Appdome’s code will decrypt the encrypted DEX files and allow the app to continue working as usual.

This obfuscation technique provides the following benefits:

1. Trying to use offline reversing techniques on the application will fail as most classes will not be found in the APK or in the AAB.
2. Decryption overhead is only incurred during the app’s first run, and even then has minimal impact.
3. Since the DEX files are encrypted, they are protected by Appdome’s Anti-Tampering.
4. In addition, any attempt to force this information out of the application using run-time methods will be thwarted by Appdome’s Anti-Debugging and other features in ONEShield.

Encrypting Dex Files on Android apps by Using Appdome

On Appdome, follow these 3 simple steps to create self-defending Android Apps that Encrypt Dex Files without an SDK or gateway:

1. Upload the Mobile App to Appdome.

   1. Upload an app to Appdome’s Mobile App Security Build System

   2. Upload Method: Appdome Console or DEV-API
   3. Android Formats: .apk or .aab

2. Build the feature: Dex File Encryption.

   1. Building Dex File Encryption by using Appdome’s DEV-API:

      1. Create and name the Fusion Set (security template) that will contain the Dex File Encryption feature as shown below:
      

      Figure 1: Fusion Set that will contain the Dex File Encryption feature
      Note: Naming the Fusion Set to correspond to the protection(s) selected is for illustration purposes only (not required).

      2. Follow the steps in Sections 2.2.1-2.2.2 of this article, Building the Dex File Encryption feature via Appdome Console, to add the Dex File Encryption feature to this Fusion Set.

      3. Open the Fusion Set Detail Summary by clicking the “...” symbol on the far-right corner of the Fusion Set, as shown in Figure 1 above, and get the Fusion Set ID from the Fusion Set Detail Summary (as shown below):

         Figure 2: Fusion Set Detail Summary
         Note: Annotating the Fusion Set to identify the protection(s) selected is optional only (not mandatory).

      4. Follow the instructions below to use the Fusion Set ID inside any standard mobile DevOps or CI/CD toolkit like Bitrise, App Center, Jenkins, Travis, Team City, Cirlce CI or other system:

         1. Build an API for the app – for instructions, see the tasks under Appdome API Reference Guide
         2. Look for sample APIs in Appdome’s GitHub Repository

   2. Building the Dex File Encryption feature via Appdome Console

      To build the Dex File Encryption protection by using Appdome Console, follow the instructions below.

      1. Where: Inside the Appdome Console, go to Build > Security Tab > TOTALData™ Encryption section
      2. How: Toggle (turn ON) Dex File Encryption, as shown below.
         If needed, Customize the Threat Notification to be displayed to the mobile end-user in a standard OS dialog notification when Appdome Encrypts Dex Files.

         Figure 3: Encrypt Dex Files option
         Note: The App Compromise Notification contains an easy to follow default remediation path for the mobile app end user. You can customize this message as required to achieve brand specific support, workflow or other messaging.

      3. When you select the Dex File Encryption you'll notice that your Fusion Set you created in step 2.1.1 now bears the icon of the protection category that contains Dex File Encryption

         

         Figure 4: Fusion Set that displays the newly added Dex File Encryption protection

      4. Optional Configuration with Dex File Encryption:

         1. Favor Loading Time

            Automatically detect and optimize obfuscation of publicly available components in order to preserve application loading time.

      5. Click Build My App at the bottom of the Build Workflow (shown in Figure 3).

   Congratulations!  The Dex File Encryption protection is now added to the mobile app

3. Certify the Dex File Encryption feature in Android Apps.

   After building Dex File Encryption, Appdome generates a Certified Secure™ certificate to guarantee that the Dex File Encryption protection has been added and is protecting the app. To verify that the Dex File Encryption protection has been added to the mobile app, locate the protection in the Certified Secure™ certificate as shown below:

   Figure 5: Certified Secure™ certificate

   Each Certified Secure™ certificate provides DevOps and DevSecOps organizations the entire workflow summary, audit trail of each build, and proof of protection that Dex File Encryption has been added to each Android app. Certified Secure provides instant and in-line DevSecOps compliance certification that Dex File Encryption and other mobile app security features are in each build of the mobile app