Dex Control Flow Relocation, Anti-Reversing for Android Apps

Learn to Prevent Static Code Analysis in Android apps, in mobile CI/CD with a Data-Driven DevSecOps™ build system.

What is Static Code Analysis?

In Android, compiled Java/Kotlin code resides in classes.dex files. Appdome’s Dex Relocation is a security feature that modifies a mobile app’s compiled code by obfuscating the logical control-flow of the app. The purpose of control-flow relocation is to make reverse engineering tools ineffective and even unusable and to make the code difficult to trace and understand. To do this, Appdome uses several techniques such as applying call obfuscation to the compiled Java code and modifying the function call targets to obscure functions.

Why Prevent Static Code Analysis in Android Apps?

Obfuscation is become a well-established preventive measure developers use against static reverse engineering attempts. What sets various obfuscation solutions apart are several things: Ease of use (e.g., specialized compilers and post-build tools), Performance (i.e., performance penalty, if any) and the reference threat level.
Since eventually all defenses can be broken, which indicates how good a defense is the amount of work, expertise and time expected to break.

Preventing Static Code Analysis on Android apps by Using Appdome

On Appdome, follow these 3 simple steps to create self-defending Android Apps that Prevent Static Code Analysis 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 Relocation.

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

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

      Figure 1: Fusion Set that will contain the Dex Relocation 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 Relocation feature via Appdome Console, to add the Dex Relocation 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 Relocation feature via Appdome Console

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

      1. Where: Inside the Appdome Console, go to Build > Security Tab > TOTALCode™ Obfuscation section
      2. How: Toggle (turn ON) Dex Relocation, 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 Prevents Static Code Analysis.

         Figure 3: Prevent Static Code Analysis 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 Relocation 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 Relocation

         

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

      4. Extra Configuration with Dex Relocation:

         1. Favor Loading Time

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

         2. Exclude Specific Classes

            List class or package prefixes to exclude and not obfuscate.

         3. No Mapping File/Script

            Changes the flow logic, but does not rename functions. No mapping file or script needed.

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

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

3. Certify the Dex Relocation feature in Android Apps.

   After building Dex Relocation, Appdome generates a Certified Secure™ certificate to guarantee that the Dex Relocation protection has been added and is protecting the app. To verify that the Dex Relocation 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 Relocation has been added to each Android app. Certified Secure provides instant and in-line DevSecOps compliance certification that Dex Relocation and other mobile app security features are in each build of the mobile app