Reversing Native Libraries

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For further information check: https://maddiestone.github.io/AndroidAppRE/reversing_native_libs.html

Android apps can use native libraries, typically written in C or C++, for performance-critical tasks. Malware creators also use these libraries, as they're harder to reverse engineer than DEX bytecode. The section emphasizes reverse engineering skills tailored to Android, rather than teaching assembly languages. ARM and x86 versions of libraries are provided for compatibility.

Key Points:

  • Native Libraries in Android Apps:

    • Used for performance-intensive tasks.

    • Written in C or C++, making reverse engineering challenging.

    • Found in .so (shared object) format, similar to Linux binaries.

    • Malware creators prefer native code to make analysis harder.

  • Java Native Interface (JNI) & Android NDK:

    • JNI allows Java methods to be implemented in native code.

    • NDK is an Android-specific set of tools to write native code.

    • JNI and NDK bridge Java (or Kotlin) code with native libraries.

  • Library Loading & Execution:

    • Libraries are loaded into memory using System.loadLibrary or System.load.

    • JNI_OnLoad is executed upon library loading.

    • Java-declared native methods link to native functions, enabling execution.

  • Linking Java Methods to Native Functions:

    • Dynamic Linking: Function names in native libraries match a specific pattern, allowing automatic linking.

    • Static Linking: Uses RegisterNatives for linking, providing flexibility in function naming and structure.

  • Reverse Engineering Tools and Techniques:

    • Tools like Ghidra and IDA Pro help analyze native libraries.

    • JNIEnv is crucial for understanding JNI functions and interactions.

    • Exercises are provided to practice loading libraries, linking methods, and identifying native functions.

Resources:

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