# Android binary libraries
## Mediacode.apk
The application contains DEX code and binary blobs.
One version for each architecture.
* armeabi: ARM 32bits no Floating Point
* mips: MIPS
* x86: intel X86 32bits
Libraries export symbols to be used through JNI.
* `nm -gD lib/x86/librrnad.so | grep JNI`
Before the binary libraries can be used, Java must load them.
* `System.loadLibrary`: argument is the library name (without lib, architecture or .so).
* `System.load`: generic object load. The argument is the full path to the object.
* **The JNI\_OnLoad method is called automatically (in the lib).**
* Allows automatic setup of data structures and generic initialization.
* May be abused if malware is present.
Without the library, the application will crash when external methods are requested.
## JNI Arguments
**Native methods support arguments from Java code.**
* Arguments are pointers to Java structures.
* Must be processed using specific methods, capable of handling the native Java types.
Native methods can also be called Java methods and classes.
* Mainly achieved by the first argument of any JNI method: JNIEnv\*.
JNIEnv\* is a pointer to a structure with a large number of functions.
* JNI Methods use it to invoke Java methods and handle Java types.
In the Java world native methods are declared:
* With the keyword native.
* Without implementation.
Easy to spot if we have the Java or Smali code.
* Java: `public native String decryptString(String)`.
* Smali: `.method public native decryptString(Ljava/lang/String;)Ljava/lang/String`
