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Android Architcture

Mar 31, 20252 min read

Tags:androidarchitecture Related to:mobilehackingbug-bountysystemsecurity See also: Signatures in Android App Android Applications

Summary

An overview of Android system and security architecture

Android System Architecture Overview

  • Android is based off linux
  • FIle Permissons is based off linux permisson model
  • Applications are isolated from each other by this permisson model

Dalvik vs Android runtime

  • dalvik was the first VM. no longer used it was replaced by android runtime

  • Android runtime trasnlates application bytecide to device instructions

  • Every app runs in its own sandboxed virtual machine similar to linux

Android IAM

  • separate user for an application(uid 10000 - 99999)

  • separate directory for an app

  • Apps communicate with each other via Content providers or Broadcast Recievers

  • Root level and system level users - full access

  • Emulators with gplay APIs dont allow root

  • Profiles

    • Users
      • Primary user - can only be removed with factory resets
      • Secondary users - can be removed with primary users
    • Guest Users
    • Kid mode
    • Work profile

Android System Architecture

The lower level acts as an enabler for the upper level to function.

Linux Kernel

  • SE Linux
  • api / sdk version - higher the better but also needs more customers so some more lower versions are included
  • Allows access to physical components via drivers

Hardware abstraction layer

  • aids access to hardware by theapplication irrespective of manufacturer
  • Can connect with external peripherals

Native C vs Android runtime

C and C++ are devices native language - doesnt require VM Devs prfer Java Now kotlin is offical std

Java API framework (APP framework)

Application layer

  • Pre installed apps
    • contacts, phone, settings etc
    • Google apps
    • Vendor apps

Android Security and Signing

Application Signing

  • Every Android app can be reverse engineered and modified - you get the byte code as apk which you convert to smalli and then to java
  • How can you prevent anyone from signing an app and publishing in playstore?
    • Public key cryptography
  • App is signed by developers key then play store verifies it and signs with its key and publishes the app for the user
  • keysign, jarsign, zip align

Secuirty Architecture

Follows a two tier model:

  • Privilege model as in linux (Unique UID and GID)
  • Permission model
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