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 Android Explained

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Malay Akechan
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PostSubject: Android Explained   Sun Oct 02, 2011 10:40 am

Android is a very fast evolving smartphone & tab platform. Let's know more about it. yes

INTRODUCTION


After the recent developments in the smart phones, they are no longer used just to make calls and send sms’s. They have been endowed with much more features than what used to be in them 5 years back. They can now take pictures, make video calls, track locations, do banking, and many more things.

With all these features, there also comes a need for security for the mobile phones. No one wants their personal information to be seen by anyone else.

Just like the Operating Systems for computers, the smartphones these days also come with an Operating System to perform advance tasks.

In this paper, I would be focussing on the Android OS, and discussing about its security, and existing malwares on this platform.

According to Google, Android is a software stack consisting of an OS, middleware and software applications. It is an open source platform developed and maintained by Google and the OHA(Open Handset Alliance) which is a consortium of over 50 different international electronics and telecom companies.

It has also got a huge user and developer community base, mainly because of its open nature. Any developer can publish his/her app on the Android Market with a self signed certificate. This makes it easier for the developers as there is no intermediate CA involved. The official application store for Android is the Android Market, which consists of over 4 billion apps.

Android is supported by a Linux Kernel 2.6.x at its core. The native libraries are in C/C++ whereas the applications are written exclusively in Java, with the layout designed in either XML or Java.

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Shown above is the Android Architecture. At the bottom core, is the linux kernel, which has been modified specially for the mobile platform. It is required for the core system services such as security, process management, memory management etc.

Above it, we have the libraries, which includes WebKit, SQLite (a powerful and lightweight database for the mobile), and OpenGL and so on. The libraries are written in C and C++ and are used by various components of the Android System.
The Android Runtime includes DVM (Dalvik Virtual Machine) and Core Libraries. The DVM is a virtual machine to run the applications. We will be looking more about the Dalvik Virtual Machine in the coming pages.

Above this, we have the Application Framework and Applications Layer. The Application Framework allows the developers to take advantage of various inbuilt functionalities of the Android Platform. All the developers have access to the same framework API that is used to build the default applications. So, developers can access each and every feature supported by the phone.

The Applications layer consists of the default applications present in-built into the phone. These include Phone, Browser, Contacts, Home and so on.

Android comes with a Android SDK which is a Development kit made for developers to create applications for the android platform. The Android also consists of an emulator and the other tools, along with the libraries necessary for the development. Emulator is a virtual phone, which you can run on your computer, to test the developed applications.

ANDROID APPLICATIONS


Android Applications or Android apk’s are basically an archive file containing all the necessary files and folders in an application. An Android application is made up of many components namely Activities, Intents, Services, Broadcast Receivers and Content Providers. We’ll be discussing each of them one by one in this paper.

ACTIVITY


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Activities are any visual screens you see and interact with in an android application. It can consist of views such as the Button View, Text view, Table View etc. Each application has a “main” activity which is the first screen presented to the user as soon as he starts the application. The Android activities follow an activity lifecycle. So, whenever an activity of an application goes to the background, it doesn’t exist, instead it just goes to an OnPause state. Also, one interesting thing to note here is that, Activity of one application can start the activity of another application, if it has the appropriate permissions.

SERVICES


Apart from the components with which you interact, there are also components in an application, which work in the background, for example a song playing in the background. For those things which need to continue even when they are not in focus, we need services. They are the components which work in the background and keep performing the necessary operations without any user interaction.
Suppose, you launch a music player application. The first screen, with which you are interacting with, is an activity. But as soon as you select a song to play, and move on to some other application, the activity goes to a pause state, but the service (the song which is playing) keeps running in the background.
Some more examples of services are network operations, file input/output etc.


CONTENT PROVIDERS


Every application needs to store its data, be it in the SQLite database or some place in the SD card or in the phone itself. For storing and retrieving the data, we need to use Content Providers.

BROADCAST RECEIVERS


As the name suggests, they receive broadcast signals made by other applications or by the phone itself and perform the action whatever they are assigned.

INTENTS


Intent is an abstract description of an operation to be performed. It can be used with startActivity to launch an Activity, broadcastIntent to send it to any interested BroadcastReceiver components, and startService (Intent) or bindService (Intent, ServiceConnection, int) to communicate with a background Service.

Intents are used by the application to bring all the other components together. It is used by an application to interact with the phone’s hardware components and other applications, or to start a service or activity with the help of broadcast receiver. Intents can also be used to call the activities of another application from one application.

ANDROID MANIFEST


All the android applications must contain a file named as AndroidManifest.xml. The xml file should be containing the following necessary things:

1. All the hardware and software permissions needed by the application.
2. The external API's if required by the application ( ex – Google Maps API )
3. The minimum Android Version needed to run the program
So, if we want to access any of the features of the phone, we have to clearly state it in the Android Manifest file. These permissions will be shown to the user, when he/she would be installing the application.

A sample AndroidManifest.xml file :

Code:
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.aditya.com"
android:versionCode="1"
android:versionName="1.0">
<uses-sdk android:minSdkVersion="8" />
<application android:icon="@drawable/icon" android:label="@string/app_name">
<activity android:name=".aditya"
android:label="@string/app_name">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
<uses-permission android:name="android.permission.SEND_SMS"></uses-permission>
</application>
</manifest>


In the above application, one can clearly see that, I’m trying to access the Send SMS feature of the phone which is stated as the permission android.permission.SEND_SMS.

The AndroidManifest file also helps a user in determining whether an application is a legitimate one or it is of malicious intentions. For example, a game application may need INTERNET permission to upload the high scores to the online server, but in no case, it should need permissions such as SEND_SMS, READ_CONTACTS or anything like that.

ANDROID SECURITY MODEL



Android puts the user in control of everything. The Android Security model is based on the fact that each application is to be run within its own DVM, which acts as a sandbox between two applications. So that, even if one of the application crashes, it won't affect the other application or the phone. This makes sure that the two applications are independent of each other; also the data of one application cannot be accessed by another application without sufficient permissions.
Dalvik Virtual Machine was created by Dan Borstein, specifically for the android platform, so that applications could run smoothly under low power processors with graphics and hardware acceleration suitable for the mobile device.

The DVM is register based instead of the standard Stack Based JVM (Java Virtual Machine). Due to this, java files of the android applications, are compiled to a Dalvik byte codes or dex (Dalvik Executables) before being packaged into the apk.

Also, if there are more than one java files, all are compiled into a single dex file to save space and memory.
When installed, each application is assigned a unique UID and GID, just like what is there in the Linux file system. Each app's UID has to be different from another application. Also, applications from a single developer could share the same UID, so that it is easy to push updates for the application, without having the user to uninstall the old version and install the new one.

I hope this would be useful. wave
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PostSubject: Re: Android Explained   Sun Oct 02, 2011 2:28 pm

Yup! Bro this is really awesome post...! yippi thanks rock
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PostSubject: Re: Android Explained   Mon Oct 03, 2011 12:29 am

Really informative, though I knew most of the parts yippi

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PostSubject: Re: Android Explained   Tue Nov 29, 2011 7:25 pm

awesome post thanks.. :)
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