The differences between a physical image and a logical extraction

There's a reason we want a physical image

All blog posts to date

Introduction	Acquisition	Analysis
Introduction	Imaging an Android Device	Examining the image
Picking a Toolkit	Live imaging an Android device	Some hidden artifacts in a physical image
	Why not load ClockworkMod or TWRP to image a device?	Using Autopsy to examine an Android image
	Identifying your Userdata Partition	Some artifacts in the /data/system/ directory
	Some non-root methods to learn about a device	Viewing SQLite Databases
	A quick note on imaging newer Android devices	Facebook for Android Artifacts
	Using Windows to Live Image an Android device	Interpreting data from apps
	Obtaining all files in the data partition without a physical image	Waze for Android forensics
		Magnet Forensics App Simulator
App Reversing	Other Topics
Reverse Engineering an Android App File	The differences between a physical image and a logical extraction
Fun with Apktool	Dirty cow
Deep dive into an app	Imaging and examining an Android car stereo
	Unpacking boot and recovery kernels
	MTPwn

Introduction This post is a request from a reader.  Thanks for the request!  If you, the reader, ever have a topic you would like to see me dive into, message me.

The question was what data do you have when you obtain a physical image instead of a logical extraction.  Great question.  First, to define a couple of working terms here.  A physical image will be the image you would obtain when following this guide on a previous blog post or using a similar tool, such as a Cellebrite UFED Physical.  A logical extraction of data is a set of data extracted using a forensic app.  For this blog, I'll reference AFLogical by viaForensics, which is a free tool you can find here and you can follow instructions for using it here.

(Please note.  In no way am I trying to bash viaForensics here.  viaForensics is a great company and I admire their work.  I'm referencing this tool as a free logical extraction tool you can download and use while pointing out the weaknesses of using logical extractions.  The fact that the tool is free should be an indication that this tool is not their premiere tool.  They have far more powerful tools and their professional services are among the best in the industry.)

So with all of the above out of the way, here we go ...

Data obtained with a physical image

The answer is everything in storage on the device.  You get every file, every database, every picture, plus also all of the slack.  For a writeup on slack space, check out this page by viaForensics.  Simply with a physical image, you get everything in storage.

There is a good reason why we always want a physical image.  Examining a physical image takes specialty tools, and I go over the basics in this blog post.  If you want to look at data records, such as text messages, you do not have a simple file to examine with all of the records.  You need to find the file storing these records, which is most likely a database, and examine the database file.  The examination process is not straightforward, but you obtain the most data.

What you do not obtain is live running memory.  Sometimes live running memory can contain important data, including decrypted data if the data in storage is encrypted.  I do not intend to go over how to image live memory simply because it is a very complicated process which sometimes does not work.

Data obtained using a logical record extraction tool

A logical record extraction tool is an app which installs on the device.  As I discussed in my post on live imaging, the imaging process requires an exploit.  In that previous post, the exploit allows for root privileges.  Root access is required to image the device, and root access is also required to read files in the /data partition, which is where user records are stored.  A logical record extraction tool does not require root access.  A logical record extraction tool uses Android APIs to extract records from the device and save them to external storage.  These APIs allow a programmer to write an app to request certain records.  The APIs do not return the actual database files but they do return the records.  For a guide on this process, check out this programming guide on how to programatically read SMS from the inbox.  Look specifically at this code snippet (from the website, I cleaned it up some to make it more readable):

if (cursor != null)
{
  try
  {
    count = cursor.getCount();
    if (count > 0)
    {
      cursor.moveToFirst();

      long messageId = cursor.getLong(0);
      long threadId = cursor.getLong(1);
      String address = cursor.getString(2);
      long contactId = cursor.getLong(3);
      String contactId_string = String.valueOf(contactId);
      long timestamp = cursor.getLong(4);

      String body = cursor.getString(5);

      if (!unreadOnly)
      {
        count = 0;
      }

      SmsMmsMessage smsMessage = new SmsMmsMessage(
      context, address, contactId_string, body, timestamp,
      threadId, count, messageId, SmsMmsMessage.MESSAGE_TYPE_SMS);

      return smsMessage;
    }
  }
}

This source code has permissions to read the SMS database.  The program goes through the database row by row and extracts the message ID, thread ID, address, contact ID, and timestamp.  All of this data goes into an “SmsMmsMessage” object.  A programmer can use this object to save the message ID, thread ID,  address, contact ID, and timestamp to a file, which effectively means all SMS records are retrieved and exported.

Here is the problem.  The APIs will give you a certain set of data.  There may be more data associated with these records which the APIs do not return.  The above code, for example, does not return any location related data associated with the message or any metadata associated with the contact or the phone number.  These extra data records will be in the database file which you can read if you obtain a physical image of the device.

The APIs also will not return any deleted records.  When an SMS message is deleted, the database file no longer retains the message.  However, if you have a physical image, you may be able to find the deleted message in slack space.  The APIs only return what records they are programmed to return; they cannot return records floating in slack space.

The logical record extraction process is incapable of extracting files in the /data partition.  You need root access to extract the actual files.  The APIs only return the records, not the files.

Also, there may not be APIs available to return data from third party apps, ranging from Facebook to third party messaging apps to web browsing apps.  If there is not an API, the data can not be retrieved using a logical record extraction app.  With a physical image, you can examine the database files associated with these apps and examine the database files.


Conclusion

In summation, you want a physical image.  The logical extraction tool is a good tool to use if you need a quick look at text messages or call logs, and it also is a good tool to use if you are unable for whatever reason to obtain a physical image of the device.  If you are doing a detailed examination of the device, you will need a physical image.

The logical extraction tools have their purposes.  I am not here to denigrate those tools by any means.  I am here to point out their limitations.

Thank you to one of my readers for suggesting this post.  If you, the reader, have a good topic you would like to see a full post on, shoot me a message and I'd be glad to oblige.

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