Practical Mobile Forensics

Chapter 1. Introduction to Mobile Forensics

In 2013, there were almost as many mobile cellular subscriptions as there were people on earth, says International Telecommunication Union (ITU). The following figure shows the global mobile cellular subscriptions from 2005 to 2013. Mobile cellular subscriptions are moving at lightning speed and passed a whopping 7 billion early in 2014. Portio Research Ltd. predicts that mobile subscribers will reach 7.5 billion by the end of 2014 and 8.5 billion by the end of 2016.

Mobile cellular subscription growth from 2005 to 2013

Smartphones of today, such as the Apple iPhone, Samsung Galaxy series, and BlackBerry phones, are compact forms of computers with high performance, huge storage, and enhanced functionalities. Mobile phones are the most personal electronic device a user accesses. They are used to perform simple communication tasks, such as calling and texting, while still providing support for Internet browsing, e-mail, taking photos and videos, creating and storing documents, identifying locations with GPS services, and managing business tasks. As new features and applications are incorporated into mobile phones, the amount of information stored on the devices is continuously growing. Mobiles phones become portable data carriers, and they keep track of all your moves. With the increasing prevalence of mobile phones in peoples' daily lives and in crime, data acquired from phones become an invaluable source of evidence for investigations relating to criminal, civil, and even high-profile cases. It is rare to conduct a digital forensic investigation that does not include a phone. Mobile device call logs and GPS data were used to help solve the attempted bombing in Times Square, New York, in 2010. The details of the case can be found at http://www.forensicon.com/forensics-blotter/cell-phone-email-forensics-investigation-cracks-nyc-times-square-car-bombing-case/. The science behind recovering digital evidence from mobile phones is called mobile forensics. Digital evidence is defined as information and data that is stored on, received, or transmitted by an electronic device that is used for investigations. Digital evidence encompasses any and all digital data that can be used as evidence in a case.

Mobile forensics

Digital forensics is a branch of forensic science focusing on the recovery and investigation of raw data residing in electronic or digital devices. Mobile forensics is a branch of digital forensics related to the recovery of digital evidence from mobile devices. Forensically sound is a term used extensively in the digital forensics community to qualify and justify the use of particular forensic technology or methodology. The main principle for a sound forensic examination of digital evidence is that the original evidence must not be modified. This is extremely difficult with mobile devices. Some forensic tools require a communication vector with the mobile device, thus standard write protection will not work during forensic acquisition. Other forensic acquisition methods may involve removing a chip or installing a bootloader on the mobile device prior to extracting data for forensic examination. In cases where the examination or data acquisition is not possible without changing the configuration of the device, the procedure and the changes must be tested, validated, and documented. Following proper methodology and guidelines is crucial in examining mobile devices as it yields the most valuable data. As with any evidence gathering, not following the proper procedure during the examination can result in loss or damage of evidence or render it inadmissible in court.

The mobile forensics process is broken into three main categories: seizure, acquisition, and examination/analysis. Forensic examiners face some challenges while seizing the mobile device as a source of evidence. At the crime scene, if the mobile device is found switched off, the examiner should place the device in a faraday bag to prevent changes should the device automatically power on. Faraday bags are specifically designed to isolate the phone from the network. If the phone is found switched on, switching it off has a lot of concerns attached to it. If the phone is locked by a PIN or password or encrypted, the examiner will be required to bypass the lock or determine the PIN to access the device. Mobile phones are networked devices and can send and receive data through different sources, such as telecommunication systems, Wi-Fi access points, and Bluetooth. So if the phone is in a running state, a criminal can securely erase the data stored on the phone by executing a remote wipe command. When a phone is switched on, it should be placed in a faraday bag. If possible, prior to placing the mobile device in the faraday bag, disconnect it from the network to protect the evidence by enabling the flight mode and disabling all network connections (Wi-Fi, GPS, Hotspots, and so on). This will also preserve the battery, which will drain while in a faraday bag and protect against leaks in the faraday bag. Once the mobile device is seized properly, the examiner may need several forensic tools to acquire and analyze the data stored on the phone.

Mobile device forensic acquisition can be performed using multiple methods, which are defined later. Each of these methods affects the amount of analysis required, which will be discussed in greater detail in the upcoming chapters. Should one method fail, another must be attempted. Multiple attempts and tools may be necessary in order to acquire the most data from the mobile device.

Mobile phones are dynamic systems that present a lot of challenges to the examiner in extracting and analyzing digital evidence. The rapid increase in the number of different kinds of mobile phones from different manufacturers makes it difficult to develop a single process or tool to examine all types of devices. Mobile phones are continuously evolving as existing technologies progress and new technologies are introduced. Furthermore, each mobile is designed with a variety of embedded operating systems. Hence, special knowledge and skills are required from forensic experts to acquire and analyze the devices.

Mobile forensic challenges

One of the biggest forensic challenges when it comes to the mobile platform is the fact that data can be accessed, stored, and synchronized across multiple devices. As the data is volatile and can be quickly transformed or deleted remotely, more effort is required for the preservation of this data. Mobile forensics is different from computer forensics and presents unique challenges to forensic examiners.

Law enforcement and forensic examiners often struggle to obtain digital evidence from mobile devices. The following are some of the reasons:

Hardware differences: The market is flooded with different models of mobile phones from different manufacturers. Forensic examiners may come across different types of mobile models, which differ in size, hardware, features, and operating system. Also, with a short product development cycle, new models emerge very frequently. As the mobile landscape is changing each passing day, it is critical for the examiner to adapt to all the challenges and remain updated on mobile device forensic techniques.
Mobile operating systems: Unlike personal computers where Windows has dominated the market for years, mobile devices widely use more operating systems, including Apple's iOS, Google's Android, RIM's BlackBerry OS, Microsoft's Windows Mobile, HP's webOS, Nokia's Symbian OS, and many others.
Mobile platform security features: Modern mobile platforms contain built-in security features to protect user data and privacy. These features act as a hurdle during the forensic acquisition and examination. For example, modern mobile devices come with default encryption mechanisms from the hardware layer to the software layer. The examiner might need to break through these encryption mechanisms to extract data from the devices.
Lack of resources: As mentioned earlier, with the growing number of mobile phones, the tools required by a forensic examiner would also increase. Forensic acquisition accessories, such as USB cables, batteries, and chargers for different mobile phones, have to be maintained in order to acquire those devices.
Generic state of the device: Even if a device appears to be in an off state, background processes may still run. For example, in most mobiles, the alarm clock still works even when the phone is switched off. A sudden transition from one state to another may result in the loss or modification of data.
Anti-forensic techniques: Anti-forensic techniques, such as data hiding, data obfuscation, data forgery, and secure wiping, make investigations on digital media more difficult.
Dynamic nature of evidence: Digital evidence may be easily altered either intentionally or unintentionally. For example, browsing an application on the phone might alter the data stored by that application on the device.
Accidental reset: Mobile phones provide features to reset everything. Resetting the device accidentally while examining may result in the loss of data.
Device alteration: The possible ways to alter devices may range from moving application data, renaming files, and modifying the manufacturer's operating system. In this case, the expertise of the suspect should be taken into account.
Passcode recovery: If the device is protected with a passcode, the forensic examiner needs to gain access to the device without damaging the data on the device.
Communication shielding: Mobile devices communicate over cellular networks, Wi-Fi networks, Bluetooth, and Infrared. As device communication might alter the device data, the possibility of further communication should be eliminated after seizing the device.
Lack of availability of tools: There is a wide range of mobile devices. A single tool may not support all the devices or perform all the necessary functions, so a combination of tools needs to be used. Choosing the right tool for a particular phone might be difficult.
Malicious programs: The device might contain malicious software or malware, such as a virus or a Trojan. Such malicious programs may attempt to spread over other devices over either a wired interface or a wireless one.
Legal issues: Mobile devices might be involved in crimes, which can cross geographical boundaries. In order to tackle these multijurisdictional issues, the forensic examiner should be aware of the nature of the crime and the regional laws.

Mobile phone evidence extraction process

Evidence extraction and forensic examination of each mobile device may differ. However, following a consistent examination process will assist the forensic examiner to ensure that the evidence extracted from each phone is well documented and that the results are repeatable and defendable. There is no well-established standard process for mobile forensics. However, the following figure provides an overview of process considerations for extraction of evidence from mobile devices. All methods used when extracting data from mobile devices should be tested, validated, and well documented.

A great resource for handling and processing mobile devices can be found at http://digital-forensics.sans.org/media/mobile-device-forensic-process-v3.pdf.

Mobile phone evidence extraction process

The evidence intake phase

The evidence intake phase is the starting phase and entails request forms and paperwork to document ownership information and the type of incident the mobile device was involved in, and outlines the type of data or information the requester is seeking. Developing specific objectives for each examination is the critical part of this phase. It serves to clarify the examiner's goals.

The identification phase

The forensic examiner should identify the following details for every examination of a mobile device:

The legal authority
The goals of the examination
The make, model, and identifying information for the device
Removable and external data storage
Other sources of potential evidence

We will discuss each of them in the following sections.

The legal authority

It is important for the forensic examiner to determine and document what legal authority exists for the acquisition and examination of the device as well as any limitations placed on the media prior to the examination of the device.

The goals of the examination

The examiner will identify how in-depth the examination needs to be based upon the data requested. The goal of the examination makes a significant difference in selecting the tools and techniques to examine the phone and increases the efficiency of the examination process.

The make, model, and identifying information for the device

As part of the examination, identifying the make and model of the phone assists in determining what tools would work with the phone.

Removable and external data storage

Many mobile phones provide an option to extend the memory with removable storage devices, such as the Trans Flash Micro SD memory expansion card. In cases when such a card is found in a mobile phone that is submitted for examination, the card should be removed and processed using traditional digital forensic techniques. It is wise to also acquire the card while in the mobile device to ensure data stored on both the handset memory and card are linked for easier analysis. This will be discussed in detail in upcoming chapters.

Other sources of potential evidence

Mobile phones act as good sources of fingerprint and other biological evidence. Such evidence should be collected prior to the examination of the mobile phone to avoid contamination issues unless the collection method will damage the device. Examiners should wear gloves when handling the evidence.

The preparation phase

Once the mobile phone model is identified, the preparation phase involves research regarding the particular mobile phone to be examined and the appropriate methods and tools to be used for acquisition and examination.

The isolation phase

Mobile phones are by design intended to communicate via cellular phone networks, Bluetooth, Infrared, and wireless (Wi-Fi) network capabilities. When the phone is connected to a network, new data is added to the phone through incoming calls, messages, and application data, which modifies the evidence on the phone. Complete destruction of data is also possible through remote access or remote wiping commands. For this reason, isolation of the device from communication sources is important prior to the acquisition and examination of the device. Isolation of the phone can be accomplished through the use of faraday bags, which block the radio signals to or from the phone. Past research has found inconsistencies in total communication protection with faraday bags. Therefore, network isolation is advisable. This can be done by placing the phone in radio frequency shielding cloth and then placing the phone into airplane or flight mode.

The processing phase

Once the phone has been isolated from the communication networks, the actual processing of the mobile phone begins. The phone should be acquired using a tested method that is repeatable and is as forensically sound as possible. Physical acquisition is the preferred method as it extracts the raw memory data and the device is commonly powered off during the acquisition process. On most devices, the least amount of changes occur to the device during physical acquisition. If physical acquisition is not possible or fails, an attempt should be made to acquire the file system of the mobile device. A logical acquisition should always be obtained as it may contain only the parsed data and provide pointers to examine the raw memory image.

The verification phase

After processing the phone, the examiner needs to verify the accuracy of the data extracted from the phone to ensure that data is not modified. The verification of the extracted data can be accomplished in several ways.

Comparing extracted data to the handset data

Check if the data extracted from the device matches the data displayed by the device. The data extracted can be compared to the device itself or a logical report, whichever is preferred. Remember, handling the original device may make changes to the only evidence—the device itself.

Using multiple tools and comparing the results

To ensure accuracy, use multiple tools to extract the data and compare results.

Using hash values

All image files should be hashed after acquisition to ensure data remains unchanged. If file system extraction is supported, the examiner extracts the file system and then computes hashes for the extracted files. Later, any individually extracted file hash is calculated and checked against the original value to verify the integrity of it. Any discrepancy in a hash value must be explainable (for example, if the device was powered on and then acquired again, thus the hash values are different).

The document and reporting phase

The forensic examiner is required to document throughout the examination process in the form of contemporaneous notes relating to what was done during the acquisition and examination. Once the examiner completes the investigation, the results must go through some form of peer-review to ensure the data is checked and the investigation is complete. The examiner's notes and documentation may include information such as the following:

Examination start date and time
The physical condition of the phone
Photos of the phone and individual components
Phone status when received—turned on or off
Phone make and model
Tools used for the acquisition
Tools used for the examination
Data found during the examination
Notes from peer-review

The presentation phase

Throughout the investigation, it is important to make sure that the information extracted and documented from a mobile device can be clearly presented to any other examiner or to a court. Creating a forensic report of data extracted from the mobile device during acquisition and analysis is important. This may include data in both paper and electronic formats. Your findings must be documented and presented in a manner that the evidence speaks for itself when in court. The findings should be clear, concise, and repeatable. Timeline and link analysis, features offered by many commercial mobile forensics tools, will aid in reporting and explaining findings across multiple mobile devices. These tools allow the examiner to tie together the methods behind the communication of multiple devices.

The archiving phase

Preserving the data extracted from the mobile phone is an important part of the overall process. It is also important that the data is retained in a useable format for the ongoing court process, for future reference, should the current evidence file become corrupt, and for record keeping requirements. Court cases may continue for many years before the final judgment is arrived at, and most jurisdictions require that data be retained for long periods of time for the purposes of appeals. As the field and methods advance, new methods for pulling data out of a raw, physical image may surface, and then the examiner can revisit the data by pulling a copy from the archives.

Practical mobile forensic approaches

Similar to any forensic investigation, there are several approaches that can be used for the acquisition and examination/analysis of data from mobile phones. The type of mobile device, the operating system, and the security setting generally dictate the procedure to be followed in a forensic process. Every investigation is distinct with its own circumstances, so it is not possible to design a single definitive procedural approach for all the cases. The following details outline the general approaches followed in extracting data from mobile devices.

Mobile operating systems overview

One of the major factors in the data acquisition and examination/analysis of a mobile phone is the operating system. Starting from low-end mobile phones to smartphones, mobile operating systems have come a long way with a lot of features. Mobile operating systems directly affect how the examiner can access the mobile device. For example, Android OS gives terminal-level access whereas iOS does not give such an option. A comprehensive understanding of the mobile platform helps the forensic examiner make sound forensic decisions and conduct a conclusive investigation. While there is a large range of smart mobile devices, four main operating systems dominate the market, namely, Google Android, Apple iOS, RIM BlackBerry OS, and Windows Phone. More information can be found at http://www.idc.com/getdoc.jsp?containerId=prUS23946013. This book covers forensic analysis of these four mobile platforms. The following is a brief overview of leading mobile operating systems.

Android

Android is a Linux-based operating system, and it's a Google open source platform for mobile phones. Android is the world's most widely used smartphone operating system. Sources show that Apple's iOS is a close second (http://www.forbes.com/sites/tonybradley/2013/11/15/android-dominates-market-share-but-apple-makes-all-the-money/). Android has been developed by Google as an open and free option for hardware manufacturers and phone carriers. This makes Android the software of choice for companies who require a low-cost, customizable, lightweight operating system for their smart devices without developing a new OS from scratch. Android's open nature has further encouraged the developers to build a large number of applications and upload them onto Android Market. Later, end users can download the application from Android Market, which makes Android a powerful operating system. More details on Android are covered in Chapter 7, Understanding Android.

iOS

iOS, formerly known as the iPhone operating system, is a mobile operating system developed and distributed solely by Apple Inc. iOS is evolving into a universal operating system for all Apple mobile devices, such as iPad, iPod touch, and iPhone. iOS is derived from OS X, with which it shares the Darwin foundation, and is therefore a Unix-like operating system. iOS manages the device hardware and provides the technologies required to implement native applications. iOS also ships with various system applications, such as Mail and Safari, which provide standard system services to the user. iOS native applications are distributed through AppStore, which is closely monitored by Apple. More details about iOS are covered in Chapter 2, Understanding the Internals of iOS Devices.

Windows phone

Windows phone is a proprietary mobile operating system developed by Microsoft for smartphones and pocket PCs. It is the successor to Windows mobile and primarily aimed at the consumer market rather than the enterprise market. The Windows Phone OS is similar to the Windows desktop OS, but it is optimized for devices with a small amount of storage. Windows Phone basics and forensic techniques are discussed in Chapter 12, Windows Phone Forensics.

BlackBerry OS

BlackBerry OS is a proprietary mobile operating system developed by BlackBerry Ltd., known as Research in Motion (RIM), exclusively for its BlackBerry line of smartphones and mobile devices. BlackBerry mobiles are widely used in corporate companies and offer native support for corporate mail via MIDP, which enables wireless sync with Microsoft Exchange, e-mail, contacts, calendar, and so on, while used along with the BlackBerry Enterprise server. These devices are known for their security. BlackBerry OS basics and forensic techniques are covered in Chapter 13, BlackBerry Forensics.

Mobile forensic tool leveling system

Mobile phone forensic acquisition and analysis involves manual effort and the use of automated tools. There are a variety of tools that are available for performing mobile forensics. All the tools have their pros and cons, and it is fundamental that you understand that no single tool is sufficient for all purposes. So understanding the various types of mobile forensic tools is important for forensic examiners. When identifying the appropriate tools for the forensic acquisition and analysis of mobile phones, a mobile device forensic tool classification system (shown in the following figure) developed by Sam Brothers comes in handy for the examiners.

Cellular phone tool leveling pyramid (Sam Brothers, 2009)

The objective of the mobile device forensic tool classification system is to enable an examiner to categorize the forensic tools based upon the examination methodology of the tool. Starting at the bottom of the classification and working upward, the methods and the tools generally become more technical, complex, and forensically sound, and require longer analysis times. There are pros and cons of performing an analysis at each layer. The forensic examiner should be aware of these issues and should only proceed with the level of extraction that is required. Evidence can be destroyed completely if the given method or tool is not properly utilized. This risk increases as you move up in the pyramid. Thus, proper training is required to obtain the highest success rate in data extraction from mobile devices.

Each existing mobile forensic tool can be classified under one or more of the five levels. The following sections contain a detailed description of each level.

Manual extraction

This method involves simply scrolling through the data on the device and viewing the data on the phone directly through the use of the device's keypad or touchscreen. The information discovered is then photographically documented. The extraction process is fast and easy to use, and will work on almost every phone. This method is prone to human error, such as missing certain data due to unfamiliarity with the interface. At this level, it is not possible to recover deleted information and grab all the data. There are some tools that have been developed to aid an examiner to easily document a manual extraction.

Logical extraction

Logical extraction involves connecting the mobile device to forensic hardware or to a forensic workstation via a USB cable, RJ-45 cable, Infrared, or Bluetooth. Once connected, the computer initiates a command and sends it to the device, which is then interpreted by the device processor. Next, the requested data is received from the device's memory and sent back to the forensic workstation. Later, the examiner can review the data. Most of the forensic tools currently available work at this level of the classification system. The extraction process is fast, easy to use, and requires little training for the examiners. On the flip side, the process may write data to the mobile and might change the integrity of the evidence. In addition, deleted data is almost never accessible.

Hex dump

A hex dump, also referred to as a physical extraction, is achieved by connecting the device to the forensic workstation and pushing unsigned code or a bootloader into the phone and instructing the phone to dump memory from the phone to the computer. Since the resulting raw image is in binary format, technical expertise is required to analyze it. The process is inexpensive, provides more data to the examiner, and allows the recovering of the deleted files from the device-unallocated space on most devices.

Chip-off

Chip-off refers to the acquisition of data directly from the device's memory chip. At this level, the chip is physically removed from the device and a chip reader or a second phone is used to extract data stored on it. This method is more technically challenging as a wide variety of chip types are used in mobiles. The process is expensive and requires hardware level knowledge as it involves the de-soldering and heating of the memory chip. Training is required to successfully perform a chip-off extraction. Improper procedures may damage the memory chip and render all data unsalvageable. When possible, it is recommended that the other levels of extraction are attempted prior to chip-off since this method is destructive in nature. Also, the information that comes out of memory is in a raw format and has to be parsed, decoded, and interpreted. The chip-off method is preferred in situations where it is important to preserve the state of memory exactly as it exists on the device. It is also the only option when a device is damaged but the memory chip is intact.

The chips on the device are often read using the Joint Test Action Group (JTAG) method. The JTAG method involves connecting to Test Access Ports (TAPs) on a device and instructing the processor to transfer the raw data stored on memory chips. The JTAG method is generally used with devices that are operational but inaccessible using standard tools.

Micro read

The process involves manually viewing and interpreting data seen on the memory chip. The examiner uses an electron microscope and analyzes the physical gates on the chip and then translates the gate status to 0's and 1's to determine the resulting ASCII characters. The whole process is time consuming and costly, and it requires extensive knowledge and training on flash memory and the file system. Due to the extreme technicalities involved in micro read, it would be only attempted for high-profile cases equivalent to a national security crisis after all other level extraction techniques have been exhausted. The process is rarely performed and is not well documented at this time. Also, there are currently no commercial tools available to perform a micro read.

Data acquisition methods

Data acquisition is the process of imaging or otherwise extracting information from a digital device and its peripheral equipment and media. Acquiring data from a mobile phone is not as simple as a standard hard drive forensic acquisition. The following points break down the three types of forensic acquisition methods for mobile phones: physical, logical, and manual. These methods may have some overlap with a couple of levels discussed in the mobile forensics tool leveling system. The amount and type of data that can be collected will vary depending on the type of acquisition method being used.

Physical acquisition

Physical acquisition of mobile phones is performed using mobile forensic tools and methods. Physical extraction acquires information from the device by direct access to the flash memory. The process creates a bit-for-bit copy of an entire file system, similar to the approach taken in computer forensic investigations. A physical acquisition is able to acquire all of the data present on a device including the deleted data and access to unallocated space on most devices.

Logical acquisition

Logical acquisition of mobile phones is performed using the device manufacturer application-programming interface for synchronizing the phones contents with a computer. Many of the forensic tools perform a logical acquisition. However, the forensic analyst must understand how the acquisition occurs and whether the mobile is modified in any way during the process. Depending on the phone and forensic tools used, all or some of the data is acquired. A logical acquisition is easy to perform and only recovers the files on a mobile phone and does not recover data contained in unallocated space.

Manual acquisition

With mobile phones, physical acquisition is usually the best option, and logical acquisition is the second-best option. Manual extraction should be the last option when performing the forensic acquisition of a mobile phone. Both logical and manual acquisition can be used to validate findings in the physical data. During manual acquisition, the examiner utilizes the user interface to investigate the contents of the phone's memory. The device is used normally through a keypad or touchscreen and menu navigation, and the examiner takes pictures of each screen's contents. Manual extraction introduces a greater degree of risk in the form of human error, and there is a chance of deleting the evidence. Manual acquisition is easy to perform and only acquires the data that appears on a mobile phone.