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