The 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 the extraction of evidence from mobile devices. All methods used when extracting data from mobile devices should be tested, validated, and well-documented:

Mobile phone evidence extraction process

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.

As shown in the preceding figure, forensics on a mobile device includes several phases, from the evidence intake phase to the archiving phase. The following sections provide an overview of various considerations across all the phases.

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 it 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. Also, while seizing the device, care should be taken not to modify any data present on the device. At the same time, any opportunity that might help the investigation should not be missed. For example, at the time of seizing the device, if the device is unlocked, then try to disable the passcode.

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. For example, if the mobile device is being searched pursuant to a warrant, the examiner should be mindful of confining the search to the limitations of the warrant.

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. For all phones, the manufacturer, model number, carrier, and the current phone number associated with the cellular phone should be identified and documented.

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 that 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. This is generally done based on the device model, underlying operating system, its version, and so on. Also, choosing tools for examination of a mobile device will be determined by factors such as the goal of the examination, resources available, the type of cellular phone to be examined, and the presence of any external storage capabilities.

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. Network isolation can be done by placing the phone in radio frequency shielding cloth and then putting the phone in airplane or flight mode. The airplane mode disables a device's communication channels, such as cellular radio, Wi-Fi, and Bluetooth. However, if the device is screen-locked, then this is not possible. Also, since Wi-Fi is now available in airplanes, some devices now have Wi-Fi access enabled in airplane mode. An alternate solution is isolation of the phone through the use of Faraday bags, which block radio signals to or from the phone. Faraday bags contain materials that block external static electrical fields (including radio waves). Thus, Faraday bags shield seized mobile devices from external interference to prevent wiping and tracking. To work more conveniently with the seized devices, Faraday tents and rooms also exist.

The processing phase

Once the phone has been isolated from 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 smallest number 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 filesystem 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. These acquisition methods are discussed in detail in later chapters.

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 has not been modified. The verification of the extracted data can be accomplished in several ways.

Comparing extracted data to the handset data

Check whether 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 that data remains unchanged. If filesystem extraction is supported, the examiner extracts the filesystem 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, the device was powered on and then acquired again, thus the hash values are different).

The documenting 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 that the data is checked and the investigation is complete. The examiner's notes and documentation may include information such as the following:

• The 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 forensic 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 usable 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.