Digital and Mobile Forensics

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In this article, Mattia Epifani and Pasquale Stirparo, co-authors of the book Learning iOS Forensics – Second Edition, would be talking mainly, if not solely, about computer forensics and computer crimes, such as when an attacker breaks into a computer network system and steals data. This would involve two types of offenses—unlawful/unauthorized access and data theft. As mobile phones became more popular, the new field of mobile forensics developed.

(For more resources related to this topic, see here.)

Nowadays, things have changed radically and they are still changing at quite a fast pace as technology evolves. Digital forensics, which includes all disciplines dealing with electronic evidence, is also being applied to common crimes, to those that, at least by definition, are not strictly IT crimes. Today, more than ever, we live in a society that is fully digitalized and people are equipped with all kinds of devices, which have different types of capabilities, but all of them process, store, and transmit information (mainly over the Internet). This means that forensic investigators have to be able to deal with all these devices.

As defined at the first Digital Forensics Research Workshop (DFRWS) in 2001, digital forensics is:


“The use of scientifically derived and proven methods toward the preservation, collection, validation, identification, analysis, interpretation, documentation, and presentation of digital evidence derived from digital sources for the purpose of facilitating or furthering the reconstruction of events found to be criminal, or helping to anticipate unauthorized actions shown to be disruptive to planned operations.”

As Casey asserted (Casey, 2011):

“In this modern age, it is hard to imagine a crime that does not have a digital dimension.”

Criminals of all kinds use technology to facilitate their offenses, communicate with their peers, recruit other criminals, launder money, commit credit card fraud, gather information on their victims, and so on. This obviously creates new challenges for all the different actors involved, such as attorneys, judges, law enforcement agents, and forensic examiners.

Among the cases solved in recent years, there were kidnappings where the kidnapper was caught—thanks to a request for ransom sent by e-mail from his mobile phone. There have been many cases of industrial espionage in which unfaithful employees were hiding projects in the memory cards of their smartphones, cases of drug dealing solved—thanks to the evidence found in the backup of mobile phones that was on computer, and many other such cases. Even the largest robberies of our time are now being conducted via computer networks.

In this article, you will learn the following:

  • Definition and principles of mobile forensics
  • How to properly handle digital evidence

Mobile forensics

Mobile forensics is a field of study in digital forensics that focuses on mobile devices. Among the different digital forensics fields, mobile forensics is without doubt the fastest growing and evolving area of study, having an impact on many different situations from corporate to criminal investigations and intelligence gathering, which are on the rise. Moreover, the importance of mobile forensics is increasing exponentially due to the continuous fast growth of the mobile market. One of the most interesting peculiarities of mobile forensics is that mobile devices, particularly mobile phones, usually belong to a single individual, while this is not always the case with a computer that may be shared among employees of a company or members of a family. For this reason, the analysis of mobile phones gives access to plenty of personal information.

Another important and interesting aspect that comes with mobile forensics, which is both challenging and frustrating at the same time for the analyst, is the multitude of different device models and the customized flavors of their operating systems available in the market. This makes it very difficult to have a single solution (either a tool or process) to address them all.

Just think of all the applications people have installed on their smartphones: IM clients, web browsers, social network clients, password managers, navigation systems, and much more, other than the classic default ones, such as an address book, which can provide a lot more information than just the phone number for each contact that has been saved. Moreover, syncing such devices with a computer has become a very easy and smooth process, and all user activities, schedules, to-do lists, and everything else is stored inside a smartphone. Aren’t these enough to profile a person and reconstruct all their recent activities, than building the network of contacts?

Finally, in addition to a variety of smartphones and operating systems, such as Apple iOS, Google Android, Microsoft Windows Phone, and Blackberry OS, there is a massive number of so-called feature phones that use older mobile OS systems.

Therefore, it’s pretty clear that when talking about mobile/smartphone forensics, there is so much more than just printouts of phone calls. In fact, with a complete examination, we can retrieve SMSes/MMSes, pictures, videos, installed applications, e-mails, geolocation data, and so on—both present and deleted information.

Digital evidence

As mentioned earlier, on one hand the increasing involvement of mobile devices in digital forensics cases has brought a whole new series of challenges and complexities. However, on the other hand, this has also resulted in a much greater amount of evidence from criminals that it is now being used to reconstruct their activities with a more comprehensive level of detail. Moreover, while classical physical evidence may be destroyed, digital evidence, most of the time, leaves traces.

Over the years, there have been several definitions of what digital evidence actually is, some of them focusing particularly on the evidentiary aspects of proof to be used in court, such as the one proposed by the Standard Working Group on Digital Evidence (SWGDE), stating that:

“Digital evidence is any information of probative value that is either stored or transmitted in a digital form.”

The definition proposed by the International Organization of Computer Evidence (IOCE) states:

“Digital evidence is information stored or transmitted in binary form that may be relied on in court.”

The definition given by E. Casey (Casey, 2000), refers to digital evidence as:

“Physical objects that can establish that a crime has been committed, can provide a link between a crime and its victim, or can provide a link between a crime and its perpetrator.”

While all of these are correct, as previously said, all of these definitions focus mostly on proofs and tend to disregard data that is extremely useful for an investigation.

For this reason, and for the purpose of this book, we will refer to the definition given by Carrier (Carrier, 2006), where digital evidence is defined as:

“Digital data that supports or refutes a hypothesis about digital events or the state of digital data.”

This definition is a more general one, but better matches the current state of digital evidence and its value within the entire investigation process.

Also from a standardization point of view, there have been, and still are, many attempts to define guidelines and best practices for digital forensics on how to handle digital evidence. Other than the several guidelines and special publications from NIST, there is a standard from ISO/IEC that was released in 2012, the ISO 27037 guidelines for identification, collection and/or acquisition, and preservation of digital evidence, which is not specific to mobile forensics, but is related to digital forensics in general, aiming to build a standard procedure for collecting and handling digital evidence, which will be legally recognized and accepted in court in different countries. This is a really important goal if you consider the lack of borders in the Internet era, particularly when it comes to digital crimes, where illicit actions can be perpetrated by attackers from anywhere in the world.

Handling of mobile evidence

In order to be useful not only in court but also during the entire investigation phase, digital evidence must be collected, preserved, and analyzed in a forensically sound manner. This means that each step, from the identification to the reporting, has to be carefully and strictly followed. Historically, we are used to referring to a methodology as forensically sound if, and only if, it would imply that the original source of evidence remains unmodified and unaltered. This was mostly true when talking about classical computer forensics, in scenarios where the forensic practitioner found the computer switched off or had to deal with external hard drives, although not completely true even in these situations. However, since the rise of live forensics, this concept has become more and more untrue. In fact, methods and tools for acquiring memory from live systems inevitably alter, even if just a little bit, the target system they are run on. The advent of mobile forensics stresses this concept even more, because mobile devices, and smartphones in particular, are networked devices that continuously exchange data through several communication protocols, such as GSM/CDMA, Wi-Fi, Bluetooth, and so on. Moreover, in order to acquire a mobile device, forensic practitioners need to have some degree of interaction with the device. Based on the type, a smartphone can need more or less interaction, altering in this way the original state of the device.

All of this does not mean that preservation of the source evidence is useless, but that it is nearly impossible in the field of mobile devices. Therefore, it becomes a matter of extreme importance to thoroughly document every step taken during the collection, preservation, and acquisition phases. Using this approach, forensic practitioners will be able to demonstrate that they have been as unintrusive as possible. As Casey states (Casey, 2011):

“One of the keys to forensic soundness is documentation. A solid case is built on supporting documentation that reports on where the evidence originated and how it was handled. From a forensic standpoint, the acquisition process should change the original evidence as little as possible and any changes should be documented and assessed in the context of the final analytical results.”

When in the presence of mobile devices to be collected, it is a good practice for the forensic practitioner to consider the following points:

  • Take note of the current location where the device has been found.
  • Report the device status (switched on or off, broken screen, and so on).
  • Report date, time, and other information visible on the screen if the device is switched on, for example, by taking a picture of the screen.
  • Look very carefully for the presence of memory cards. Although it is not the case with iOS devices, generally many mobile phones have a slot for an external memory card, where pictures, chat databases, and many other types of user data are usually stored.
  • Look very carefully for the presence of cables related to the mobile phone that is being collected, especially if you don’t have a full set of cables in your lab. Many mobile phones have their own cables to connect to the computer and to recharge the battery.
  • Search for the original Subscriber Identity Module (SIM) package, because that is where the PIN and PIN unblocking key (PUK) codes are written.
  • Take pictures of every item before collection.

Modifications to mobile devices can happen not only because of interaction with the forensic practitioner, but also due to interaction with the network, voluntarily or not. In fact, digital evidence in mobile devices can be lost completely as they are susceptible to being overwritten by new data, for example, with the smartphone receiving an SMS while it is being collected, thus overwriting possible evidence previously stored in the same area of memory as the newly arrived SMS, or upon receiving a remote wiping command over a wireless network. Most of today’s smartphones and iOS devices can be configured to be completely wiped remotely.

From a real case:

While searching inside the house of a person under investigation, law enforcement agents found and seized, among other things, computers and a smartphone. After cataloguing and documenting everything, they put all the material into boxes to bring them back to the laboratory. Once back in their laboratory, when acquiring the smart phone in order to proceed with the forensics analysis, they noticed that the smartphone was empty and it appeared to be brand new. The owner had wiped it remotely.

Therefore, isolating the mobile device from all radio networks is a fundamental step in the process of preservation of evidence. There are several ways to achieve this, all with their own pros and cons, as follows:

  • Airplane mode: Enabling Airplane mode on a device requires some sort of interaction, which may pose some risks of modification by the forensic practitioner. This is one of the best possible options since it implies that all wireless communication chips are switched off. In this case, it is always good to document the action taken with pictures and/or videos. Normally, this is possible only if the phone is not password-protected or the password is known. However, for devices with iOS 7 or higher, it is also possible to enable airplane mode by lifting the dock from the bottom, where there will be a button with the shape of a plane. This is possible only if the Access on Lock Screen option is enabled from Settings | Control Center.
  • Faraday’s bag: This item is a sort of envelope made of conducting material, which blocks out static electric fields and electromagnetic radiation completely isolating the device from communicating with external networks. It is based, as the name suggests, on Faraday’s law. This is the most common solution, particularly useful when the device is being carried from the crime scene to the lab after seizure. However, the use of Faraday’s bag will make the phone continuously search for a network, which will cause the battery to quickly drain. Unfortunately, it is also risky to plug the phone to a power cable outside that will go inside the bag, because this may act as antenna. Moreover, it is important to keep in mind that when you remove the phone from the bag (once arrived in the lab) it will again be exposed to the network. So, you would need either a shielded lab environment or a Faraday solution that would allow you to access the phone while it is still inside the shielded container, without the need for external power cables.
  • Jamming: A jammer is used to prevent a wireless device from communicating by sending out radio waves along the same frequencies as that device. In our case, it would jam the GSM/UMTS/LTE frequencies that mobile phones use to connect with cellular base stations to send/receive data. Be aware that this practice may be considered illegal in some countries, since it will also interfere with any other mobile device in the range of the jammer, disrupting their communications too.
  • Switching off the device: This is a very risky practice because it may activate authentication mechanisms, such as PIN codes or passcodes, that are not available to the forensic practitioner, or other encryption mechanisms that carry the risk of delaying or even blocking the acquisition of the mobile device.
  • Removing the SIM card: In most mobile devices, this operation implies removing the battery and therefore all the risks and consequences we just mentioned regarding switching off the device; however, in iOS devices this task is quite straightforward and easy, and it does not imply removing the battery (in iOS devices this is not possible). Moreover, SIM cards can have PIN protection enabled; removing it from the phone may lock the SIM card, preventing its content from being displayed. However, bear in mind that removing the SIM card will isolate the device only from the cellular network, while other networks, such as Wi-Fi or Bluetooth, may still be active and therefore need to be addressed.
  • The following image shows a SIM card extracted from an iPhone with just a clip; image taken from http://www.maclife.com/:

Summary

In this article, we gave a general introduction to digital forensics for those relatively new to this area of study and a good recap to those already in the field, keeping the mobile forensics field specifically in mind. We have shown what digital evidence is and how it should be handled, presenting several techniques to isolate the mobile device from the network.

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