Methodology, Mobile Devices, Research

Categorization of embedded system forensic collection methodologies

There are many classifications as far as forensic data collection is concerned, but much of it is still a de facto and Wild West when it comes to naming convention. This is especially true in the embedded system area.

When I refer to embedded systems, I think of specialized devices, sometimes in a larger system or machine. Embedded systems usually have at least one microprocessor with dedicated program, and limited options to extract the information in a sound forensic way. Cell phones, smart phones, tablets, DVD and BluRay players, advanced digital watches, TVs, cars, elevators, and even washers & dryers can have embedded systems.

I would like to suggest a more structured way to represent data collection methods for such systems. As this is a work in progress, I look forward to constructive criticisms that can benefit the forensics community.

The classification is broken down into six methodologies.

  1. Manual acquisition
  2. Logical acquisition
  3. Pseudo-physical acquisition
  4. Support-port acquisition
  5. Circuit read acquisition
  6. Gate read acquisition

Each methodology has their shortcomings and benefits. I categorized these into four areas, and ranked them in a scale of 1 to 10, with one for “least” and 10 “most”.

Destructiveness is the impact on the target device, and how likely that everything is fully functional after data collection.

Technical & Training is the required understanding and education in the area required to attempt the methodology.

Cost is simply the expenses involved with the resources required, such as equipment, tools and consumables, to attempt the methodology.

Forensically Sound, the final measurement, is how likely the original data is modified, knowingly or not.

Acquisition Methodology Comparison

Acquisition Methodology Comparison

Manual

This is the oldest and least training and equipment required methodology. The examiner takes advantage of the devices display and user interface and a camera to record all relevant information, as much as possible. The target device may record all display and user interface activity, and update system data as normal housekeeping.

Example: Secure cell phone in holding bracket, then using the keypad scroll through all relevant items while taking pictures of the cell phone with an external camera. A commercial product used for this kind of acquisition is Paraben Project-A-Phone.

Logical

Logical acquisition method is where the device’s operating system (OS) is in full control of what can be accessed, and provides the method to transfer the data. The examiner connects the device to a forensic workstation, and using various software packages communicates with the OS on the target device. The OS may record the connection, and communication on the target device, and update system data as normal housekeeping.

Example: Connect cell phone’s external port to USB port, using proprietary cable. Run Software to initiate serial communication with device, and request information from device using proprietary and device specific commands. A software, such as BitPim would be used for this type of acquisition.

Pseudo-Physical

The process of pseudo-physical collection involves forcing program code onto the target device in some way which allows access to most data areas. The code may only provide access and takes advantage of the target device’s OS to provide communication, or is a complete replacement of the OS with just collection functionality. Thereafter, the examiner connects the device to a forensic workstation, and using various software packages communicates with the program code, or the OS on the target device. The OS may record the connection, and communication on the target device, and update system data as normal housekeeping. The forced-on code may also impact the information on the target device.

Although often touted as physical acquisition by almost all vendors, this process is not, in my opinion, truly physical as most forensics examiners expect it to be. Most forensics examiners think “bit-by-bit” when they hear “physical”. In my experience, this is not the case, as unallocated and slack areas of the storage are not collected.

Example: Target device is connected to the forensic workstation with a USB to proprietary serial cable. The target device is placed in Device Firmware Update (UDF) mode. The software on the forensic workstation at this time may load a special program code onto the target device. The code allows the software on the forensic workstation to access most information on the target device. Sometimes the target device’s UDF mode software provides the communication features.

Support-Port

Most mass produced electronic devices have ports for testing the electronics, or for updating firmware on various onboard integrated circuitry. These “ports” can be implemented as user accessible ports such as a USB, RS232 or even some pin and socket connector (Molex), non-user accessible ports including pin headers or insulation-displacement connector, and finally test connection pads that appear on the printed circuit assembly (PCA).

To access these ports, almost all small electronics require disassembly, often voiding the manufacturer’s warranty. Once the device is disassembled, the port must be identified on the PCA, and the specific communication protocol must also be found. Communication is established with the specific storage circuitry, and data is requested. This data is then stored for further analysis.

The most often used protocols are Boundary Scan (often referred to by the standardizing group name Joint Test Action Group [JTAG]), Inter-Integrated Circuit (I2C), Serial Peripheral Interface (SPI), Enhanced Synchronous Serial Interface (ESSI), Controller Area Network (CAN), Local Interconnect Network (LIN), and Background Debug Mode (BDM).

Example: The target device is disassembled, and test access points (TAP) are located. Leads are soldered or clamped onto the TAP, and connected to a protocol specific universal asynchronous receiver/transmitter (UART). This device in turn is connected to a USB port of the forensic workstation. Specialized software using circuit-specific commands instructs the on-board device to download data from the circuit. The returned data is stored on the forensic workstation. No information is stored or written to the target device besides the temporary instructions.

Circuit Read

For this acquisition methodology, the integrated circuits (IC) such as memory chips are desoldered from the PCA and data is extracted using chip specific pin-out and communication. This is often referred to as “chip-off” process.

There are several critical points with this method, including the possibility to permanently damage the IC during desoldering, dealing with stacked ICs (3D packaging) or monolithic configuration.

In this particular method, the IC is removed, socketed or soldered, and specific signals are sent to extract the data from the specific chip, using specialized software.

Example: The target device is disassembled, and data storage ICs are located. Pin out information, and timing details for communication with the IC is researched. Target device is preheated, and then the specific ICs are desoldered. The ICs are either placed in temporary sockets, or leads are soldered to appropriate pins. The socket or leads are connected to a communication device using proper communication protocol, such as a Transistor-Transistor Logic (TTL), which in turn is connected to the forensics workstation.

Specialized software using IC-specific commands instructs the socket to download data from the IC. The returned data is stored on the forensic workstation. No information is stored or written to the target IC.

Gate Read

This methodology requires both equipment, and chemicals that are usually not found in most digital forensics labs. The process involves the removal of the target IC in similar fashion as the Circuit Read acquisition methodology. Instead of attempting to communicate with the IC through electronic signals, the chip is literally sliced into multiple layers, to expose each original semiconductor lithographic layer, and information is reverse engineered from the layers.

The layers are measured in nanometers (1 x 10-9 m) or a billionth of a meter. Each layer is removed, photographed, and then reverse engineered from the photograph. The process is as much guess work as it is a very high level understanding of IC internals and IC lithography. The process works best with planarized chips. The steps of the process are device depoting or package removal, delayering, imaging, annotation, schematic, organization and finally analysis.

Example: The target device is disassembled, and data storage ICs are located. Pin out information for the IC is researched. Target device is preheated, and then the specific ICs are desoldered. The IC is bathed in chemicals to remove potting, or encasing. At this point, the only remaining items are the leads to a piece of silicon die. The leads are noted and photographed. The die using lapping (or other very precise slicing or abrasion method) removes each layer, and photographed. The layers are stacked in software, and reverse engineered using the shape, color density and interconnection of the layers. This process requires identification amongst other things the N-type, P-type silicon, the gates, power and ground.

Reference:

  Manual Logical Pseudo-Physical Support-port Read Direct Circuit Read Gate Read
Destructiveness 1 1 2 3 5 10
Technical & Training 1 2 3 5 6 9
Cost 1 2 3 3 5 7
Forensically Sound 1 2 5 8 9 7

Rankings are on a scale of 1 to 10, with one for “least” and 10 “most”. Ex. Most destructive would be a 10; Least costly would be a 1.

Discussion

2 thoughts on “Categorization of embedded system forensic collection methodologies

  1. Interesting,however I am highly doubtful about the last one method being practical for normal forensic practices.

    Posted by George Fan | June 19, 2013, 9:37 am
    • Thanks for your comment. I believe I started the section on “gate read” stating that “[t]his methodology requires both equipment, and chemicals that are usually not found in most digital forensics labs”. Additionally the scores are marked as most destructive, most technical, requires most training, and most costly.

      What is a “normal” lab in the digital forensic science field? Are you referring to a corporate lab working on eDiscovery, internal HR, Intellectual Property theft, or competitive intelligence cases? Is it a commercial lab contracting with law firms dealing with civil cases? Is the lab at a sheriff’s office dealing with petty crime, larceny, traffic cases, misdemeanors and a few felonies? Is the lab at a national law enforcement agency, tracking gangs, narco-trafficers, or large scale white-coller crime? Or, is it a lab for a federal or defense agency investigating international terrorism or cyber crime?

      I agree. I am also highly doubtful about the “Gate Read” being practical for “normal” forensic practices. Yet, it is not impossible, it is performed in some labs regularly, and it has been demonstrated even in “garage-labs”.

      Posted by tapolyai | June 19, 2013, 12:39 pm

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