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Patent: Improvements in or relating to digital forensics.

Buchanan, William J; Graves, Jamie; Bose, Niladri


Jamie Graves

Niladri Bose


WO 2010029346 20100318


The present disclosure relates to improvements in or relating to digital forensics, and in particular to new methods and apparatus for digital forensic analysis of digital computing systems.

Locard's Exchange Principle best describes the fundamental theory of Forensic Science; that it is impossible to commit a crime without leaving a trace. Due to this interchange, it is possible for this evidence to be collected and analysed to establish the cause of an incident. The discipline of forensic science relates specifically to a scientific methodology to collect, preserve and analyse this information.

Forensic science has been developing ever since the late 18th Century, when the original forensic scientists practiced medicine, usually in order to analyse the cause of disease, by performing autopsies. The use of forensic analysis for criminal investigations was derived from this medical background, due to the teaching of thorough, evidence-based reasoning, leading to the development of fingerprint evidence as well as contemporary specialisations that have evolved, such as ballistics, DNA analysis, toxicology, and so on.

Although digital computing devices and systems process and store virtual, not physical material, Locard's principle still applies. The discipline of digital forensics is well established, and is starting to diverge into specialties. The term "digital forensics" is understood herein to refer to investigative or analytical activity relating to any digital computing system ("DCS"), where a DCS is any device that manipulates, stores or otherwise processes digital information. For example, computers of all types, mobile telephones, personal digital assistants (PDA's), media players, set-top

boxes, games consoles, televisions, and all associated network components such as routers, switches, hubs, servers, and broadcast equipment, are encompassed by the term DCS. This list is not exhaustive and is provided for purposes of illustration only.

In the early days of digital forensics, the methodologies that were used in traditional forensics fields were directly applicable. The evidence gathered from computer systems were generally indicative of tangible crimes committed outside of the computer environment. Computers themselves were rarely the target for criminal activity, but instead were used to store evidence relating to crimes, such as fraud. However with the increasing importance and popularity of computers, IT systems themselves are, increasingly, the target of criminal activity. It is now possible for crimes to be committed in virtual domains, with electronic and logical trails of evidence relating to events that only occurred in an abstract, or technical sense. With the increasing sophistication of those individuals capable of breaking into a computer system (intruders), and the ubiquity of high value data stored on digital systems, those tasked with investigating these crimes adopted investigative techniques identical to those they were tracking. Whereas the ballistics expert or fingerprint analyst possess skill sets different from the individuals who committed the crimes, those working with networked systems have to learn and adopt skills identical to those of the individuals being investigated.

Internetworked systems are not the only technology being abused by criminals, or implicated in criminal activities. PDA's, phones and digital cameras can all contain vital clues as to the circumstances surrounding a crime. With the continued convergence of technologies, the number and variation of devices will merge and expand into new and even more sophisticated equipment. For example, the convergence of the digital

camera and mobile phone leads to interesting trails of evidence. These changes and re-definitions of a computing device occur frequently, and rapidly. The almost limitless configuration of devices capable of storing and processing digital information, results in an ecosystem of variant devices, with differing levels of openness, methods of interaction, and so on. This poses a significant challenge for the computer forensic investigator, who must keep their skill-set updated and relevant. This has been addressed, in part, by the obvious need of sub-disciplines within this domain. These can be referred to as Computer Forensics, PDA Forensics, Network Forensics, and so on. The term "Digital Forensics" (DF) encompasses all of these particular sub-disciplines. While various bodies have attempted to offer formal definitions of the field of digital forensics, the term as used in this description has a slightly broader meaning as discussed above, because as will be apparent the teaching of this disclosure can be applied to any investigative or analytical activity relating to any DCS.

The problems facing DF do not solely relate to the variance within DCS. Different domains require distinctive outcomes from an DF investigation. Broadly, these domains can be defined as: civilian; and organisational. The civilian context normally involves law enforcement agencies investigating individual(s) with the intent of solving or prosecuting an alleged crime. The distinction between these two domains is the manner in which crime is detected, reported, and controlled. Within the civilian context, crimes are reported to, and therefore investigated by law enforcement agencies. Within the organisational context, a crime can constitute a number of different types of digression, all of which have differing levels of severity. A circumvention of an IT usage policy would be a transgression against the organisation. Depending on the policy circumvention, it may also break various laws. If this is the case, the

organisation may seek prosecution, which means the necessary law enforcement groups would be contacted. Yet, the organisation may chose to deal with the policy circumvention itself, as it may have the necessary technical and operational expertise to conduct such an investigation in- house, or possibly because of the sensitive nature of the operations or incident that occurred.

A DF investigation may be carried out in a variety of circumstances. Temporal aspects of the investigation can be the determining factor. The traditional view is of the post-event analysis of a computer-related crime. This may be the analysis of records left on a computer system that has been used in connection with a criminal offence, for example, the examination of a computer system that contains documents relating to a drugs offence. With the rise of more technologically sophisticated crimes, it may be the case that digital forensic techniques are deployed during the time in which a crime is being committed. As a consequence, the tools and procedures during an investigation differ, along with the reasons for launching an investigation, and the desired outcome of an investigation, for example the primary objective of a law enforcement operative is to successfully prosecute the criminal and the forensic activity is usually carried out after the event of a crime, while the primary objective of a military outfit would be to maintain continuity of operations and the forensic activity may carried out during the performance of a crime.

The methodology employed to investigate an incident will depend on a number of factors, some of which have just been highlighted. One of the principle concerns of the security domain is with intrusion detection, that is, prevention mechanisms and detection techniques for preventing unauthorised access to data. The security domain classically employs a model of policy circumvention which can broadly be defined as: 1.

Reconnaissance; 2. Foot-printing; 3. Enumeration; 4. Probing for weaknesses; 5. Penetration; 6. Gaining the objective; 7. Clean-up.

However, within the DF domain, unlike that of security, these steps do not offer the entire framework. The methodological context and investigative model for a security domain are not always applicable to the DF domain.

The following example illustrates how, in particular, policy context influences how different systems would view an event where users A and B are users on a computer network. A logs in from a given node, N, to run the program EXAMPLE to access ?'s customer record.

Detection of Attack: Is the program EXAMPLE an attack tool? Are the actions taken by the user A part of an Attack?

Detection of Intrusion: Is the user A really logged in? Does A normally log in from node N at this time? Does A normally run program EXAMPLE to access B s customer record? Is A allowed to do so?

Detection of Misuse: Is A supposed to be running program EXAMPLE? Is A supposed to be accessing ?'s customer record? Has A run other programs that make running EXAMPLE a policy violation? Has A accessed other records that make accessing ?'s records a policy violation?

Analysis: What is happening on the system? Where is node W? Who is using the account called "A" and where is that user located? What program is being called by the name EXAMPLE? What part of the database is being accessed by the label "?'s customer

record" and where is it being stored? What changes happen as a result of this action?

Thus, it can be seen that the goal of an investigation is coloured by a particular context. The same is true for DF.

Accordingly, the investigative model for DF differs in some respects from that of the security domain. The DF investigative model borrows from fundamental forensic science models, but also incorporates domain specific knowledge.

The manner in which an investigation proceeds is normally modelled, or predefined, in order to ensure the investigator fulfils core investigative procedures, as well as the different considerations that must be made in each set of circumstances. Context should be considered, and can be important at various different stages of the investigation. As we will see, there are a number of different factors that must be considered, as they will have a fundamental impact on the investigation and its outcomes. A good model of investigation can provide a basis for common technology, aid implementation and testing of new technology, and provide a common basis for technology and information sharing. The fundamental methodology used in digital forensics is the same as used in forensic science, comprising the following stages:

Acquisition - Obtain the data from the device(s) under investigation. The quantity of information gathered will vary depending on the size of disk analysed or the size of distributed system under investigation.

Analysis - Construct a hypothesis about the events leading up to and after the incident. Use the evidence collected from the environment in order to confirm or refute the original hypothesis. Construct a new hypothesis as necessary and reiterate the process.

Presentation - The findings of the investigation will be written up and presented in a manner that is easy to understand, explaining, with reference to the evidence collected, the conclusions. All abstracted terminology should be explained in detail.

The evidence collected during an investigation must be handled in an appropriate manner, as it is this material that will be used to prove, or disprove a hypothesis constructed and can be inculpatory or exculpatory.

Due to the ad-hoc nature in which digital forensics has evolved, a wide array of guidelines have been published by law enforcement agencies, governments, system administrators and even system intruders. There are various inconsistencies between and deficiencies with these models. However, the stages of a DF investigation will generally include the following:

1. Preparation - This stage involves implementing and establishing proper audit and controls in order to detect an incident. This will be defined in a policy, agreed upon by management, which takes into consideration legal aspects and the goals of the organisation in question. In addition, this phase consists of collecting the appropriate tools, establishing techniques and training personnel to use these tools and techniques.

2. Incident Response - This stage consists of identifying an incident from the auditing systems, or any other indications available. This phase will consist of establishing the extent of the intrusion, and preparing a response according the goals of the organisation.

3. Data Collection - At this stage, data should be preserved, if possible, from contamination. All necessary information from both the physical and logical crime scene should be recorded using standardized and accepted procedures.

4. Data Analysis - Construct a hypothesis about the events leading up to and after the incident. Use the evidence collected from the environment in order to confirm or refute the original hypothesis. Construct a new hypothesis as necessary and reiterate the process.

5. Presentation of Findings - Findings of the investigation will be written up and presented in a manner that is easy to understand, explaining, with reference to the evidence collected, the conclusions. All abstracted terminology should be explained in detail.

6. Incident Closure - Depending on the findings or intent of the investigation, criminal proceedings may be initiated, disciplinary hearings may be conducted, or a review of IT policy may be undertaken.

Another key aspect of a DF investigation is the nature of the data itself. Every DCS creates, stores or manipulates digital information which form

the basis of digital evidence. DCS's create a diverse range of data other than those familiar to an everyday unskilled end user. For every text document created and saved to a hard disk, or for every data packet routed from one end of the Internet to the other, a voluminous amount of data relating to each activity is created, manipulated and discarded. Some of this information is useful, and can be used in a variety of ways, from debugging an application, to signalling that various equipment or applications are working in a correct manner. Indeed, some of this record keeping by digital computing systems is desired as it allows the operators to gather situational awareness, which generally comes in the form of a log file or audit event. Low-level system events, as well as application- specific events, all generate records by the use of various event logging mechanisms provided by the relevant Operating System (OS), or in a bespoke manner by the application itself. This situational awareness can provide the system administrator with enough information to understand when a particular user has logged in, or for the software engineer to have an indication of the last error message a piece of software generated.

All data produced by a system is regarded by the DF investigator as possible evidence. The terms "data" and "digital evidence" will henceforth be used interchangeably. The potential richness of this pool of data can be limited by, or even diluted, by the type of data that gets logged, and which particular software system logs it. Depending on context and intent, the scarcity, or over-abundance of data can be either be beneficial, detrimental, or both. Because the digital evidence used for an investigation can originate from many different sources, and not just the output of a security monitoring system, we will define the general term Logging Entity (LE) which will be used to cover all forms of digital collection and logging apparatus.

Data gathered has a number of characteristics which need to be taken into account when designing a DF methodology and system.

Firstly, data can be viewed at different levels of abstraction. This is also known as the complexity problem. At the lowest form, data is generally incomprehensible to humans, as it is a series of one's and zero's. It takes a great deal of skill to view data in this manner, and although not impossible, is not an efficient or a desirable form of analysis. The operating system or application will generally translate this form of data into a human-readable format.

One of the best examples of the complexity problem can be outlined using HTML. HTML is a mark-up language that defines the layout and look of a Web page. At its lowest


Buchanan, W. J., Graves, J., & Bose, N. (2010). Patent: Improvements in or relating to digital forensics

Publication Date 2010
Deposit Date Jan 28, 2011
Peer Reviewed Peer Reviewed
Keywords System call information; sequence format; sequence alignment; security; forensics;
Public URL