Introduction
Computer forensics is collecting, analyzing, and reporting digital information in a legally permissible way. It can detect and prevent crime in any dispute where evidence is stored digitally. Computer forensics has comparable examination stages to other forensic disciplines and faces similar issues.
About this guide
This guide discusses computer forensics from a neutral perspective. It is not linked to particular legislation or intended to promote a specific company or product. It is not written in the bias of either law enforcement or commercial computer forensics. It is aimed at a non-technical audience and provides a high-level view of computer forensics. This guide uses the term “computer,” but the concepts apply to any device capable of storing digital information. Where methodologies have been mentioned, they are provided as examples only and do not constitute recommendations or advice. Copying and publishing the whole or part of this article is licensed solely under the Creative Commons terms – Attribution Non-Commercial 3.0 license.
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Uses of computer forensics
There are few areas of crime or dispute where computer forensics cannot be applied. Law enforcement agencies have been among the earliest and heaviest users of computer forensics and have often been at the forefront of developments in the field. Computers may constitute a ‘scene of a crime,’ for example, with hacking [ 1] or denial of service attacks [2], or they may hold evidence in the form of emails, internet history, documents, or other files relevant to crimes such as murder, kidnap, fraud, and drug trafficking. It is not just the content of emails, documents, and other files that may interest investigators but also the ‘meta-data’ [3] associated with those files. A computer forensic examination may reveal when a document first appeared on a computer, when it was last edited, when it was later saved or printed, and which user carried out these actions.
More recently, commercial organizations have used computer forensics to their benefit in a variety of cases, such as;
- Intellectual Property Theft
- Industrial espionage
- Employment disputes
- Fraud investigations
- Forgeries
- Matrimonial issues
- Bankruptcy Investigations
- Inappropriate email and internet use in the workplace
- Regulatory compliance
Guidelines
For evidence to be admissible, it must be reliable and not prejudicial, meaning that admissibility should be at the forefront of a computer forensic examiner’s mind at all stages of this process. One set of guidelines widely accepted to assist is the Association of Chief Police Officers Good Practice Guide for Computer-Based Electronic Evidence, or ACPO Guide for short. Although the ACPO Guide is aimed at United Kingdom law enforcement, its main principles apply to all computer forensics in whatever legislature. The four main principles from this guide have been reproduced below (with references to law enforcement removed):
- No action should change data held on a computer or storage media, which may be subsequently relied upon in court.
- When a person finds it necessary to access original data held on a computer or storage media, they must be competent to do so and give evidence explaining the relevance and implications of their actions.
- An audit trail or another record of all processes applied to computer-based electronic evidence should be created and preserved. An independent third party should be able to examine those processes and achieve the same result.
- The person in charge of the investigation ensures that the law and these principles are adhered to.
- In summary, no changes should be made to the original. However, if access/changes are necessary, the examiner must know what they are doing and record their actions.
Live acquisition
Principle 2 above may raise the question: In what situation would changes to a suspect’s computer by a computer forensic examiner be necessary? Traditionally, the computer forensic examiner would make a copy (or acquire) information from a device that is turned off. A write-blocker[4] would create an exact bit-for-bit copy [5] of the original storage medium. The examiner would work then from this copy, leaving the original demonstrably unchanged.
However, sometimes it is not possible or desirable to switch a computer off. Switching a computer off may not be possible if doing so would result in considerable financial or other loss for the owner. Switching a computer off may not be desirable if doing so would mean that potentially valuable evidence may be lost. In both these circumstances, the computer forensic examiner would need to carry out a ‘live acquisition,’ which would involve running a small program on the suspect computer to copy (or acquire) the data to the examiner’s hard drive.
By running such a program and attaching a destination drive to the suspect computer, the examiner will make changes and additions to the computer state that were not present in his actions. Such actions would remain admissible as long as the examiner recorded their efforts, was aware of their impact, and could explain their actions.
Stages of an examination
For this article, the computer forensic examination process has been divided into six stages. Although they are presented in their usual chronological order, it is necessary to be flexible during an examination. For example, during the analysis stage, the examiner may find a new lead, which would warrant further computers being examined and mean a return to the evaluation stage.
Readiness
Forensic readiness is an important and occasionally overlooked stage in the examination process. Commercial computer forensics can include educating clients about system preparedness; forensic examinations will provide stronger evidence if a server or computer’s built-in auditing and logging systems are switched on. For examiners, there are many areas where a prior organization can help, including training, regular testing, and verification of software and equipment, familiarity with legislation, dealing with unexpected issues (e.g., what to do if child pornography is present during a commercial job) and ensuring that your on-site acquisition kit is complete and in working order.
Evaluation
The evaluation stage includes receiving clear instructions, risk analysis, and allocating roles and resources. Risk analysis for law enforcement may involve assessing the likelihood of a physical threat entering a suspect’s property and how best to deal with it. Commercial organizations also need to be aware of health and safety issues, while their evaluation would also cover the reputational and financial risks of accepting a particular project.
Collection
The main part of the collection stage, acquisition, has been introduced above. If the purchase is done on-site rather than in a computer forensic laboratory, this stage would include identifying, securing, and documenting the scene. Interviews or meetings with personnel who may hold information relevant to the examination (which could consist of the end-users of the computer and the manager and person responsible for providing computer services) would usually be carried out at this stage. The ‘bagging and tagging’ audit trail would start here by selling any materials in unique tamper-evident bags. Consideration must also be given to securely and safely transporting the material to the examiner’s laboratory.
Analysis
The analysis depends on the specifics of each job. The examiner usually provides feedback to the client during research, and from this dialogue, the study may take a different path or be narrowed to specific areas. The analysis must be accurate, thorough, impartial, recorded, repeatable, and completed within the timescales available and resources allocated. There are myriad tools available for computer forensics analysis. We believe the examiner should use any tool they feel comfortable with as long as they can justify their choice. A computer forensic tool’s main requirement is that it does what it is meant to do, and the only way for examiners to be sure of this is to test and calibrate the tools they use before analysis regularly. Dual-tool verification can confirm result integrity during analysis (if with the tool ‘A,’ the examiner finds artifact ‘X’ at location ‘Y,’ then tool ‘B’ should replicate these results.)
Presentation
This stage usually involves the examiner producing a structured report on their findings, addressing the initial instructions’ points and subsequent instructions. It would also cover any other information the examiner deems relevant to the investigation. The report must be written with the end reader in mind; in many cases, the report reader will be non-technical, so the terminology should acknowledge this. The examiner should also be prepared to participate in meetings or telephone conferences to discuss and elaborate on the report.
Issues facing computer forensics
Computer forensics examiners’ issues can be divided into three broad categories: technical, legal, and administrative.
Encryption – Encrypted files or hard drives can be impossible for investigators to view without the correct key or password. Examiners should consider that the key or password may be stored elsewhere on the computer or on another computer to which the suspect has had access. It could also reside in a computer’s volatile memory (known as RAM [6], usually lost on computer shut-down; another reason to consider using live acquisition techniques as outlined above.
Increasing storage space – Storage media holds ever greater amounts of data, which for the examiner means that their analysis computers need to have sufficient processing power and available storage to deal with searching and analyzing enormous amounts of data efficiently.
