Digital forensics (sometimes known as digital forensic science) is a branch of forensic science encompassing the recovery and investigation of material found in digital devices, often in relation to computer crime. The term digital forensics was originally used as a synonym for computer forensics but has expanded to cover investigation of all devices capable of storing digital data. With roots in the personal computing revolution of the late 1970s and early 1980s, the discipline evolved in a haphazard manner during the 1990s, and it was not until the early 21st century that national policies emerged.

Digital forensics (sometimes known as digital forensic science) is a branch of forensic science encompassing the recovery and investigation of material found in digital devices, often in relation to computer crime. The term digital forensics was originally used as a synonym for computer forensics but has expanded to cover investigation of all devices capable of storing digital data. With roots in the personal computing revolution of the late 1970s and early 1980s, the discipline evolved in a haphazard manner during the 1990s, and it was not until the early 21st century that national policies emerged.

Digital forensics investigations have a variety of applications. The most common is to support or refute a hypothesis before criminal or civil (as part of the electronic discovery process) courts. Forensics may also feature in the private sector; such as during internal corporate investigations or intrusion investigation (a specialist probe into the nature and extent of an unauthorized network intrusion).

The technical aspect of an investigation is divided into several sub-branches, relating to the type of digital devices involved; computer forensics, network forensics, forensic data analysis and mobile device forensics. The typical forensic process encompasses the seizure, forensic imaging (acquisition) and analysis of digital media and the production of a report into collected evidence.

As well as identifying direct evidence of a crime, digital forensics can be used to attribute evidence to specific suspects, confirm alibis or statements, determine intent, identify sources (for example, in copyright cases), or authenticate documents. Investigations are much broader in scope than other areas of forensic analysis (where the usual aim is to provide answers to a series of simpler questions) often involving complex time-lines or hypotheses.

• Forensic process

A digital forensic investigation commonly consists of 3 stages: acquisition or imaging of exhibits, analysis, and reporting. Ideally acquisition involves capturing an image of the computer’s volatile memory (RAM) and creating an exact sector level duplicate (or “forensic duplicate”) of the media, often using a write blocking device to prevent modification of the original. However, the growth in size of storage media and developments such as cloud computing have led to more use of ‘live’ acquisitions whereby a ‘logical’ copy of the data is acquired rather than a complete image of the physical storage device. Both acquired image (or logical copy) and original media/data are hashed (using an algorithm such as SHA-1 or MD5) and the values compared to verify the copy is accurate.

During the analysis phase an investigator recovers evidence material using a number of different methodologies and tools. In 2002, an article in the International Journal of Digital Evidence referred to this step as “an in-depth systematic search of evidence related to the suspected crime.” In 2006, forensics researcher Brian Carrier described an “intuitive procedure” in which obvious evidence is first identified and then “exhaustive searches are conducted to start filling in the holes.”

The actual process of analysis can vary between investigations, but common methodologies include conducting keyword searches across the digital media (within files as well as unallocated and slack space), recovering deleted files and extraction of registry information (for example to list user accounts, or attached USB devices).

The evidence recovered is analysed to reconstruct events or actions and to reach conclusions, work that can often be performed by less specialised staff. When an investigation is complete the data is presented, usually in the form of a written report, in lay persons’ terms.

• Application

Digital forensics is commonly used in both criminal law and private investigation. Traditionally it has been associated with criminal law, where evidence is collected to support or oppose a hypothesis before the courts. As with other areas of forensics this is often as part of a wider investigation spanning a number of disciplines. In some cases the collected evidence is used as a form of intelligence gathering, used for other purposes than court proceedings (for example to locate, identify or halt other crimes). As a result, intelligence gathering is sometimes held to a less strict forensic standard.

In civil litigation or corporate matters digital forensics forms part of the electronic discovery (or eDiscovery) process. Forensic procedures are similar to those used in criminal investigations, often with different legal requirements and limitations. Outside of the courts digital forensics can form a part of internal corporate investigations.

A common example might be following unauthorized network intrusion. A specialist forensic examination into the nature and extent of the attack is performed as a damage limitation exercise. Both to establish the extent of any intrusion and in an attempt to identify the attacker. Such attacks were commonly conducted over phone lines during the 1980s, but in the modern era are usually propagated over the Internet.
The main focus of digital forensics investigations is to recover objective evidence of a criminal activity (termed actus reus in legal parlance). However, the diverse range of data held in digital devices can help with other areas of inquiry.

Attribution Meta data and other logs can be used to attribute actions to an individual. For example, personal documents on a computer drive might identify its owner.

Alibis and statements Information provided by those involved can be cross checked with digital evidence. For example, during the investigation into the Soham murders the offender’s alibi was disproved when mobile phone records of the person he claimed to be with showed she was out of town at the time.

IntentAs well as finding objective evidence of a crime being committed, investigations can also be used to prove the intent (known by the legal term mens rea). For example, the Internet history of convicted killer Neil Entwistle included references to a site discussing How to kill people.

Evaluation of sourceFile artifacts and meta-data can be used to identify the origin of a particular piece of data; for example, older versions of Microsoft Word embedded a Global Unique Identifer into files which identified the computer it had been created on. Proving whether a file was produced on the digital device being examined or obtained from elsewhere (e.g., the Internet) can be very important.

Document authentication Related to “Evaluation of source,” meta data associated with digital documents can be easily modified (for example, by changing the computer clock you can affect the creation date of a file). Document authentication relates to detecting and identifying falsification of such details.

Limitations One major limitation to a forensic investigation is the use of encryption; this disrupts initial examination where pertinent evidence might be located using keywords. Laws to compel individuals to disclose encryption keys are still relatively new and controversial.

Legal considerations The examination of digital media is covered by national and international legislation. For civil investigations, in particular, laws may restrict the abilities of analysts to undertake examinations. Restrictions against network monitoring, or reading of personal communications often exist. During criminal investigation, national laws restrict how much information can be seized. For example, in the United Kingdom seizure of evidence by law enforcement is governed by the PACE act. During its existence early in the field, the “International Organization on Computer Evidence” (IOCE) was one agency that worked to establish compatible international standards for the seizure of evidence.

In the UK the same laws covering computer crime can also affect forensic investigators. The 1990 computer misuse act legislates against unauthorised access to computer material; this is a particular concern for civil investigators who have more limitations than law enforcement.

An individuals right to privacy is one area of digital forensics which is still largely undecided by courts. The US Electronic Communications Privacy Act places limitations on the ability of law enforcement or civil investigators to intercept and access evidence. The act makes a distinction between stored communication (e.g. email archives) and transmitted communication (such as VOIP). The latter, being considered more of a privacy invasion, is harder to obtain a warrant for. The ECPA also affects the ability of companies to investigate the computers and communications of their employees, an aspect that is still under debate as to the extent to which a company can perform such monitoring.

Article 5 of the European Convention on Human Rights asserts similar privacy limitations to the ECPA and limits the processing and sharing of personal data both within the EU and with external countries. The ability of UK law enforcement to conduct digital forensics investigations is legislated by the Regulation of Investigatory Powers Act.

• Digital evidence

When used in a court of law digital evidence falls under the same legal guidelines as other forms of evidence; courts do not usually require more stringent guidelines. In the United States the Federal Rules of Evidence are used to evaluate the admissibility of digital evidence, the United Kingdom PACE and Civil Evidence acts have similar guidelines and many other countries have their own laws. US federal laws restrict seizures to items with only obvious evidential value. This is acknowledged as not always being possible to establish with digital media prior to an examination.

Laws dealing with digital evidence are concerned with two issues: integrity and authenticity. Integrity is ensuring that the act of seizing and acquiring digital media does not modify the evidence (either the original or the copy). Authenticity refers to the ability to confirm the integrity of information; for example that the imaged media matches the original evidence.The ease with which digital media can be modified means that documenting the chain of custody from the crime scene, through analysis and, ultimately, to the court, (a form of audit trail) is important to establish the authenticity of evidence.

Attorneys have argued that because digital evidence can theoretically be altered it undermines the reliability of the evidence. US judges are beginning to reject this theory, in the case US v. Bonallo the court ruled that “the fact that it is possible to alter data contained in a computer is plainly insufficient to establish untrustworthiness.” In the United Kingdom guidelines such as those issued by ACPO are followed to help document the authenticity and integrity of evidence.

Digital investigators, particularly in criminal investigations, have to ensure that conclusions are based upon factual evidence and their own expert knowledge. In the US, for example, Federal Rules of Evidence state that a qualified expert may testify “in the form of an opinion or otherwise” so long as: (1) the testimony is based upon sufficient facts or data, (2) the testimony is the product of reliable principles and methods, and (3) the witness has applied the principles and methods reliably to the facts of the case.

The sub-branches of digital forensics may each have their own specific guidelines for the conduct of investigations and the handling of evidence. For example, mobile phones may be required to be placed in a Faraday shield during seizure or acquisition to prevent further radio traffic to the device. In the UK forensic examination of computers in criminal matters is subject to ACPO guidelines. There are also international approaches to providing guidance on how to handle electronic evidence. The “Electronic Evidence Guide” by the Council of Europe offers a framework for law enforcement and judicial authorities in countries who seek to set up or enhance their own guidelines for the identification and handling of electronic evidence.

• Investigative tools

The admissibility of digital evidence relies on the tools used to extract it. In the US, forensic tools are subjected to the Daubert standard, where the judge is responsible for ensuring that the processes and software used were acceptable. In a 2003 paper Brian Carrier argued that the Daubert guidelines required the code of forensic tools to be published and peer reviewed. He concluded that “open source tools may more clearly and comprehensively meet the guideline requirements than would closed source tools.” In 2011 Josh Brunty stated that the scientific validation of the technology and software associated with performing a digital forensic examination is critical to any laboratory process. He argued that “the science of digital forensics is founded on the principles of repeatable processes and quality evidence therefore knowing how to design and properly maintain a good validation process is a key requirement for any digital forensic examiner to defend their methods in court.” ”

BranchesDigital forensics investigation is not restricted to retrieve data merely from the computer, as laws are breached by the criminals and small digital devices (e.g. tablets, smartphones, flash drives) are now extensively used. Some of these devices have volatile memory while some have non-volatile memory. Sufficient methodologies are available to retrieve data from volatile memory, however, there is lack of detailed methodology or a framework for data retrieval from non-volatile memory sources. Depending on the type of devices, media or artifacts, digital forensics investigation is branched into various types.

1. Computer forensics
   • The goal of computer forensics is to explain the current state of a digital artifact; such as a computer system, storage medium or electronic document. The discipline usually covers computers, embedded systems (digital devices with rudimentary computing power and onboard memory) and static memory (such as USB pen drives).
   • Computer forensics can deal with a broad range of information; from logs (such as internet history) through to the actual files on the drive. In 2007 prosecutors used a spreadsheet recovered from the computer of Joseph E. Duncan III to show premeditation and secure the death penalty. Sharon Lopatka’s killer was identified in 2006 after email messages from him detailing torture and death fantasies were found on her computer.
2. Mobile device forensics
   • Mobile device forensics is a sub-branch of digital forensics relating to recovery of digital evidence or data from a mobile device. It differs from Computer forensics in that a mobile device will have an inbuilt communication system (e.g. GSM) and, usually, proprietary storage mechanisms. Investigations usually focus on simple data such as call data and communications (SMS/Email) rather than in-depth recovery of deleted data. SMS data from a mobile device investigation helped to exonerate Patrick Lumumba in the murder of Meredith Kercher.
   • Mobile devices are also useful for providing location information; either from inbuilt gps/location tracking or via cell site logs, which track the devices within their range. Such information was used to track down the kidnappers of Thomas Onofri in 2006.
3. Network forensics
   • Network forensics is concerned with the monitoring and analysis of computer network traffic, both local and WAN/internet, for the purposes of information gathering, evidence collection, or intrusion detection. Traffic is usually intercepted at the packet level, and either stored for later analysis or filtered in real-time. Unlike other areas of digital forensics network data is often volatile and rarely logged, making the discipline often reactionary.
   • In 2000 the FBI lured computer hackers Aleksey Ivanov and Gorshkov to the United States for a fake job interview. By monitoring network traffic from the pair’s computers, the FBI identified passwords allowing them to collect evidence directly from Russian-based computers.
4. Forensic data analysis
   • Forensic Data Analysis is a branch of digital forensics. It examines structured data with the aim to discover and analyse patterns of fraudulent activities resulting from financial crime.
5. Database forensics
   • Database forensics is a branch of digital forensics relating to the forensic study of databases and their metadata. Investigations use database contents, log files and in-RAM data to build a timeline or recover relevant information.