Practical Forensic Imaging

by Bruce Nikkel

Reviewed by Scar de Courcier, Forensic Focus

Any book that begins with a foreword by Eoghan Casey is almost guaranteed to be a vital and immensely useful read in the field of digital forensics, and Practical Forensic Imaging is no exception.

The need to securely preserve digital evidence is of the utmost importance to any investigator, particularly in criminal cases where findings may need to be upheld in a courtroom situation. Despite the huge impact of this subject matter, however, there have been precious few books on the topic to date. Luckily, Practical Forensic Imaging steps in now to fill the gap.The idea behind the book is not only to give forensic examiners a thorough understanding of why securing digital evidence is so important, but to provide practical steps that aid this process in a way that is scalable and financially viable.

Quite a laudable goal, and yet Nikkel’s textbook does just that. Particularly in the light of new amendments to US Federal Rules of Evidence that mean digital evidence will have to be self-authenticating, it is crucial to ensure that evidence is preserved in such a way that its authenticity will not be called into question should a case come to court.

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One thing I particularly enjoyed about this book was its practicality. It is sometimes a feature of digital forensics textbooks that they are excessively caught up in the theory behind their subject matter, with very few step-by-step exercises for the reader to follow. While theory is of course important, from the point of view of a less experienced practitioner in particular it is invaluable to have a clearly defined process to follow, and to refer back to when necessary.

Having said that, the book does assume a certain amount of prior knowledge. It focuses on Linux tools, so a level of familiarity with the Unix and Linux shell environment is highly desirable. However, it is nonetheless an accessible read and would not be too hard to follow even for the less technical among us.

Practical Forensic Imaging begins with an overview of the history of digital forensics – impressively crammed into a single chapter, it is therefore a bit of a skim-read over the past several decades of computing, but it lays the groundwork nicely for the rest of the book to follow. I particularly liked how, in each timeframe being described, the author brought the reader’s attention back to how this was relevant to the task at hand: that of securing digital evidence. Digital media being the labyrinthine beast it is today, it is easy to get bogged down in detail and forget the goal. Nikkel doesn’t fall into this trap at all, keeping the book remarkably concise for something that spans such a huge amount of subject matter.

As mentioned previously, Practical Forensic Imaging is aimed at the intermediate investigator who already has a certain amount of technical knowledge. However, the first chapter gives an overview of storage media (with pictures) which will prove useful to ambitious new forensic analysts who have less experience but a willingness to apply themselves to the task of learning.

Introductions out of the way, we now approach the meatier bit of the book. Linux is introduced as a forensic acquisition platform, with a nod to its disadvantages and drawbacks as well as the reasons Nikkel ultimately chose it. A small introduction to Linux file systems follows, although again this does assume a basic level of prior knowledge.

A short history of, and introduction to, various digital forensic image formats follows in the next chapter. This provides a handy go-to guide but does not go into much depth.

The bulk of Practical Forensic Imaging is then taken up with the nitty-gritty of how to get the job done. At each step of the way, Nikkel very precisely walks the line between describing why a particular step is important, and yet not getting so caught up in it as to make the book inscrutably dense or overly long. (Indeed, at just 292 pages, Practical Forensic Imaging will hardly cram your shelf.)

Lines of code are included where relevant, and are easy to pick out from the rest of the text. The use of bulleted lists makes it particularly readable, and illustrations accompany many of the suggestions throughout the book.

One thing I would have liked to see is a few more real-life examples. Sometimes it can be useful to understand how something was done correctly (or not!), but this is more a matter of personal preference than a flaw in the book itself.

Chapters 4 through 7 give the reader everything they might need to set up their system, acquire and securely preserve digital evidence, including brief forays into remote acquisition and RAID systems. Disk cloning and duplication is also covered, and near the end of the book we encounter a section on how to securely wipe storage drives.

The final couple of chapters deal with those less common yet still important situations we encounter from time to time – encrypted file systems, virtual machines, extracting subsets of forensic images, and so on.

The book ends with a call to digital forensic investigators not to be complacent when it comes to the preservation of evidence – something I think we can all agree is of great importance.

Practical Forensic Imaging by Bruce Nikkel is, in summary, a useful reference for digital forensic analysts and will make an excellent addition to any bookshelf.

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