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First printing, March 2005 Dedication THIST BOOK IS DEDICATED TO MY GRANDPARENTS, HENRI, GABRIELLE, ALBERT, AND RITA Foreword Computer forensics is a relatively new field, and over the years it has been called many things: "computer forensics," "digital forensics," and "media analysis" to name a few. It has only been in the past few years that we have begun to recognize that all of our digital devices leave digital breadcrumbs and that these breadcrumbs are valuable evidence in a wide range of inquiries. While criminal justice professionals were some of the first to take an interest in this digital evidence, the intelligence, information security, and civil law fields have enthusiastically adopted this new source of information. Digital forensics has joined the mainstream. In 2003, the American Society of Crime Laboratory Directors–Laboratory Accreditation Board (ASCLD–LAB) recognized digital evidence as a full-fledged forensic discipline. Along with this acceptance came increased interest in training and education in this field. The Computer Forensic Educator's Working Group (now known as the Digital Forensic Working Group) was formed to assist educators in developing programs in this field. There are now over three-dozen colleges and universities that have, or are, developing programs in this field. More join their ranks each month. I have had the pleasure of working with many law enforcement agencies, training organizations, colleges, and universities to develop digital forensic programs. One of the first questions that I am asked is if I can recommend a good textbook for their course or courses. There have been many books written about this field. Most take a targeted approach to a particular investigative approach, such as incident response or criminal investigation. Some tend to be how-to manuals for specific tools. It has been hard to find a book that provides a solid technical and process foundation for the field . That is, until now. This book is the foundational book for file system analysis. It is thorough, complete, and well organized. Brian Carrier has done what needed to be done for this field. This book provides a solid understanding of both the structures that make up different file systems and how these structures work. Carrier has written this book in such a way that the readers can use what they know about one file system to learn another. This book will be invaluable as a textbook and as a reference and needs to be on the shelf of every digital forensic practitioner and educator. It will also provide accessible reading for those who want to understand subjects such as data recovery. When I was first approached about writing this Foreword, I was excited! I have known Brian Carrier for a number of years and I have always been impressed with his wonderful balance of incredible technical expertise and his ability to clearly explain not just what he knows but, more importantly, what you need to know. Brian's work on Autopsy and The Sleuth Kit (TSK) has demonstrated his command of this field—his name is a household name in the digital forensic community. I have been privileged to work with Brian in his current role at Purdue University, and he is helping to do for the academic community what he did for the commercial sector: He set a high standard. So, it is without reservation that I recommend this book to you. It will provide you with a solid foundation in digital media. Mark M. Pollitt President, Digital Evidence Professional Services, Inc. Retired Director of the FBI's Regional Computer Forensic Laboratory Program Preface One of the biggest challenges that I have faced over the years while developing The Sleuth Kit (TSK) has been finding good file and volume system (such as partition tables, RAID, and so on) documentation. It also has been challenging to explain to users why certain files cannot be recovered or what to do when a corrupt file system is encountered because there are no good references to recommend. It is easy to find resources that describe file systems at a high level, but source code is typically needed to learn the details. My goal for this book is to fill the void and describe how data are stored on disk and describe where and how digital evidence can be found. There are two target audiences for this book. One is the experienced investigator that has learned about digital investigations from real cases and using analysis tools. The other is someone who is new to the field and is interested in learning about the general theory of an investigation and where digital evidence may exist but is not yet looking for a book that has a tutorial on how to use a specific tool. The value of the material in this book is that it helps to provide an education rather than training on a specific tool. Consider some of the more formal sciences or engineering disciplines. All undergraduates are required to take a couple of semesters of physics, chemistry, or biology. These courses are not required because the students will be using all the material for the rest of their careers. In fact, software and equipment exist to perform many of the calculations students are forced to memorize. The point of the classes is to provide students with insight about how things work so that they are not constrained by their tools. The goal of this book is to provide an investigator with an education similar to what Chemistry 101 is to a chemist in a forensics lab. The majority of digital evidence is found on a disk, and knowing how and why the evidence exists can help an investigator to better testify about it. It also will help an investigator find errors and bugs in his analysis tools because he can conduct sanity checks on the tool output. The recent trends in digital investigations have shown that more education is needed. Forensic labs are being accredited for digital evidence, and there are debates about the required education and certification levels. Numerous universities offer courses and even Master's degrees in computer forensics. Government and university labs are conducting theoretical research in the area and focusing on future, as well as current, problems. There are also peer-reviewed journals for publishing research and investigation techniques. All these new directions require in-depth knowledge outside of a specific tool or technique. The approach of this book is to describe the basic concepts and theory of a volume and file system and then apply it to an investigation. For each file system, this book covers analysis techniques and special considerations that the investigator should make. Scenarios are given to reinforce how the information can be used in an actual case. In addition, the data structures associated with volume and file systems are given, and disk images are analyzed by hand so that you can see where the various data are located. If you are not interested in parsing data structures, you can skip the data structure chapters. Only non-commercial tools are used so that you can download them for free and duplicate the results on your systems. Roadmap This book is organized into three parts. Part 1 provides the basic foundations, and Parts 2 and 3 provide the technical meat of the book. The book is organized so that we move up the layers of abstraction in a computer. We start by discussing hard disks and then discuss how disks are organized into partitions. After we discuss partitions, we discuss the contents of partitions, which are typically a file system. Part 1, "Foundations," starts with Chapter 1, "Digital Investigation Foundations," and discusses the approach I take to a digital investigation. The different phases and guidelines are presented so that you know where I use the techniques described in this book. This book does not require that you use the same approach that I do. Chapter 2, "Computer Foundations," provides the computer foundations and describes data structures, data encoding, the boot process, and hard disk technology.