 presenting the case must be able to prove the following:
- The data is authentic.
- The copy of the data used for analysis is reliable.
- The data was not modified during acquisition or analysis (chain of custody).
- The tools used to analyze the data are valid tools.
- Sufficient evidence, both incriminating and exculpatory, has been acquired and analyzed to support the proffered conclusion.
- The conclusions drawn are consistent with the data collected and analyzed.
- People involved in the collection and analysis of the data are properly trained and qualified to do their job.
This doesn’t sound easy, and it isn’t. Details on how to assure that all of these requirements are met are covered in greater detail in later chapters. For now, suffice it to say that it is essential that they be fulfilled.
A cardinal rule of digital investigation is that the original data must never be touched. For many years, the standard rule has been that a forensically sound copy of the original be made and that the examination and analysis of data be performed on the forensic copy. In terms of nonvolatile media, such as hard disks, removable media, and optical disks, this is still the rule. Devices should always be mounted as read-only in order to assure that no data is modified or overwritten during the process of mounting the device. Hard disk duplicators are designed specifically for this purpose, and in Windows systems, a simple modification of the registry allows USB devices to mount read-only.
Legal issues might arise if there is any possibility that media used to store images may have been contaminated. Be aware of that possibility and either have new media available for collection or be certain that previously used media has been forensically wiped.
In many cases, it becomes essential that copies of data be acquired through a process of live acquisition. This is the case when it becomes necessary to capture the contents of memory from a running system, to acquire log files from network devices that cannot be brought down, or to archive information from network servers or storage appliances that defy the making of a forensic copy. If it is not possible, for any reason, to create a forensically sound copy, it is essential that the investigator document the reasons such a copy could not be made and record as accurately as possible the state of the evidentiary source before and after acquisition.
Storage of preserved information becomes part of the chain of custody process, and care must be taken that all data and devices collected during this phase are properly documented and tracked. Be able to verify that there was never a possibility for evidence to become tainted through outside tampering, corruption, or improper procedure.
The process of examining data increases in scope and complexity every year. Whereas 1.44MB floppy disks were once the repository for stolen and illicit data, investigators these days are presented with flash drives the size of key fobs that hold 64 or more gigabytes of data and hard disks that store in excess of a terabyte. To make matters worse, the data is not likely to sit on a porch swing in plain view for anyone to see. Investigators will find it necessary to look for evidence in unallocated space left behind by deleted files. Hidden partitions, slack space, and even registry entries are capable of hiding large quantities of data. Steganography can hide documents inside of an image or music file. So essentially, the investigator is given an archive the size of the Chicago Public Library and asked to find a handwritten note on the back of a napkin tucked somewhere inside of a book.
Data carving tools and methods of looking for evidentiary material have evolved, and depending on the nature of the case, the investigator’s tool kit will require having several utilities. For criminal cases requiring forensically sound presentation, it is critical that the tools used to examine data be those considered valid by the courts. There are a few commercially available software suites approved for evidentiary use. Among