Keynote Speech: California Sciences Institute
AbstractThis paper summarizes a keynote speech on moving toward a science of digital forensic evidence examination. The basic premise is that we are not a normal science today, and to get to normal science, we need to make progress in many areas. The question at hand, is where we should move as a community, and how we should get from here to there.
In making progress in the science of digital forensic evidence examination, it may be helpful to look at the advancement of science in other areas. In most areas of science, a scientific methodology consists of four basic elements; (1) studying the past and current theories, methods and their experimental basis; (2) identifying inconsistencies between current theories and repeatable experimental outcomes, (3) hypothesizing a new theory and performing experiments to test the new theory, and (4) publishing the results. However, in an area where there is no pre-existing scientific theory, a new methodology, theory, experimental basis, and perhaps even a new physics has to be built from scratch. In the case of DFE examination, only one attempt has been made to do this so far, and this paper is substantially about that attempt. Where no citation appears in this paper, it is to be assumed to cite .
failures of forensics. Recent failures have been quite dramatic. For example, in the Madrid bombing case, and the US FBI declared that a fingerprint from the scene demonstrated the presence of an Oregon attorney. However, that attorney, after having been arrested, was clearly demonstrated to have been on the other side of the world at the time to question. The side effect is that fingerprints are now being challenged as valid scientific evidence across the land, and around the world.  A similar situation exists in cases where forensic examiners have done a poor job, and testified in numerous cases, typically for the prosecution. The inability to effectively challenge evidence by such supposedly experts through a scientific methodology and inquiry process makes this sort of evidence extremely problematic, and all the more so because of the limits of human integrity involved in this case. In case after case, when the details are examined, forensic evidence seems to come up short under close scrutiny, and if competently challenged. The solution is simple. Build and apply real science, and the truth will out.
Digital evidence consists entirely of sequences of binary values that we call bits. Thus, in this limited field, we do not deal with the physical nature of normal space, but we operate in a very different space.
The physics of DFE is different than that for matter and energy, and thus the normal assumptions that are made with respect to the way the world works do not apply, or don't apply in the same way, to DFE. Among the substantial differences are, without limit:
Unlike most physical evidence, which is very often transfer evidence and sometimes trace evidence, DFE is always trace evidence, but essentially never transfer evidence.
DFE is normally latent in nature in that it can only be observed through the use of tools. This then implies a multitude of requirements surrounding those tools and their use.
As a "scientific" approach, the theories are not casual theories, but "scientific theories". That means that:
The "theories" of DFE lead to a form of a physics of digital information. Many of them are based on existing widely accepted mathematical knowledge, but some are still somewhat conjectures from computer engineering, computer science, finite mathematics, and related areas.
Input sequences to digital systems produce outputs and state changes as a function of the previous state. To the extent that the state or outputs produce stored and/or captured bit sequences, these form traces of the event sequences that caused them. Thus the definition of a trace may be stated as: "A set of bit sequences produced from the execution of a finite state machine.
We generally think of the physical space we live in as a space that diverges with time, with any given initial conditions in history producing a wide variety of possible future outcomes. As a result, when looking at a physical trace, at least theoretically, we could identify a unique historical event sequence that produced such a trace. But the digital space converges with time, so that instead of the one to many relation that we see in the physical world, we see a many to one relation in the digital world. That means that a very large number of potentially very different input sequences and initial states may produce identical traces (i.e., from subsequent states and sequences). Almost any digital trace we identify could be the result of a large number of different historical event sequences, and the number of those sequences increases dramatically with the passage of time (i.e., execution of FSMs). Thus the traces from digital mechanisms are not, in general, unique as to the input sequences that produced them.
Another less mathematical sort of problem is the relationship between the unlimited granularity of the physical world in both time and space and the finite granularity of the digital world in both time and space. Because of this difference, at the interface between the physical and digital world there is a discontinuity, and quite minor differences are exaggerated near the discontinuity while major differences are ignored away from the discontinuity. The limited sensor and actuator capacity of the devices that convert between the digital and physical world also largely prevent the exchange of a wide variety of information that is potentially probative, as well is making a wide variety of forgeries at the interface far easier than they might otherwise be. This then also implies that input sequences do not directly demonstrate what non-digital events sequences may have produced them. As a result, additional effort is required to attribute traces to real-world causes and forgery is potentially far easier in the digital space than in the physical space.
While a great deal of time and effort could be spent discussing in substantial detail the rather large number of such differences, many of these are already documented and the larger implication of these examples is that digital forensic evidence is the result of processing with FSMs, and that inherently limits its potential utility for providing probative information regarding real-world events. DFE examiners must take these limitations into account when undertaking their examinations, and when testifying about the results of those examinations. These limitations are due directly to the limits of DFE and the methodologies used to understand and work with it.
Dr. Frederick Cohen, Ph.D., and his associates have recognized expertise in E-Discovery, Digital Forensics, Computer Security, Information Protection, and Related Areas. For more than 30 years, they have served as an unbiased independent source of top quality information in this arena.
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