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\begin{document}
\SACJtitle{Digital Forensic Science: A Manifesto}
\SACJauthor{Martin S Olivier}{}
\SACJaddress{Department of Computer Science, University of Pretoria}

\SACJabstract{
Forensic examination of evidence holds the promise of making
claims about the truth of certain propositions with the inherent
accuracy and reliability that characterises scientific
endeavours.  The propositions may relate to the artefacts
examined or related artefacts.  The nature of propositions about
which claims can be made depend on the extent to which given
propositions fall within the ambit of scientific knowledge and on
the extent to which the examined evidence is suitable for the
application of established science.  A continuing series of
incidents illustrate that in many forensic disciplines that
promise is not met --- often because some branch of forensic
science happen to not being scientific at all.  In fact, serious
assessments of forensic science have shown that many (if not
most) branches of forensic science are not scientifically valid.

Digital forensic science is one of the newest members of the
family of forensic sciences.  A number of reasons for concern
exist that it is following in the footsteps of its more
established footsteps and repeating many of the mistakes of those
other branches of forensic science.

This viewpoint is written in the form of a manifesto that is
situated in the current discourse about digital forensic science
and practice.  If challenges the current developments in digital
forensic science by positing a number of demands that digital
forensic science have to meet to be deemed scientific.  The
demands are posited as necessary, but not sufficient to ensure
that digital forensic science uses science to contribute to
justice.  Appropriate responses to the manifesto is a change in
digital forensic developments or an informed debate about the
issues raised in the manifesto.
}

%\SACJclassification{Computer forensics; Computing profession; Law}

\SACJkeywords{Digital forensic science; Foundational science}

\SACJmaketitle



\section{Introduction}
Most members of the public probably had a rather vague notion of
forensic science until various TV shows --- starting with CSI ---
carried an image of a forensic utopia into our living rooms on a
weekly basis.  In general we were impressed --- to the extent
that jurisdictions where juries are used had to deal with the
so-called new \emph{CSI-effect}: Juries wanted the detailed and
authoritative evidence they got used to in their favourite shows
in order to make what should have been simple decisions during
their deliberations.  Unfortunately, the reality did not match
these expectations.

While many reports suggested that much of the forensic science
on these shows was in principle realistic (apart from the speed
at which tests results became available) the computer scientists
(and some technically inclined computer users) amongst us were
usually not impressed when digital evidence needed to be
recovered.  The ability to type a command or two to trace the
exact physical location from which some message was received was
often beyond what we could accept as science fiction.

However, the disillusion was not limited to the situations where
jury members (or victims of crimes) learned that forensic science
could often not provide the answers.  A more significant problem
emerged.  In reality, various groups of people knew about these
problems for many years, but a wider audience became aware of
them as the popular media carried reports about the problem more
frequently.  The problem was that forensic science was not always
as reliable as touted.  Actually, it was worse: Much of what was
used as forensic science had no scientific basis.  These
knowledge soon enough made news headlines.  Stories of innocent
people who were wrongfully convicted based on flawed forensic
conclusions and spent much of their lives in prison (or were
executed) before being exonerated are indeed stories of human
tragedy, and deserve to be told.  Such stories also sell
newspapers.  Unfortunately, such stories cast a shadow of doubt
over forensic science in general.  The greatest tragedy of all
occurs when forensic practice in general deserves such a blanket
of distrust.

Comparative bullet-lead analysis and microscopic hair analysis
are just two examples of `forensic sciences' that were
discredited.  Currently bite-mark analysis is clinging to straws
to regain some of its former reputation.  Are finger prints
unique?  Do we really interpret blood spatter correctly?  The
simple answer, using science as a yardstick, is \emph{no}.  In a
September 2016 report (cited below) the US President's Council of
Advisors on Science and Technology reconfirmed what is obvious to
so many: They found that of the seven forensic disciplines they
assessed, almost none could be deemed to be founded on science.
(Their study only considered pattern comparison methods.)

And yet, we use those methods to send people to jail and (in some
jurisdictions) to justify capital punishment.

Much research has been done in the field of digital forensics
over the past decade.  However, where so few of the forensic
disciplines --- despite their practitioners' best efforts --- are
not scientific, self-reflection is indicated for all disciplines.

The document below describes the context and then highlights some
course adjustments that should be made if digital forensic
science want to be a `real' science.  As we move more of our
daily lives into the digital realm, and realise that we have not
yet (as of September 2016) worked out how to do proper forensic
science in the physical world (with some notable exceptions), the
need to think about digital forensic science has become
imperative.




\section{The challenge of a digital forensic science}

Forensics is the application of science to determine facts that
contribute to reaching a just deciding in a legal case.  While
the focus is on the law, these insights are also required in some
other situations, such as when the root cause of, say, an
aviation accident needs to be determined with sufficient
certainty to prevent similar accidents in future whenever
possible.  As a society we often rely on science to make informed
decisions about important matters.  In the safety and efficacy of
medicine, the prediction of severe weather conditions, the safety
of new technologies and the determination of the root cause of
disastrous accidents scientific answers are preferred over other
forms of knowledge; in fact legislation often requires scientific
proof of, for example, the safety and efficacy of a new
medication before the medicine can be registered and offered for
sale.

Forensic science, in
principle, enables one to make similar informed decisions in
courtrooms and elsewhere where the law is to be applied.
However, if forensic science is not scientific but a pretence of
science, trust in the endeavour is misplaced.  Note that even
where the word \emph{forensics} (rather that the phrase
\emph{forensic science}) is used, the notion of science is
implied; almost every academic paper on, for example, digital
\emph{forensics} contain some definition that invokes science as
an inherent foundation of such forensics.

Mistakes (or even deception) occur in all forms of testimony in
legal matters.  However, mistakes in the context of
forensics introduce a systematic bias with far-reaching effects
on the justness of the justice system.  It is not hard to find
extensive lists of examples where forensic evidence was wrong and
possibly lead to an incorrect determination that an accused was
guilty or innocent.  The Innocence
Project\footnote{\texttt{http://www.innocenceproject.org/}} is a
good starting point to find such examples; it should be noted
that they make extensive use forensic science --- in particular
of DNA evidence --- to exonerate the
wrongfully convicted.  Arguably the most thorough critique of
forensic science (including recommendations about reforming the
discipline) is the US National Academies of Science report
\cite{nas} on the state of forensic science.  In its assessment of
various forensic disciplines it repeatedly finds that the
specific discipline is not grounded in science.  A more recent
report by the US President's Council of Advisors on Science and
Technology \cite{obama} finds that almost none of the (selected)
forensics disciplines it examined meets the requirements of
scientific foundational validity.  They highlight, in particular,
that ``an expert's expression of \emph{confidence} based on
personal professional experience or expressions of
\emph{consensus} among practitioners about the accuracy of their
field is no substitute for error rates estimated from relevant
studies'' \cite[p.6]{obama}.

The discourse in digital forensics has only seen limited
self-reflection about the use of science (or scientific methods)
in its activities \cite{dft}.  While some notable exceptions
exist, the few published claims that digital forensics is indeed
scientific are often based on a limited understanding of science.

Interactions in the world in which we live increasingly occur in
the digital realm; hence one would expect that criminal
activities (and civil disputes) will increasingly rely on
evidence obtained from the digital domain.  The purpose of this
manifesto is to --- in a rather informal manner --- reflect on
the inherent qualities that a discipline needs to meet to be
viewed as a forensic \emph{science}.

For the sake of brevity we simply posit that large parts of work
done under the digital forensics label ought not to be considered
forensic science. Often a debate about such a statement reveals
that different parties in the debate view the notion of science
differently.  However, while some differences in opinion about
the nature of science will always exist, an activity cannot
simply be `designated' as scientific based on some notion of
science.  Both reports referred to above emphatically reject
various disciplines claims to be scientific --- despite the
strong belief in some of those communities that their work is
indeed scientific.

Below a number points pertinent to a digital forensic science are
raised as a basis for reflection.  They are not intended to form
a comprehensive argument about the nature of digital forensic
science, but are a reaction to some common themes in current
research in the discipline; some points provide basic background
information; others are introduced to either support or oppose
some prevalent lines of thought in the research literature.  The
manifesto provided in the final section of this paper should
similarly be seen as a document situated in the current state of
digital forensics and the current discourse of its
`scientificness'.  It is hoped that the manifesto will have some
impact on the future course of digital forensic science --- if
not by correcting inappropriate lines of inquiry, then by a
deeper reflection of how the discipline ought to proceed.

\section{Musings about digital forensic science: 
Truth, scientific truth and legal truth}
\subsection{Some remarks on when and where the `science' is
performed}

If forensic science is the use of science to help answer disputes
in legal and related matters a question that arises is when this
science is actually performed.  Consider, as a comparative
example, the amount of science that underlies the operation of a
modern motorcar.  However, this does not make the average driver
a scientist.  Most mechanics won't be deemed scientists.  In
fact, very few car service facilities or repair shops would
employ any scientists.  This does not imply that such places and
people do not possess extensive expertise in their specific
domain.  In fact, it will not raise many eyebrows if such a
person is called as an expert witness in some case.  However, the
person will not be able to testify as a scientist.

In the realm of forensic science the following scenario is
common: After extensive research a test is developed to detect
the presence of some substance in, say, blood.  Then
a device is developed to execute the test.  In a
particular case a phlebotomist will typically draw blood from an
individual, put it in the device and obtain a reading (or
printout) from the device.  A phlebotomist is not a scientist
(and, in particular, not a forensic scientist): In the UK there
is no formal qualification required to become a phlebotomist.
Very few states in the US require phlebotomists to hold any
particular qualification.  In a court case they can attest to the
fact that they labelled the blood samples correctly and operated
the test device according to standard operating procedures.
However, they are not qualified to offer any conclusions to the
court based on the results reported by the device.  Science
occurs during the development of the test.  Evidence on the
interpretation of the result (as well as on the accuracy of the
results) will have to be given by a scientist, who knows and
understands the operation of the underlying science.  In the case
of forensic laboratories the report of the test will (officially)
be prepared and signed by such a qualified scientist.

Note that the example in the previous paragraph does not imply
that the actual `testing' in the laboratory never requires a test
to be performed by a scientist.  The point is merely that many
people involved in a forensic science process on a daily basis are not 
(and need not be) scientists.   The process itself (and hence is
development) needs to be scientifically sound; the scientific
laws that underlie the process needs to be understood (and
justified) by the developers of such a process.

Checking authenticity of data using a hash function is an example
where `science in a box' may be used by a technician in a digital
forensic science laboratory. However, if the similarity is
disputed --- say due to new results about hash collisions --- the
active involvement of the scientist may be required.  A hash
function, after all, maps an infinite number of inputs to a
limited number of outputs --- and hence there will be hashes
corresponding to an infinite number of inputs.  Hence, using a
hash to claim uniqueness is far from obvious and needs an
underlying scientific basis before conclusions may be drawn that
a given match is unique.

\subsection{On the truths}

Science is a quest for truth.  The law, when considering
disputes, often need to determine facts.  Facts are claims that
are true.  It was inevitable that at some point the paths of
science and law would meet.

As the first step in a project to reassess scientific truth for
application in law it is necessary to recognise that 
two different notions of truth are involved: Scientific
truth is truth that helps to explain our world.  As we learn more
about the world, the truth often needs to be adjusted.  But these
adjustments are not arbitrary.  As an example, science explains
how aerodynamic forces impact on a body that moves through air.
Such knowledge can be used to design wings that cause sufficient
lift to keep aeroplanes in the air.  If it is at some point
determined that the scientific theories were not perfect when
that plane was built, that plane will not suddenly stop flying.
The old truth was `good enough'.  The new truth is (hopefully)
better.  (Formally: it has more explanatory power.)

Legal truth, on the other hand, is whatever the court decides.
Such a truth typically remains a legal truth unless it is changed
through some judicial process (such as an appeal to a higher
court or a change in legislation).  Legal truth is often deemed
as absolute (unless changed in such an explicit manner).  Case
law (that may date back to Roman times) in common law jurisdictions 
are deemed law until it is changed by a party authorised to do
so.

When a scientific truth changes, the use of an older scientific
truth in legal proceedings is often still `good enough'.
Problems arise where the older theory was not `good enough', but
where it was wrong.  The question to ask when science is used in
legal proceedings is not whether that science is perfect, but
whether it is sufficiently reliable. Newtonian physics, for
example, is adequate to consider the trajectory of a bullet even
though such physics is often deemed to have been replaced by
relativity theory.  In a dispute about the height of some
building or the boundaries of some property Euclidean geometry or
traditional trigonometry may be used, even when it (in principle)
proceeds from the assumption that the earth is flat.  On the
other hand, to use an old example, the discovery of the
(non-existent) planet Vulcan was a mistake (and the theory that
`predicted' its existence was corrected by relativity theory).

While science does not claim to be infallible; it is not hard to
find examples of scientific theories that were incorrect.
However, many of the failed forensic disciplines were not based
in science.  When a `non-scientific' forensic science discipline
fails it does not represent a failure of science.  It does
represent a failure of the legal system which never explored the
grounds on which such a discipline made claims that purported to
be based on forensic science.

To testify, is to express propositions that one believes to be
true.  The belief that a certain proposition is true may, in the
case of an eye-witness, be based on the fact that the witness
observed what is being testified to.  However, courts limit the
nature of the grounds on which beliefs may be based to accept the
belief as testimony.  Most courts will not accept a
belief based on divine revelation as evidence.  Similarly, what
one has heard from someone else is normally not admissible as
evidence and typically rejected as \emph{hearsay}.  In some
situations knowledge of those who have experience of actions,
context or other related matters may share their expertise with
the court as expert testimony, which may assist the court to
better understand the matter at hand.

Forensic science, in contrast, bases its belief in the truth of a
given proposition on science.  

To illustrate, the ballistic
trajectory of a projectile will follow after launch is known in
physics.  It can be calculated based on characteristics of the
projectile, the speed and direction at which it is launched, the
impact of gravity and a number of other variables.  The accuracy
with which the trajectory is calculated does not depend on the
experience of the person who performs the calculation.  It uses
theories formulated by people who may have died centuries ago and
could therefore be seen as hearsay evidence.\footnote{The first
use of scientific evidence in English Law occurred in
\emph{Folkes v. Chadd and Others (1782)} (often referred to as
the Wells Harbour case). In summary it determined that ``In an
action of trespass for cutting a bank, where the question is,
whether the bank, which had been erected for the purpose of
preventing the overflowing of the sea, had caused the choking up
of a harbour, \emph{the opinions of scientific men \emph{[sic]},
as to the effect of such an embankment upon the harbour, are
admissible evidence\ldots}''  (emphasis added)
\cite[p.157]{harbour}.} The calculation of such ballistic tables
(also known as range tables) was a routine component of artillery
used in battle.  In fact, one of the prime reasons the
development of computers became important at the time of the
Second World War was to automate such calculations.

While such a calculation is relatively simple to perform for
application in the artillery context, it may be much harder to
perform in the forensic context, where reconstruction may be the
aim.  In the forensic context muzzle elevation angle, muzzle
velocity, barometric pressure, wind speed and even the original
size and shape of the projectile may be known to a reasonable
degree of accuracy.  However, to reconstruct the trajectory (or,
at least, determine the area from which it was fired), the same
theories --- momentum, gravity, drag, drift and other
factors --- are used.  However, compared to those who fire the
projectile, the forensic examiner cannot readily determine the
values of all these variables at the time of firing, which makes
the computation significantly more complex and introduces errors,
which the forensic examiner will (hopefully) be able to quantify.

It is worth pointing out that software behaves in a very similar
manner.  It is typically easy to predict what a program will do;
however, reconstructing what a program did is much harder.  A
number of pertinent questions should be asked:
\begin{enumerate}
\item Are there scientific theories (akin to those use in ballistic
	trajectory calculation) that are useful to
	understand (or predict what will happen in)
	the digital realm?
\item To what extent (if at all) are such theories used in the
	reconstruction of events in a computing context in the
	current discourse on digital forensics?
\end{enumerate}
The latter question may be reformulated as follows: What are the
scientific theories that a digital forensic scientist can use to
justify that his or her testimony is true?  Do these theories
meet the requirements of foundational scientific validity?



\subsection{On the origins of forensic science}
An alternative route to explore the nature of forensic science is
an exploration of the roots of forensic science.  This section
explores two aspects of these roots: it explores the semantics of
the phrase and the original recognised use of science in a court
case.


Prediction observes a phenomenon (the `cause') and predicts an
outcome (the `effect').  Therefore, if $A$ (predictably) causes
$B$, and $A$ is the only cause of $B$, then if $A$ and $B$
happened, one can infer that $A$ caused $B$.  Stated differently,
$A$ now \emph{explains} $B$.  This is exactly how forensic
science uses laws to explain phenomena; forensic science is often
defined as a scientific analysis performed to determine the root
cause of one or more events.

Locard, by many seen as the father of forensic science, formulated
what has become known as \emph{Locard's exchange principle}; in
his 1934 book \emph{La Police et les M\'ethodes Scientifiques}
he formulates it as ``Any action of a human \ldots{} cannot
unfold without leaving some mark''
\cite[p.7]{locard}\footnote{This is a direct translation as he
formulates it in the cited book: ``Toute action de l'homme
\ldots{} ne peut pas se dérouler sans laisser quelque marque.''}
It has been formulated in a number of ways --- often in the short
form: every contact leaves a trace.  While this principle is not
a scientific law, it works remarkably well, and in many ways seem
even more valuable in the digital realm.  If we know from science
that contact between $X$ and $Y$ leaves some trace $T$, observing
$X$, $Y$ and $T$ may enable us to explain $T$ (assuming the usual
caveats about determining causes form what are deemed to be
effects).

For such an explanation to be accepted as testimony, law 
is required to make two concessions: (a) It needs to recognise
some notion of scientific truth, that may be conveyed from one
scholar to another in a `hearsay' fashion and that science has
checks and balances in place to ensure 'truth'; these checks and
balances override the need to hear (and cross examine) the
original scholar to determine (`legal') truth; and (b) the
scientist is allowed to \emph{conclude} that the presence of $A$
explains the occurrence of $B$. This latter concession is
important because the law prefers to hear the `facts' and then
reach its own conclusions.  Now some  conclusion, based on
science rather than the law, effectively becomes a fact in the
legal process.

\subsection{On the digital}
Science, truth and reality in a sense form a triad:  Science
helps us to discover the truths about the reality in which we
live.  Conversely, if science make correct claims (in particular,
correct predictions) about the reality in which we live, science
has uncovered truth.  (Postmodernists will arguably disagree
here, but it is not clear that postmodern forensic science is
possible\ldots)

Now enter the digital realm.  This is an environment that seems
to be human-made.  Many of its prominent concepts, such as
\emph{cyberspace} derived from science fiction.  Reality in this
context may be virtual --- that is, reality may be `unreal'.
Yet, despite these idiosyncrasies, we have moved into this world
lock, stock and barrel.  Whether one views this `cyberspace' as an
alternative world, or just use the Internet for shopping, banking
and talking, does not matter.  The digital is integrated in our
lives (or vice versa).  If things in life go wrong we often need
 to prove claims we make.  And, in this integrated world, many
relevant events may have happened on the digital side.

Hence, it is no surprise that a branch of forensics --- digital
forensics --- developed to find truths about what happened in the
digital sphere.  But, unlike the physical world, there seem to be
very few rules that constrain the digital world.  In the physical
world, the rules of physics enable us to predict what will happen
(or explain what happened).  Is there a basis on which we can
make such claims about the digital space?

This is the purpose of this text: to explore \emph{which}
questions about the digital space can be answered in a scientific
manner, so that we can demonstrate a scientific truth for our
claims --- in particular claims that may be useful as evidence in
a legal context.

\subsection{Digital forensic science}
From the preceding it is rational to question whether a digital
forensic science can ever exist.  Most (if not all other)
forensic sciences deal with natural phenomena, natural substances
or human nature (which is also natural in some senses at least).
Even human-made tools are made of natural materials that will,
when it interacts with any other natural material, behave in a
predictable manner.  Note that the word ``natural'' is used
loosely here: plastics and other synthetic materials exhibit
``natural'' characteristics --- that is, in contact with other
materials, they will (to a lesser or greater degree) react in a
predictable manner --- this artificial material possesses (an
artificial) ``nature''.

In contrast, computing and the various artefacts produced by it
are as close to alchemy that humanity has ever come.  It is
trivial to program a computer to, for any inputs $x_i$, generate
any desired outputs $y_j$; it is almost equally simple to modify
`trusted' software to produce arbitrary outputs for given inputs
--- unless security mechanisms are in place that will ensure that
the software cannot be modified. Stated differently, it seems one
needs to build systems that are so secure (and correct) that they
perform as reliably and as consistently as a law of nature does.
However, I think very few experts would be willing to stake their
reputations on such an assumption that a piece of software in
infallible (or even, say, 99.999\% reliable).

The shift from digital forensic science to computing in the
previous paragraph may not seem logical.  However, digital
evidence is --- as Fred Cohen \cite{cohen-examine} so aptly
states --- a bag of bits\footnote{Perhaps the phrase ``bag of
of bit sequences'' would have been more apt.}
out of which the examiner has to extract some `evidence'.
Evidence (or, at least, meaning) may be inferred from one of only
two processes:  (1) If the bits through some justifiable process
can be arranged to form some meaningful artefact,\footnote{The
term \emph{artefact} is meant to refer to something digital
produced for later use; it forms the traces available to the
digital forensic examiner.  In a number of forensic science
branches the term \emph{artefact} refers to something
artificially introduced into a photograph or recording that was
not part of what was originally recorded; in those branches
artefacts are ignored as artificial additions to recorded
observations.} then meaning
has obviously been found.  Alternatively, (2) if the bag of bits
is the result of some computational process it may sometimes be
possible to make claims about the inputs to that process and/or
the process itself.  

\newtheorem{conjecture}{Conjecture}

\begin{conjecture}
\label{con:claims}
Digital forensic science claims can only assume one or both of
two forms, namely
\begin{enumerate}
\item That the digital data examined is an example of a
specific class of artefact; and/or
\item That the digital data examined proves or disproves a claim
that the data was the result of specific data transformed by a
specific computational process.
\end{enumerate}

More formally these two claims may be stated as follows:
\begin{enumerate}
 \item For some `recognised category' $C$ (to be elaborated on later)
and some sequence of bits $s$, the digital forensic scientist
can \emph{conclude} that $s \in C$; and/or
 \item That, given some computational process $P$, some inputs
$x$ and some output $s$, the digital forensic scientist may
\emph{conclude} that, depending on the specific values, $P(x)$
did or could have produced $s$.
\end{enumerate}

\end{conjecture}

Both of these claims, for the sake of simplicity, have been stated
in a somewhat more limited form than intended.  This will be
addressed below.  In fact, it will be shown that in this limited
form the conjecture has much wider application than what it may
seem initially.

To `prove' this conjecture in one direction (namely that
scientific forensic claims can indeed take one --- or both --- of these
two forms) examples will suffice.  However, conjectures are
conjectures because they cannot (yet) be proven; to convince the
reader that the examples to be provided are indeed correct, we
need conjecture \ref{con:design} to be introduced below.
`Proving' the conjecture in the other direction is harder and may
indeed be shown to be false.

Note that conjecture \ref{con:claims} refers to
digital forensic science specifically; it excludes branches of
forensic science that may deal with digital artefacts, but where
the claims made are not in the digital realm.  Thus the intention
here is \emph{not} to deal with, for example, voice recognition
or authorship attribution of a recording or a document,
respectively, that happen to be in a digital format.  Those
branches include `natural' properties (such as the properties of
human speech or the vocabulary and style used to compose a
document), and hence do not face the same challenges as `pure'
digital forensic science.

A defence of the conjecture that these are the only two valid
forms of scientific forensic claims will be attempted later.
However, to proceed in the former direction (that there are
indeed two forms) another conjecture is required, which will be
colloquially formulated as follows.

\begin{conjecture}
\label{con:design}
In digital forensic science the notion of `intelligent design'
will often be sufficient to correctly classify an artefact.  The
degree of certainty with which this can be done depends on the
nature of the class.
\end{conjecture}

To illustrate this conjecture, suppose that an investigator
obtains a set of bytes for which some reasonable grounds exist to
infer that a subset of the bytes are intended to be interpreted
in a given fashion.  To make this concrete, suppose one obtains a
sequence of bytes from a system that are purportedly a JPG file.
The claim (or hypothesis) that it is a JPG file may come from the
file extension (if the file name is available), the initial bytes
of the sequence and/or a variety of other clues.  Conjecture
\ref{con:design} claims that we are able to determine whether the
sequence of bytes  indeed a JPG file or not and make that
claim with a specified degree of certainty.

Of course, if the sequence of bytes conforms to all syntactic and
semantic requirements for a JPG file, it opens in a variety of
JPG viewers and (possibly) yields an identifiable picture, the
sequence is a JPG structure, without any doubt.  The only source
of uncertainty is whether it existed on the medium from which it
was retrieved as a JPG file.  This is where the level of
certainty needs to be determined.  It is extremely unlikely that
a random sequence of bytes from a medium will form a JPG image.
On the other hand, in the unlikely case that one tries all
permutations of subsets of bytes on a medium, the likelihood that
one of the permutations will conform to the JPG specifications
increases dramatically.

Another example may be useful: if one recovers an 8-bit value
from a medium it obviously can be a member of the class of 8-bit
unsigned binary numbers.  The question whether it existed (that
is, was used) as an 8-bit binary number on the system in question
can only be answered after much more context has been studied.

Clearly, the likelihood of error does depend on the complexity
of the artefact being examined: A JPG file has a header which not
only has a standard format, but also has fields that impact the
interpretation of the remainder of the file.  As noted, when
opening the file in an image viewer one would normally expect to
see an intelligible image.  If this is true of a file confidence
grows that we indeed have a JPG file.  A series of additional
checks may be desired, such as the EXIF metadata to increase
confidence --- if required.  In contrast, other formats may have
much less inherent structure (effectively, much less
redundancy/meaning) and it may be much harder (or even
impossible) to confirm whether they exhibit ``intelligent design
traits.''


It is now time to return to conjecture \ref{con:claims} to fulfil
the promise that examples of the two conjectured claims would be
provided.  For the first form the example alluded to will
suffice: If the investigator obtains a file that claims to be a
JPG image (say, through its extension), determines that it
conforms to the rules and specifications of a valid JPG image
and, when opened with an image viewer displays what is clearly a
picture, the conclusion that the file is indeed a JPG file is
obvious.  The contents of that file can then be reproduced in a
form that will enable the court (or some suitably qualified
expert) to make its findings.  In some instances the digital
forensic scientist will be qualified to do this, given the second
form of conjecture \ref{con:claims}.

As an example of the second form of conjecture \ref{con:claims}
consider the case where the computational process $P$ is the
calculation of a known reliable hash function and the input $x$ is a
sequence of bytes.  Then the digital forensic scientist may
conclude that $s$ is (or is not) the hash of $x$.  To say that
$P(x)=s$ is straightforward; however, the intention is also to
conclude that $P^{-1}:s \mapsto x$, which needs to be qualified
by the confidence (or error rate) of such a claim, because this
is inherently a probabilistic claim.  However, note that this
example does not suggest that $P$ should be a standard,
well-studied computation: $P$ may, for example, also be a piece
of malware never encountered before.

Given the fact that these two forms of claims are used on a daily
basis in digital forensics no further elaboration is required to
substantiate their utility.  What needs attention is their
sufficiency and (eventually) a stronger justification that there
is a scientific basis for (some) such claims.

\section{The manifesto}

The manifesto that follows represents the insights that may be
gained from the discussion in the preceding sections.  Not every
point contained in the manifesto can be deduced in full from the
preceding discussion, though.


\newcounter{mi}


\subsection*{Forensic science}
\begin{enumerate}

\item The term \emph{forensics} refers to forensic
\emph{science}.  Any notion of a non-scientific forensics
contradicts a generally held understanding in the academic
literature and by the general public of forensics; such a notion
would inherently cause confusion.

\item The utility of science in forensic science is the ability
of science to explain phenomena.  The explanatory ability of
science is inherently related to its ability to (correctly)
predict.

\item The reliability (or accuracy) of a forensic science (or
forensic discipline) is limited by the accuracy with which the
underlying science can predict.

\item The term \emph{science} is contested and the problem of
demarcating science remains critical.  Philosophy of science
provides many useful insights.  In addition, standard scientific
practices, such as peer review, provide a practical basis for
demarcation.  Both the appropriate nature and appropriate
practice are necessary elements to denote an activity as
scientific.

\item Forensic science ultimately has to explain why an event is
seen as the root cause of other events.  Forensic science
therefore needs to be a science (or an application of a science
or based on a science) that (a) can justifiably claim to be a
science, and (b) has explanatory --- and hence --- predictive
abilities.

\setcounter{mi}{\theenumi}

\end{enumerate}

\subsection*{The digital realm}

\begin{enumerate}

\setcounter{enumi}{\themi}

\item Computing is used in many branches of forensic science,
such as matching exemplar fingerprints with those stored in an
extensive database or visualising physical phenomena in various
ways.  The fact that computers (and, hence, digital
representations of phenomena) are used does not imply that
digital forensics is being used.  In these cases computing is
used to support some forensic test.  If a category descriptor is
required for such computing the phrase \emph{forensic computing}
accurately reflects the activity.

\item The phrase \emph{digital forensics} is commonly used to
describe an examination of digital artefacts that \emph{exist as
digital artefacts} (rather than physical artefacts that have been
converted to digital).  To emphasise the point, fingerprints that
have been transferred to paper are not examined as paper
forensics; similarly fingerprints that have been converted to a
digital representation do not form part of digital forensics.
One possible characterisation of digital forensics is that it
examines events (or traces of events) that happened in
the digital realm; the purpose of digital forensics then is to
determine the root cause of, or to reconstruct events that
happened in cyberspace.

\setcounter{mi}{\theenumi}

\end{enumerate}

\subsection*{Examinations and investigations}

\begin{enumerate}

\setcounter{enumi}{\themi}

\item  Forensic examinations punctuate investigations.
An investigation (such as a police investigation or a
criminal investigation) typically includes many activities that
are not scientific (and that cannot be scientific).
Investigators often follow leads that are wrong or based on
unreliable evidence.  Decisions about which leads to follow and
when to abandon a specific line of investigation are often not
based on objective criteria.  The investigators' experience,
intuition, the legal requirements of obtaining search warrants
and other permissions, the behaviour of those implicated by a
case and many other factors determine the course of the
investigation.  It is expected that the investigation will
uncover relevant facts.  The role of forensic science is to test
hypotheses (or theories) that arise for (scientific) factuality.
Investigators work with leads that range from unlikely to proven
facts.  The bar for considering something a lead is low, but
leads may differ in strength; a strong lead may need much
stronger grounds to be considered a strong lead.  Do note that a
proven fact may be a weak lead --- if it, for example, turns out
to be irrelevant.

\item The phrase \emph{forensic investigation} is usually a
misnomer.  This phrase is often used when disasters (such as
airplane accidents) are investigated.  The phrase may allude to
the fact that forensic science often fulfils a major function in
such investigations.  However, such investigations include many
non-scientific aspects (such as interviews with eyewitnesses and
survivors).  Forensic examinations are typically conducted by
laboratories best equipped for the specific test to be performed;
in the case of major disasters, many forensic facilities in many
countries may be involved, where each focusses on one component,
one category of residue or some other specific facet of the
investigation.  The investigators work on the investigation; the
forensic laboratories conduct their specific analysis limited to
the question raised by the investigation team.  In order to
minimise confusion it is best to explicitly distinguish between
forensic examinations (or forensic analyses) within the context
of a (non-forensic) investigation.

\item With forensics being inherently scientific, care should be
taken to not refer to non-scientific procedures as forensic
activities. Many forensic processes may be useful during an
investigation, but the converse is not always true.  Forensic results will, in
general, be admissible in court as evidence; leads and
investigation results will not be admissible as facts unless
sufficiently corroborated.  Hence the investigator should clearly
understand the difference between the two.

\item The word \emph{evidence} should similarly be used with
care.  On the one hand, evidence may be whatever is collected in
relation to a crime --- whether it will have probative value or
not.  On the other hand \emph{evidence} may be something that
proves a claim; this is the type of evidence handed up in a court
of law.  In a forensic context, evidence ought to have the
second meaning --- (forensic) evidence will, in particular, be
evidence about which forensic truth claims are made.

\setcounter{mi}{\theenumi}

\end{enumerate}

\subsection*{Independence}

\begin{enumerate}

\setcounter{enumi}{\themi}

\item The use of forensic science is not limited to the criminal
justice system; many forensic disciplines (with digital forensics
a prime example) ought to be of use in civil matters, internal
hearings and other contexts where such evidence may contribute to
justice.  This ought to be reflected in the language used to
report research results; words such as crimes, guilt and
innocence should be used judiciously in research on the topic
since they ultimately affect the research agenda and application
of forensic research.  (This neither means that these words
should be avoided at all costs, nor that work that is of
particular use in a given context --- criminal, civil, or other
--- should be discouraged; however, many forensic procedures will
be applicable to more than one context, and its exposition and
development should not be limited by unnecessary suggestions of
context through examples, terminology or other potential biases.)

\item 
Forensic science is ultimately in the service of justice, rather
than specific users of forensic science.  Too often forensic
science research focuses on its use in law enforcement (as the
most prominent example).  Digital forensic science will often be
more useful in corporate contexts than most other forensic
disciplines.  Care should therefore be taken that the digital
forensic research agenda is balanced and serves the interests
both of those with and without access to resources.  One way of
gaining neutrality is for any work that produces a mechanism to
prove some proposition $p$ to also reflect on how to prove
$\overline{p}$ or to prove some proposition $q$ that could serve
as a rebuttal of the claim that $p$ happened.

\item
Neither law enforcement, nor the corporate sector is, in general,
equipped to do scientific research (with some significant
exceptions).  Hence the `natural home' for forensic science
research are the traditional research institutes, such as
universities.  Funding from industry or law enforcement should be
recognised as possible sources of bias (if not in the research
itself, then in the research agenda).  Hence any such sponsorship
should be explicitly declared as potential conflicts of
interest.\footnote{This is standard practice in most medical
research and already strictly enforced by the American
Academy of Forensic Sciences.}  In the ideal world digital
forensic research will be funded by government or other bodies
for whom a slogan such as a better life for all is inherently a
call for justice, rather than the simplifying the task of law
enforcement or big business.  They now need to put their money
where their mouths are.

\setcounter{mi}{\theenumi}

\end{enumerate}


\section{Conclusion}
Manifestos are often written by authors who deem it necessary to
assume and express a position at a time when they perceive a
danger that, if such an option is not expressed an opportunity
will be lost to impact the direction of some discourse.
The use of the term \emph{manifesto} indicates strong
convictions of the author that (a) an adjustment of the course is
necessary and (b) the issues that are raised in the manifesto are
those that ought to be high on the agenda for reflection. In this
sense a manifesto is contextual; it speaks to the current
discourse, rather than provide a conclusive, comprehensive
perspective on the topic at hand.

In this manner, this manifesto does not attempt to define digital
forensic science.  Its intention is to highlight necessary (but
not sufficient) aspects of a digital forensic science.

A manifesto is, by nature, a conviction set forth by its
author(s).  A conviction does not claim to be absolutely correct,
but issues a challenge to others participating in the discourse
to engage in further discussion on the points raised in the
manifesto.  As a conviction, it is a call for change in the
current course of events.  The weaknesses of forensic science
have made newspaper headlines over many years, but the news media
are often easily dismissed as being more interested in
sensationalism, rather than facts.  However, when organisations,
such as the US National Academies of Science and the United
States President's Council of Advisors on Science and Technology
raises serious concerns about the absence or lack of
(foundational) science in most of the forensic science
disciplines that they considered, it is a loud and clear signal
that introspection is required.  The two reports issued by these
organisations that were cited above say very little about digital
forensic science, it does not absolve the digital forensic
community from introspection and a well-considered response.
Hopefully the manifesto above posits claims that will indeed lead
to reconsideration of the `old' answers to the issues raised
(where) such answers exist, and reflection on the `new' issues
raised.

\section*{Acknowledgements}
I would like to thank a number of colleagues, friends and family
members who read, commented on (and critiqued) the manifesto.
They will remain anonymous to protect them from the mob of
digital forensic practitioners who may be upset by the
publication of this manifesto.  You know who you are.  I
appreciate your assistance.

\bibliography{dfm}



\end{document}

