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\chapter{Physical Security in Quantum Key Distribution}
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\section{Cryptography in the Age of Quantum Computers}

For a decade or two now, Quantum Computing has been creating a buzz that nobody in Computer Science and adjacent fields
could evade. Originating in the 1980ies as a highly academic fusion applying concepts from Computer Science in Quantum Physics,
% FIXME citation
its concepts have long found their way into popular science articles. Quantum Computing encompasses a model of
computation that is fundamentally different from the \emph{classical}\footnote{
    In Quantum Computing, the term \emph{classical} is used as the complement of \emph{quantum}, and refers to the
    digital computers we know and (maybe) love. This terminology stems from the distinction between classical and
    quantum physics.}
digital circuits that underly all of modern computing. While at first this might seem like a step backwards into the era
of early 1900s analog computing,
% FIXME citation
the capabilites of a future quantum computer promise to far outpace those of contemporary classical computers. Key to
this improved processing capability is a property called \emph{Quantum Parallelism}. What this refers to is the fact
that a quantum computer's internal state can simultaneously represent a multitude of states of a classical, digital
computer, and the quantum computer can operate on all those states at once using a single quantum operation.

Applying Quantum Parallelism to practical problems is far more complicated than, e.g., translating a digital circuit
solving some equation to a quantum circuit, but for certain problems we already know \emph{quantum algorithms} that
for large inputs solve these problems much faster than any classical computer ever could. Two of these algorithms, one
by Shor % FIXME citation
and one by Grover % FIXME citation
are what caused most of the buzz around the field of quantum computing, because they spell trouble for a large part of
modern cryptography.

Besides the computational speed-up promised by Quantum Parallelism, there is one more interesting aspect of Quantum
Computing where it radically deviates from classical computing. The reason modern cryptography exists is that when we
transmit (or store!) classical information through some channel (or storage!) that we do not control, there is nothing
we can do to prevent an attacker from reading this information. Even with cryptography we cannot prevent this, but
cryptography gives us tools to very effectively make whatever information the attacker is able to read useless to them.

A basic principle of Quantum Physics is the \emph{No-Cloning Theorem}, which states that it is impossible to create an
identical, independent copy of an arbitrary, unknown quantum state. % FIXME citation
An implication of this theorem is that when we encode classical information into quantum states in just the right way,
we can make it so that an attacker atttempting to eavesdrop on our quantum information can only actually read this
information by destroying it in the process. This property can be exploited to replace a number of classical asymmetric
primitives in interactive settings, % FIXME citation, check if interactive only
the most popular application of which is replacing an asymmetric Diffie-Hellman key exchange % FIXME citation
with a quantum process called Quantum Key Distribution that yields much of the same properties.

In the past decades, the field of cryptography has been fundamentally shaped by the development of Quantum Computing and
Quantum Key Distribution. However, the popular conception that all of today's cryptography will be broken and that we
have to start from scratch is not accurate. Quantum Computing poses an unique threat to modern cryptography, and Quantum
Key Distribution is a promising new tool, but the practical implications of both are much more subtle than how they may
be portrayed. In the remainder of this chapter, we will look into the practical implications of these quantum
technologies, and we will come to two major conclusions: First, that while the underlying cryptographic primitives will
change, apart from some minor engineering issues cryptography as a whole will remain largely the same. Second, that
while Quantum Key Distribution is hailed as a revolution for network security, its practical advantages will remain far
short of how it is usually conceptualized, and hardware security will assume a pivotal role in the practical security of
Quantum Key Distribution systems that is a stark departure from its relative irrelevance in today's applied
cryptography.

Building on these conclusions, we will end this chapter with a study of a use case that illustrates a practical design
for a secure network employing Quantum Key Distribution. Relying on both established classical and quantum primitives
with known security properties we will elaborate how one can construct a large-scale network from those primitives
that provides practical security to its users that goes beyond the (surprisingly limited) extents of quantum security
proofs.

\subsection{Computational Assumptions and Information\Hyphdash Theoretic Security}

In the past paragraphs we have briefly mentioned that Quantum Computing provides a significant speed-up that can be
applied to solve many cryptographic problems fast enough for it to become a problem, but we have not elaborated on what
that means in practice. In this section, we will attempt to provide concrete numbers to quantify the threat that both
Shor's and Grover's algorithm pose to modern cryptography.

Shor's algorithm allows for the factorization of large numbers in polynomial time on a quantum
computer, a problem whose hardness (or the hardness of variants of which) is the foundation for the vast majority of
today's asymmetric cryptography.

While Shor's algorithm attacks the foundations of most modern asymmetric cryptography, Grover's algorithm can be applied
to hash functionss and symmetric cryptography. Fundamentally, Grover's algorithm is a search algorithm that allows a
quantum computer to find one target entry out of an \emph{unstructured} list of $N$ source entries in
$\mathcal{O}\left(\sqrt{N}\right)$ time instead of the $\mathcal{O}\left(N\right)$ time that a classical computer would
require for an exhaustive search. Applied to cryptography, we model the key space of a symmetric cipher as the
unstructured list that is input to the algorithm, and set it to search for the key that results in the successful
decryption of a given ciphtertext.

An important nuance applying these algorithms to cryptography is that while both provide significant speed-ups over
classical computers, the speed-up of Shor's algorithm is exponential and effectively breaks most modern asymmetric
cryptography as it erases the asymmetric nature of the underlying mathematical problem. That is, for an asymmetric
cryptosystem susceptible to Shor's algorithm, there is no set of parameters that is large enough to be safe.

In contrast to this, while Grover's algorithm radically speeds up the breaking of a symmetric cryptosystem, this
speed-up is only quadratic. In practice this means that it halves the security level % FIXME definition, citation of sec. lvl
of a given symmetric cipher. While this is bad news for applications that parameterize these symmetric primitives to a
security level at the lower end of what is considered secure today, the advantage provided by Grover's algorithm can
easily be compensated by doubling key size. Longer key sizes require more storage or bandwidth for the additional bits
and result in slightly slower operation of the cipher, but this additional cost is easily manageable even without any
improvement in today's hardware.

\section{The Physics of Quantum Computing}

\section{Quantum Key Distribution}

\section{Quantum Networking}

\section{Securing QKD Networks with Inertial HSMs}

\section{Outlook}

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\appendix

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