The Bridge - February 2018 - 18

Feature

Quantum
Cryptography and
Side Channel Attacks
by: Colin Lualdi, Stephen Pappas, Daniel Stack,
and Brandon Rodenburg
Quantum Information Science Group,
The MITRE Corporation

ABSTRACT
Quantum key distribution (QKD) promises a
theoretically unbreakable cryptosystem by employing
the probabilistic nature of quantum measurement
over mutually unbiased bases, making it superior
to classical cryptosystems threatened by the advent
of quatnum computing. However, it has been
shown that QKD systems are vulnerable to side
channel attacks due to engineering and technical
imperfections in practical implementations. This
article presents a general overview of quantum
cryptography, beginning with a comparison of
classical and quantum cryptography is threatened
by quantum computing. A basic discussion of QKD's
security characteristics and implementation details is
given. An example side channel attack is introduced
with the avalanche photodiode backflash attack. The
authors provide a brief overview of their experiment
to investigate this attack, and report results indicating
the ability for this attack to, in principle, succeed.

1 INTRODUCTION
Quantum key distribution (QKD) promises a
theoretically unbreakable cryptosystem by employing
the probabilistic nature of quantum measurement
over mutually unbiased bases. Its security lies
in the impossibility of an eavesdropper to gain
access to the quantum keys without revealing their
presence due to the destructive nature of quantum
measurement, as measured by the quantum bit
error rate (QBER) [1]. However, it has been shown

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that QKD systems possess security vulnerabilities
due to engineering and technical imperfections
in practical implementations. Side channel attacks
exploiting various points of accidental information
leakage have been described in the literature, with
examples including the photon number splitting and
faked states attacks [1, 2, 3].
Many of those systems employ avalanche
photodiodes (APDs) as a part of the process
of measuring photon states to create a secure
key as described by QKD protocols like BB84
[1]. Avalanches of charge carriers following
photodetection events in silicon and InGaAs
APDs are known to emit secondary photons as
a consequence of carrier relaxation [4, 5]. These
photons, or backflashes, may be coupled back
into the quantum channel and detected by an
eavesdropper who could potentially deduce the
states of the original information-carrying photons
measured by the legitimate QKD receiver without
affecting the QBER and thus remain hidden [6, 7].

2 QUANTUM CRYPTOGRAPHY
2.1 The One-Time Pad
Many encryption systems varying in complexity
and security have been invented. However, in the
context of quantum cryptography the one-time pad
plays an important role. The principle behind this
cryptosystem (also known as the Vernam cipher)
is simple, yet it is extremely effective at securing
information [9]. Suppose we have a system where
we assign a numerical value to each letter in
the alphabet:
Letter
Number

A
1

B
2

C
3

...
...

Y
25

Z
26

We can use this to obtain numerical equivalents of
ordinary text. For instance C A T would translate to
3 1 20. Suppose a person, Alice, wishes to send
her friend, Bob, the message C A T but she does
not want anyone to intercept her message and read
its contents. So she and Bob agree to use a one-



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Contents
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