IEEE - Aerospace and Electronic Systems - March 2021 - 46

An Introduction to Practical Quantum Key Distribution
insecurities in the practical implementation of the detector. Trojan-horse attacks, as described in [136], are one
such attack. In standard QKD protocol's, components of
Alice and Bob's QKD apparatus must be precisely configured in accordance with the bases they will be using. In a
Trojan-horse attack, Eve gains information on these configurations by transmitting a high intensity pulse of light
into the apparatus of one of the parties and measuring the
reflected light after it has passed back through the apparatus to gain information on the system. In [136], it was
shown that an attacker could use commonly available
materials to carry out such an attack against commercially
available QKD devices (specifically the Clavis2 from
IDQuantique-we discuss commercial QKD devices in
the " Current Commercial Systems " section) and successfully recover Bob's bases choices while remaining undetected. In the same paper, it was shown that a viable
method for protecting against these attacks could be realized by introducing a filter to the QKD apparatus to block
pulses of light with high intensities, a key component in
remaining undetected in this attack.
In some cases, even if precautions have been taken to
protect against possible exploits, an attacker may still be
able to disable these precautions by damaging components
by use of a laser, as shown in [137]. One key example of
the damage that can be dealt and the resulting possible
exploits is the desensitization of detectors that can cause
not only a denial of service attack, but can potentially
open the door for a Trojan-horse [136] attack, allowing an
attacker to recover the entire QKD key.
In one such attack, by thermally blinding avalanche
photodiodes (APDs) used in commercial QKD systems [138], Eve was able to recover the entire secret key.
In these commercial systems, precautions were already
taken to prevent Eve from blinding detectors (making
them unable to detect single photons, instead only detecting bright pulses), however in [138], a more complicated
blinding attack was proposed in which the photon-detectors were subjected to heating, which bypassed the precautions put in place. After blinding Bob's detectors, Eve
measures Alice's signals in whatever basis she chooses
and sends the result to Bob as a bright pulse. As a result of
the blinding, Bob can only detect these pulses if his basis
choice agrees with Eve's, and so his raw key will be fully
known to Eve. This attack does not at all contribute to the
error rate used to detect an attack in common QKD
attacks, and it is not yet clear how best to protect against
this and the broader range of blinding attacks. One possible solution, briefly mentioned in [138], is to design protocols and implementations that can operate in both singlephoton mode and bright pulse mode.
Finally, this class of attack was in fact realized in realistic conditions against the 290 m QKD network in Singapore in 2012 [139]. Eve's attacking apparatus, small
enough to fit in a briefcase, was able to reliably control
46

Bob's measurement results while introducing a negligible
amount of extra loss in the channel and only a small
amount (well below the tolerance level) of noise. As a
result, Eve was able to gain full information on the final
secret key against two parties engaging in a QKD session
without either party detecting her interference.
It is possible to close the side channel used in [139],
and it is usually possible to close other side channels that
could be utilized in similar attacks through various filters
and countermeasures. However, alternatively, one may
move to various device independent models, discussed
earlier, which closes these side-channels by moving certain " weak points " (such as measurement devices) to the
adversary itself.

CURRENT COMMERCIAL SYSTEMS
At this point QKD technology has advanced to a point
where several commercial companies exist producing a
wide-range of quantum cryptographic devices. In this section, we survey several of the major efforts in this commercial area. This is done to provide the reader with a
view as to the upcoming progress of actual QKD technology beyond academic experimental implementations and
to show that this technology has the potential to seriously
impact every-day communication. We make no claims as
to the benefits of one company's products over another
nor do we survey their advantages and disadvantages. We
merely survey what is currently possible from a commercial perspective. We also survey recent telecom investments in QKD technology and applications. Note that this
is a rapidly changing area and this survey was conducted
in early 2019.

ID QUANTIQUE
One of the largest commercial supplier of QKD solutions
is the Swiss company ID Quantique. They provide a range
of devices for single photon detection, quantum random
number generation, and QKD designed to be easily integrated with current data center technology. We summarize
the product lines offered using the information provided
on their website [60] below.

RANDOM NUMBER GENERATION
True random number generation is vital to achieve security in a variety of systems, including QKD. For this reason random number generators based on properties of
quantum mechanics, are currently being commercialized.
ID Quantique first introduced random number generators, based on quantum effects, in 2001. Currently they
offer their " Quantis " line of products as their quantum random number generator solutions. These products make use

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MARCH 2021

IEEE - Aerospace and Electronic Systems - March 2021

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