*: An alternative title is obviously: Safe By The Bell.

**TL;DR.** Quantum cryptography offers the best of both worlds: security *and* quantum weirdness. In particular, quantum key distribution (QKD) is often claimed to offer “unconditional security based on the laws of physics”, a claim which is easily seen to be false. However, QKD does seem to be qualitatively more powerful than anything classical cryptosystems have to offer. This essay takes a critical look at the interplay between QKD and traditional cryptography by visiting the necessary background and summarizing the relevant concepts and context from a layman’s point of view.

## 1. Introduction

In the modern information age where computers are all around us, the need for computer security is greater than ever. It is no exaggeration to say that just about all our communication and financial infrastructure is protected by cryptography, the branch of computer security that deals specifically with protecting information. Calls to conduct government elections and identity management electronically should take their computer security requirements into careful consideration, or risk sacrificing national sovereignty and personal freedom to the whims of sufficiently sophisticated attackers — attackers who are sure to keep their algorithmic and technological advantages secret, thus making the estimation of security a very subtle game indeed.

Quantum technologies, *i.e.*, devices that exploit quantum mechanical properties in a way that classical devices cannot mimic, are one recent development that is bound to dramatically affect the cryptographic landscape. While the end-goal of large-scale quantum computers is, despite significant resources devoted to the project, (presumably) not yet technologically feasible, detectors that measure the polarity of individual photons are already commercially available. Perhaps surprisingly, this on-going proliferation of quantum technology is causing a rift in the cryptographic community. On the one hand, the physicists are hailing the advent of photon detectors as enabling quantum key distribution and, by proxy, as the key to generating unconditional security — security beyond the traditional computational problems that are merely assumed, without proof, to be intractable. On the other hand, the mathematicians reject this notion of security guaranteed by physics; instead they prefer to stick to traditional computing hardware and design classical cryptographic algorithms to resist the end-game attacks by large-scale quantum computers.

Which side is right? The computer scientists, of course, because only they offer the proper computational perspective on the interplay between quantum mechanics and hard mathematical problems. More importantly, it is computer science and its parent, information theory, that offer the only rigorous definitions of security, namely in terms of confidentiality and authenticity. It makes sense then, to take a computer-scientific approach to assess the physicists’ and mathematicians’ claims. This essay does just that — it considers quantum key distribution from the point of view of theoretical computer science and cryptography by offering the layman reader an introduction to the subject matter and the necessary background. In the process we cover security-by-assumption; the strong Church-Turing hypothesis; quantum computation and entanglement; non-local-realism; Bell inequalities; and many more violations of common sense.

## 2. Cryptography

Cryptography is the science of protecting sensitive information against being read, modified, or otherwise used for nefarious purposes. While the use of cryptography dates back to the Ancient Greeks and beyond, the scientific investigation into this field of study is much newer, coinciding in large part with the information theory and computer science revolutions of the Twentieth Century. This coincidence is no accident as computers are experts at rapid information processing and are consequently the natural allies both of those who routinely generate and use sensitive information, but also — especially — of those who would attack its cryptographic safeguards.

### 2.1. One-Time Pad and Computational Security

One major result at the onset of this revolution is the proof by Claude Shannon — inventor of the eponymous information theory — that the one-time pad offers security against computationally unbounded adversaries^{1}. Informally, by adding enough noise to a message, it becomes completely indecipherable to the eavesdropper, even though the intended receiver might be capable of reproducing the exact same stream of noise and thus recover the message. The label “unconditional security” is often applied when it is implicitly understood that the attacker fits the mathematical model and sees only the ciphertext in transit. By contrast, no such guarantee is offered against the attacker who violates this model by having access not only to the ciphertext in transit but also to another channel on the side — such as a watchful eye over the sender’s shoulder when the noise is added, or its equivalent in terms of a power consumption trace.

Unfortunately, unconditional security comes at the rather cumbersome cost of secret keys that must be at least as large as the message. Moreover, these keys must be distributed ahead of time, oblivious to the length of the message that is only chosen afterwards. In response, cryptographers have designed ciphers that mimic the one-time pad except for requiring only a relatively small key, independent of the message length. These ciphers do not offer unconditional security but *computational security*. That is to say, security only holds under an additional assumption to the effect that certain computational problems are hard to solve. This strategy of security by assumption may seem odd at first sight. However, considering that since the very successful standardization of the Belgian-designed AES algorithm^{2}, the worldwide effort by cryptanalysts to break it have led essentially nowhere, it is fair to say that the entire computer security community is on board with this assumption. Moreover, any meaningful result on computational hardness would imply $\mathbf{P} \neq \mathbf{NP}$, the biggest unproven conjecture in all of computer science. This result would not only prove one of the seven millennium problems; it would also confirm mathematician’s suspicions that the other six are actually hard. Requiring a mathematical development of this magnitude seems like an unreasonably high bar for secure cryptosystems to pass.

### 2.2. Diffie-Hellman Key Establishment

Unfortunately, the shift towards computational security does not solve everything. Two parties wishing to communicate securely must still agree on a key beforehand, or else the eavesdropper will intercept the key and decrypt all subsequent traffic. Moreover, suppose $n$ parties wish to communicate; then there must be $n(n-1)$ shared keys for a complete graph. This strategy does not scale well.

A revolutionary solution to this problem was proposed by Diffie and Hellman in 1976^{3}, and for which they since won the Turing award. Their protocol, now called Diffie-Hellman key agreement, relies on the algebraic properties of groups as well as on the computational difficulty of computing discrete logarithms in them.

Consider for example $\left(\mathbb{Z}/p\mathbb{Z}\right)^*, \times$, the multiplicative group of integers modulo a large prime $p$. The *discrete logarithm problem* asks to compute $x$ from $g$ and $g^x \,\mathsf{mod}\, p$; there is no known efficient algorithm to perform this task on regular computers. Two parties, Alice and Bob, can use this group to establish a shared secret key by sending messages over an insecure channel; the eavesdropper cannot obtain this secret key.

The protocol proceeds as follows. First, Alice and Bob agree on a group element $g$ which will serve as the base number. Alice chooses a random integer $a \in \{0,…,p-1\}$ and sends $g^a \,\mathsf{mod}\, p$ to Bob; Bob mirrors this action by choosing a random integer $b \in \{0,…,p-1\}$ and sends $g^b \,\mathsf{mod}\,p$ to Alice. The group’s associative property guarantees that Alice and Bob can combine their own secret information with the message from the other to obtain a new group element $k=(g^a)^b=(g^b)^a \,\mathsf{mod}\,p$ which they use as the secret key for subsequent communication. On the other hand, the passive eavesdropper sees $g, g^a \,\mathsf{mod}\, p, g^b \,\mathsf{mod}\,p$ but this is not enough information to find $k$ efficiently. While there is no proof that obtaining $k$ from these values requires as much work as solving the discrete logarithm problem, in practice only groups are used in which this is widely believed to be the case.

Unfortunately, the Diffie-Hellman key agreement protocol only protects users from passive adversaries,* i.e.*, adversaries who are content to only eavesdrop. By contrast, active adversaries who are capable of blocking messages and injecting them, are also capable of undermining the security of the protocol using a so-called *man-in-the-middle attack*. In short, the man-in-the-middle masquerades as Bob to establish a key with Alice; and masquerades as Alice to establish a key with Bob. Alice and Bob both think they have established a secret key with each other, but in reality they have both only established a secret key with the adversary. Any subsequent messages that are encrypted under one key are first decrypted by the adversary and then encrypted again under the other key before they are passed on.

The way security proofs get around this problem is by assuming it away. In particular, the authenticated links model explicitly states that the adversary is only passively observing and not blocking or modifying traffic^{4}. The way this is solved in practice is by pre-distributing keys used for signature verification. Alice and Bob sign all their outgoing messages; they verify the authenticity of the incoming messages by verifying the attached signatures. The adversary cannot forge signatures on new messages because he is ignorant of the secret key. However, this solution requires that keys be distributed before communication begins, defeating the purpose of requiring no pre-shared information in the first place.

It is possible to separate signature verification from signature generation by requiring different keys for these operations. The *private key*, kept secret, is necessary for generating signatures, whereas the *public key*, broadcast to the world, is necessary for verifying them. The public keys themselves may be signed by yet other private keys and through a sequence of hierarchical hops reach a central authority. This strategy, called public key infrastructure, at least solves the scaling problem by requiring only a constant number of authority keys to be shared (as opposed to $O(n^2)$), although this solution comes at the cost of introducing a single point of failure.

### 2.3. The Quantum Revolution

Of course, any proposition about security that simultaneously reduces to computational hardness assumptions and claims to offer security in the real world, implicitly relies on a notion of computations that are physically realizable and at an acceptable cost. The first headway into answering this question was made by computer science pioneers Alonzo Church’s and Alan Turing’s formalizations of *computation*^{5} ^{6} and the subsequent formulation of the *Church-Turing hypothesis*. Properly interpreted as a qualitative proposed law of nature, this hypothesis states that any process occurring in nature can be simulated by a Turing machine, a mathematical abstraction of what it means to compute due to Turing and inspired by Church’s lambda calculus. No physical experiments conducted thus far have indicated that the hypothesis as stated is false. Consequently, this has led computer scientists and physicists to pose a stronger version of this hypothesis, stating that all natural processes can be simulated *efficiently* ^{7} By contrast, this strong Church-Turing hypothesis seems to be — most emphatically — probably false.

In the 1990’s a series of results by pioneers in the field of quantum computation give strong evidence that quantum computers — computers that exploit the peculiar properties of quantum particles — are capable of solving some computational problems asymptotically more efficiently than classical computers. This string of results on quantum algorithms culminated in Shor’s celebrated work, which shows that the integer factorization and discrete logarithm problems, while difficult on classical computers, are in fact *easy* on quantum computers^{8}. Not only is Shor’s algorithm strong evidence that the strong Church-Turing hypothesis is false; it is also practically relevant as it breaks the security of the Diffie-Hellman protocol described above. The situation is even worse: switching to RSA or to elliptic curve cryptography (ECC) will not help as RSA crucially relies on the difficulty of the integer factorization problem and elliptic curve cryptography on the general formulation of the discrete logarithm problem — both of which are still susceptible to Shor’s attack. In fact, in the foreseeable future when large-scale quantum computers exist, most public-key cryptography widely deployed today will become insecure. More importantly, information encrypted today can be stored for the future at very low cost until a point at which quantum computers do exist; from this point of view, Diffie-Hellman, RSA and ECC are *already* insecure.

In response to this threat, researchers are designing and developing post-quantum cryptosystems — cryptographic algorithms that can be executed on traditional computers but promise to offer security even against quantum computers^{9}. The way these cryptosystems accomplish this feat is by reducing their security properties to other computational hardness assumptions, notably different from the integer factorization and discrete logarithm problems and hence very arguably immune to Shor’s attack. While the selection of post-quantum cryptosystems is large enough to sustain a scientific discipline in and of itself, they suffer from two significant disadvantages. One, they are inefficient in one way or another: either the public key, message size, or computational requirements are rather large, thus making their adoption an unattractive course of action. Two, they rely on relatively under-studied cryptographic hardness assumptions; anyone who uses a post-quantum cryptosystem over RSA or ECC is consequently making a larger gamble with respect to security. Nevertheless, in some cases such as hash-based signatures, the security can be reduced to assumptions which are already commonplace; cryptographic assumptions so minimalistic that *if* they turn out to be false, no computationally-secure cryptography whatsoever would be possible in the first place.

## 3. Quantumness

Quantum particles behave in undeniably peculiar ways. For instance, the complete description of a system of $n$ qubits — *i.e.*, $n$ independent binary properties of quantum particles — requires a vector of $2^n$ complex numbers with Euclidean length one; we refer to this vector as the system’s *state* and denote it using Dirac ket notation: $|a\rangle$. By contrast, the complete description of n classical binary variables requires $n$ bits. In this sense, a quantum system is much more akin to a *probability distribution* over strings of $n$ bits; a complete description of such a probability distribution requires $2^n$ real numbers between zero and one and whose *sum* is one (as opposed to its sum-of-squares).

This comparison of quantum states to probability distributions is no accident. Every quantum state associates a complex-valued *amplitude* to each observable event. By squaring the modulus of this number, one obtains the probability of measuring this outcome. However, upon employing measurement, the quantum state *collapses*; it assumes amplitude one in the component associated with the observed event and amplitude zero in all other components. While this sounds familiar to sampling classical probability distributions, the surprising fact is that quantum states *exist* — it is possible to define and perform experiments whose observations cannot be explained without recourse to this exponentially-large complex-valued state vector. By contrast, classical probability distributions are merely a method invented by humans to cope with hidden variables that do have a definite but unknown value.

In the terminology of quantum physicists and quantum computer scientists, an impure quantum state constitutes a *superposition* of values. Moreover, by manipulating the particles used to represent these qubits it is possible to generate *constructive or destructive interference* —* i.e.*, cause the amplitudes add or cancel out. Moreover, it is possible to generate quantum states where measuring one particle only partially collapses the state, because the other particles still remain unmeasured. This phenomenon is known as *entanglement* and has generated much controversy in the past.

### 3.1. The EPR Paradox and the Bell Inequality

Entanglement means that measuring one particle can influence the probability distribution associated with measuring another particle. In other words, one cannot reason meaningfully about counterfactual observations because the world in which the experiment is performed is fundamentally different from the world in which it isn’t. If this idea makes you uncomfortable, you are in good company. Albert Einstein famously rejected this notion, insisting that Nature had to follow the principle of *realism*: reality does not change depending on whether you observe it or not. Phrased differently, the moon does not pop into existence the second you decide to look at it.

Together with co-authors Podolsky and Rosen, Einstein formulated a seemingly devastating blow to the theory of quantum mechanics^{10}. The EPR thought experiment takes two entangled particles and separates them in space by a large enough distance. Then quantum mechanics predicts that the act of measuring one particle will exert an influence on the other particle, as borne out by a different probability distribution associated with measuring it, *faster than the speed of causality*. While Einstein understood that this principle could not be used to transmit information — thus not violating the second fundamental postulate of special (and general) relativity outright — this sense of influence-faster-than-causality certainly violates the *spirit* of relativity, and gave rise to the fantastically playable phrase “spooky action at a distance”.

The EPR paradox subtly sneaks in a second assumption about Nature, in addition to that of realism. In order for this “spooky action” to have to *travel* in the first place, the information describing one particle’s state, before and after measurement, has to be *localized* — independent of other events elsewhere. Phrased differently, the moon exists whether or not the sun is shining. This principle of *locality*, in conjunction with its brother realism, is known as *local-realism*; the contradiction predicted by quantum mechanics is known as *non-local-realism*.

Twenty-nine years after the publication of the EPR paradox, a physicist by the name of John Stewart Bell showed that local realism is not just a philosophical ponderance; it is a falsifiable hypothesis about nature^{11}. In other words, it is possible to perform experiments whose measurement results will indicate whether we live in a local-realist world or not. The intuition is as follows. If Einstein’s conception of quantum mechanics is correct, then there exists information hidden inside a quantum system that determines the measurement outcome. By preparing the system in a controlled manner, this hidden information is guaranteed to follow a probability distribution, and by performing the experiment many times the observed variables are guaranteed to follow that probability distribution, *even if these observables measure in large part the same property*.

By contrast, *quantum mechanics precludes any such probability distribution* where the same property can be independently sampled by two different observables. The first measurement will influence the state and the second measurement measures a different state, hence giving rise to other borne probabilities. Unfortunately, this state collapse makes the system look as though it was behaving conformant to local-realism the entire time and any measurement following the collapse cannot decisively indicate non-local-realism. Bell’s crucial insight was that the world-distinguishing information is not located in individual measurement variables, but rather their *correlation*, or more generally, their mutual relations.

Consider this excellent example due to Sakurai^{12}, of a quantum system consisting of two perfectly entangled photons having an unknown but equal polarization. The photons are sent to Alice and Bob, at opposite ends of the universe. Their polarizations are measured along one of three axes, $x$, $y$ and $z$, each $120^\circ$ apart and in the plane but such Alice’s set of axes coincides with Bob’s. Obviously, if they choose to measure along the same axis, they are guaranteed to obtain the same result, 1 (the photon detector fires) or 0 (no activity is registered).

If it is true that the photons’ polarization is completely fixed and independent of the measurement at either end of the universe, then whatever preparation device that produces the photons follows a probability distribution $p(x,y,z)$ describing the probability of observing the photon at any of the three axes. Being a probability distribution, the sum over all events of that event’s probability equals one:

$$ \sum_{x \in \{0,1\}} \sum_{y \in \{0,1\}} \sum_{z \in \{0,1\}} p(x,y,z) = 1 \enspace .$$

Moreover, Alice and Bob’s measurements don’t *influence* the state of the quantum system; they merely *obtain information about it*. Consequently, their joint statistics may be used to qualify this probability distribution, for example in the following way.

$$ {\rm Pr}[x=y] = p(0,0,0) + p(0,0,1) + p(1,1,0) + p(1,1,1) $$

$$ {\rm Pr}[x=z] = p(0,0,0) + p(0,1,0) + p(1,0,1) + p(1,1,1) $$

$$ {\rm Pr}[y=z] = p(0,0,0) + p(1,0,0) + p(0,1,1) + p(1,1,1) $$

The sum of the probabilities of these non-mutually-exclusive events must thus satisfy $$ {\rm Pr}[x=y] + {\rm Pr}[x=z] + {\rm Pr}[y=z] = 1 + 2p(0,0,0) + 2p(1,1,1) \geq 1 \enspace . $$

In the intuitive words of Preskill, “whenever three coins are on the table, and each one is either an H or a T, then at least two of the three have to be the same”^{13}. However, Alice and Bob experimentally confirm that ${\rm Pr}[x=y] = {\rm Pr}[x=z] = {\rm Pr}[y=z] = (\mathsf{cos} \, 120^\circ)^2 = 0.25$ and hence

$$ {\rm Pr}[x=y] + {\rm Pr}[x=z] + {\rm Pr}[y=z] < 1 \enspace , $$

thus violating the Bell inequality.

In practice, experiments prefer to use the less accessible CHSH inequality^{14}. However, the same principle is used: EPR local-realism and quantum entanglement predict different measurement statistics. Since Bell’s original 1964 paper, there have been many variants of his equation and even more experimental validations — that is to say: *violations* of the inequality — deciding in favor of the non-local-realism of quantum mechanics beyond reasonable doubt.

### 3.2. Quantum Key Distribution

So if we can’t observe particles without disturbing them, how about using this to our advantage? Specifically: certain actions are quantum mechanically impossible; can we put these impossible tasks on the only path between the attacker and our sensitive information, in the same sense that computationally infeasible tasks block this path now? The answer is yes, it is possible to use quantum mechanics to protect information. It turns out entanglement isn’t even required.

In 1984, two researchers by the name of Bennet and Brassard proposed what is now known as the BB84 protocol: a mechanism for key distribution relying on the polarization of photons^{15}. Alice chooses a string of bits from which to derive the key. She then sends a string of individual photons where each bit of her secret bit string is encoded in the photons’ polarization, with respect to rectilinear ($+$) or diagonal ($\times$) bases as determined by random coin tosses. Bob measures the incoming photons with respect to rectilinear or diagonal bases as determined by his own random coin tosses. Whenever Bob chooses the same basis as Alice, there is a small probability of the photon not being measured at all, but if it *is* measured it is guaranteed to produce the same bit Alice sent. On the other hand, if the photon is measured in opposite bases, Bob’s measured bit will be uniformly random.

After measuring the photons with respect to random bases, Bob announces these chosen bases to Alice over a public classical channel, along with an indication which bits were actually received. Alice responds by identifying which choices of basis were correct, thus indicating which of Bob’s measured bits are identical to Alice’s. A few of these correct bits are also exchanged just to guarantee with high probability that there is no eavesdropper on the quantum channel.

Security is guaranteed by the following rationale. Suppose an eavesdropper named Eve is observing the photons on the quantum channel with respect to her randomly chosen bases. And suppose she sends out new photons towards Bob with the polarization that she measures. Then whenever Alice and Bob used the same basis, there is a $1/2$ chance that Eve’s basis is opposite. If this is the case, Bob’s measurement will be the same as Alice’s with probability $1/2$. So by exchanging $k$ agreed upon bits in the last step, Alice and Bob reduce the probability of a successful eavesdropper to $(3/4)^k$. Table 1 is drawn straight from BB84’s paper and summarizes the steps involved.

In 1991, a couple of years after the BB84 protocol, Artur Ekert proposed another construction for key distribution based on quantum entanglement^{16}. While Ekert’s protocol is rather more difficult to realize physically, the security guarantees it offers are vastly different from the ones offered by the BB84 protocol, so it is certainly worthwhile to take both protocols into account.

In Ekert’s protocol, the quantum channel consists of a source of pairs of entangled particles sent to opposite ends of the channel. The emitted particles are in the singlet state: $|S\rangle = 1/\sqrt{2} |\uparrow\downarrow\rangle – 1/\sqrt{2} |\downarrow\uparrow\rangle$, guaranteeing that whatever spin is measured at one end, the other end must measure the opposite spin provided the same basis is used.

Both Alice and Bob measure the particles headed their way in random bases. After this measurement, they communicate over a public classical channel to determine in which cases both Alice and Bob registered a measurement in the same basis. At this point, they release the measurements obtained when their measurement bases were unequal; this allows them to determine statistical variables for which the CHSH inequality (an inequality of the Bell family) should be violated if the particles are properly entangled.

An eavesdropper who measures the particles’ spin in random bases just before the particle reaches Bob and then sends out a new particle with the measured spin, cannot violate the Bell inequality. Consequently, this eavesdropper will be detected with very high probability.

The reliance on Bell’s inequality removes the requirement to trust the device that produces the quantum particles to be honest: malicious devices cannot pass the Bell test. Consequently, this type of quantum key distribution is called *device-independent*.

### 3.3. Authentication

At this point we have glossed over a key assumption on the adversary: he can look, but not touch. In other words, the adversary is assumed to be somewhat passive. While he is allowed to read messages, his cannot block, modify, or create them.

Of course, in a realistic scenario, the adversary is capable of active intervention. In this case, he can perform the man-in-the-middle attack: impersonate Bob in the eyes of Alice, and impersonate Alice in the eyes of Bob. Both Alice and Bob think they’re establishing a secure connection to the other, but in reality they are only establishing a secure connection to Eve, the (wo)man-in-the-middle. Even the Bell test cannot help because at most it will indicate that Alice’s connection to Eve and Bob’s connection to Eve were both generated using properly entangled particles.

In practice, the only way to block a man-in-the-middle attack is to authenticate the messages that are sent over the public classical channel. This variant of digital signatures allows the receiver, who knows the secret key, to verify beyond doubt that the messages were sent by the other owner of the secret secret key and that the messages were not modified in transit. Under the assumption that the adversary does not know this secret key, an authentication scheme thus offers two properties: *entity authentication*, the sender is who they say they are; and *data integrity authentication*, the data has not been tampered with. Unfortunately, any secure authentication scheme requires a pre-shared secret key, thus cancelling in large part the very point of quantum key distribution in the first place. Moreover, the authentication schemes used in practice, for example in internet protocols such as TLS, rely once again on hard problems to guarantee security. But the whole *point* of QKD was to not have to rely on computational problems at all!

Fortunately, though, the outlook for quantum key distribution is not quite as bleak. There exist unconditionally secure authentication schemes whose security does not rely on the intractability of computational problems, such as the Wegman-Carter scheme^{17}. The reason they are not used in TLS, for example, is because they consume secret key material as they are used, in the same sense that the one-time pad consumes secret key material as it is used. Use the same secret bits twice and the security of the scheme is broken. However, in the case of authentication and in contrast to encryption, *the length of the secret key can be shorter than the processed data*. This means that it is possible to use a short shared secret key to authenticate quantum key distribution which then generates a longer shared secret key. From this point of view, the term *quantum key distribution* is a misnomer; it should be* quantum key-length extension*.

## 4. Criticisms of QKD

Quantum key distribution can obviously offer security properties that classical cryptography is incapable of offering. So why is the reception of quantum cryptography lukewarm at best among traditional cryptographers? We explore several possible reasons.

### 4.1. Implementation Issues

Quantum key distribution is not just an idea; it is supposed to be implemented in practice using physical matter. Consequently, it might be very difficult or even impossible to engineer this physical matter into behaving according to the protocol. Conversely, the adversary against which the protocol is supposed to protect might have access to the physical device through side-channels that the security model does not account for.

The possibility of side-channel attacks was observed in the very first practical demonstration of the BB84 protocol, which the authors describe as being “unconditionally secure against any eavesdropper who happened to be deaf! :-)”^{18}. Indeed, the listening adversary could hear the polarization filters rotating and would be able to infer the chosen bases from the sound. Another example considers the difficulty of generating a laser pulse containing exactly one photon. But if the pulse contains multiple photons, the adversary can get away with measuring the polarity of one of the superfluous ones, leaving the one photon that travels all the way to Bob intact. A final example considers an implementation attempting to get around the rotating filters problem by generating photons of different polarization by different sources. Unfortunately, this implementation does not take into account that different sources have a unique spectral signature, which grants the adversary access to another side channel leaking information on the chosen bases. An excellent survey of practical issues is due to Scarani and Kurtsiefer^{19}.

It is clear that requiring specialized hardware for instantiating quantum key distribution protocols generates its own set of specific problems, each one requiring significant engineering considerations to circumvent. By contrast, traditional cryptography is by design implementable on standard computer hardware. The all-too-common pitch that quantum key distribution offers “security guaranteed by the laws of physics” is clearly false if the adversary has side-channel access to the QKD devices. Likewise, the security of traditional cryptographic algorithms breaks down whenever the attacker has side-channel access to the computing device — but in this case at least the notion of “side-channel access” is well-defined and empirically bounded: power consumption, timing information, acoustic and electromagnetic wave emanation, voltage of probed wires … and that’s about it. In order to instantiate secure quantum key distribution, one has to take into account many more, vastly different, and unpredictable side-channel attacks.

### 4.2. Quantum Eavesdropper

It is tempting to compare the adversary in the quantum key distribution protocols with the passive adversary in classical key distribution protocols. However, this comparison does not quite hold. The passive adversary in classical cryptographic protocols is allowed to observe data in passing; he is merely not allowed to modify. By contrast, in the quantum case *observing is modifying*; by making observation and modification inseparable, quantum key distribution is introducing a model that is simultaneously weaker and stronger than the passive model of classical protocols, albeit for good reason (*i.e.*, the laws of physics).

Except, observation is equated to *measurement*, which necessarily collapses the state. However, especially in the realm of quantum phenomena there exist manipulations which are more subtle than measurement but nevertheless akin to observation. In particular, consider the quantum computer^{20} that, having access to its own, secret qubit prepared in the state $|s\rangle = |0\rangle$ manipulates the photon heading to Bob by performing the computation $|b,s\rangle \mapsto |b,s\oplus{}b\rangle$. This quantum operation is unitary and does not violate the no-cloning theorem because after the computation is done the eavesdropper’s qubit is not independent of the passing photon; by contrast, it is perfectly entangled with it. Once Bob makes known the bases in which he measured, then the quantum computer uses the same basis to measure its secret qubit, thus obtaining the exact same bit Bob obtained.

Ekert’s protocol implicitly deals with this attack as it is a subcase of the explicitly considered situation in which three entangled particles rather than two are emitted by the source. Bell’s inequality comes to the rescue: the statistical test cannot be satisfied if three particles are entangled rather than two. In other words, not only can the Bell test distinguish between *some entanglement* and *no entanglement*; it can also distinguish between some entanglement and *even more entanglement*. While the adversary can get away with eavesdropping on a few particles in this manner, the probability of this misbehavior being noticed rises with the number of ‘observed’ particles. This is precisely the reason why quantum key distribution protocols are often enhanced with another “privacy amplification” step, in which the string of shared secret bits of which the adversary might know some individual bits is transformed into a bitstring of which the adversary does not know any individual bits but instead some minimal amount of general information.

However, the BB84 protocol is vulnerable to this attack precisely because there is no Bell test. More importantly, there cannot be a Bell test in the BB84 protocol because there is no entanglement; there is no “third coin” whose non-existence must be indicated by the statistical information. Phrased differently, there is no counterfactual event whose occurrence would have caused different observations. The measurement statistics in this protocol conform to local realism.

However, it is not clear that even if quantum computers can be built that they should also be capable of intercepting photons, computing on the qubit represented by its polarization, and send out a photon with a polarization matching a given qubit. Nevertheless, this possibility is implied by the conjectured possibility of quantum routers and relays that enable quantum key distribution across many hops and storing them in quantum memory for a little while at each point. Moreover, the alarming fact is that this attack is *not ruled out by quantum physics*. Quantum key distribution is, after all, one of the proposed solutions to the threat of quantum computers and their break of widely deployed public key infrastructure. It would seem that the *claims of security against quantum computers are restricted only to device-independent QKD*.

### 4.3. Nothing Wrong with Computational Security

Perhaps most fundamentally, the chief selling point of quantum key distribution attests to a deep-seated distrust in computational hardness assumptions — a distrust that cryptographers, in general, are unlikely to share. It is fairly reasonable to assume that the discrete logarithm and integer factorization problems may one day be easy; Shor’s algorithm predicts as much. However, it is another thing altogether to conjecture that AES will one day be easy to break or that collisions to SHA2-256 will one day be found. Cryptographers will not take the people who make these claims seriously unless they are preceded by breakthrough results. However, anyone who prefers the clearly-not-unconditional security of quantum key distribution over the merely-computational security offered by AES or SHA2 is implicitly making exactly these claims. QKD proponents who don’t want to make these claims even implicitly must consequently argue for quantum key-length extension *until* the shared key reaches 256 bits, after which point computational cryptography over the classical channel is perfectly adequate. But in this case the usefulness of QKD is restricted to the world in which it *is* feasible to establish shared and perfectly secret strings of less than 256 bits but for some reason it is *not* feasible to do the same for a string of 256 bits or longer.

Nevertheless, there is eminent justification for skepticism regarding the conjectured computational difficulty of the problems underlying post-quantum cryptosystems for public key encryption and Diffie-Hellman key agreement. These problems are after all new and under-studied, and don’t secure large monetary rewards for whoever solves them. However, perhaps one day, after resisting many years of attempted cryptanalysis and being duly recognized by national and international standardization bodies, a subset of these problems can achieve the same status as AES and SHA2 — and inspire the same confidence. Indeed, not only is this likely to happen, it is the desirable outcome. At that point, the same criticism applies to quantum key distribution: what *advantage* does it offer over a classical cryptosystem relying on hard problems that are trusted anyway?

## 5. Conclusion

Quantum physics speaks to the imagination because it is so viciously counter-intuitive. It rejects many rules of thumb that make perfect sense in the macro world, such as counterfactual events having well-determined outcomes; or such as information having to travel in order for one event to exert influence on another. It is all too easy to ascribe some mystical properties to the workings of quantum mechanics because to the uninitiated it truly is indistinguishable from magic.

To a lesser degree, the field of cryptography is also familiar with this mysticism. Cryptography deals with problems that are *difficult by design* — they must be, because otherwise they would be ineffective. In the eyes of popular science it takes many years of studying complex mathematical formulas before being able to grasp this spirit of difficulty.

It is perfectly understandable then, that the combination *quantum + cryptography* speaks to the imagination beyond what either branch is capable of by itself. It is a self-selling sales pitch that lodges itself firmly in the pitchee’s mind in that special place allocated for whatever thought is capable of producing a sense of wonderment about the world. No doubt this enticing nature has helped secure funding on the topic for many years to come.

Except, that’s not how science works. It is wrong and unscientific to worship the priests for informing us about the truth. It is a disservice to all three fields — quantum information theory, cryptography, and the overlap — to ascribe importance to the technology of quantum key distribution due to its perceived mystical properties. The researchers in these fields hold themselves to the highest (and most definitely anti-mystical) standards of verifiability and reproducibility and to pretend otherwise is to level an insult. Either quantum key distribution produces material benefits or it does not, and the quality of this invention should be assessed accordingly.

Without a doubt, we have seen that quantum key distribution is capable of offering security properties that no classical cryptosystem can offer. However, the setting in which these special security properties become relevant is rather artificial and constrained. By contrast, the promulgation of quantum key distribution technologies seems to focus on the BB84 variant, which introduces a whole host of new security hazards, to the point where the claimed security “benefit” is very dubious indeed.

In the end, it will not be the theoretical computer scientist who decides which technologies are useful and which are not — nor should it be. It will instead be the engineer who plots graphically the implementation cost on one axis against the security requirements on the other, and comes to a conclusion based on considering the relative positions of all competing options. Quantum key distribution may well occupy a location on this graph that justifies its existence. But until quantum key distribution proponents take the existence of this cost-benefit-graph into account and pinpoint QKD’s location on it, QKD is bound to remain only a theoretical exercise with the occasional proof of concept supporting its physical possibility.