Quantum Encryption in Simple Terms

The humanity has been long faced with the problem of exchanging messages confidentially. Military plans, commercial developments, state secrets, sensitive personal information have to be protected against evil intentions. But how can all of these be kept safe? Let us discuss some of the ways in more detail.

How Can You Communicate Messages Safely?

Ensuring the secure transmission of information remains one of the key challenges of the modern era. All existing methods can be divided into three fundamental categories, with each one having their unique characteristics and limitations.

The first category is connected with creating a completely secure communication channel. This strategy is among the most technologically complex, and its practical implementation requires ever-increasing resources due to the progressive development of information interception capabilities.

Secondly, you can conceal the very fact of transmission. This approach, known as steganography, or shorthand (Fig 1), has been used with varying degrees of success throughout history. As technology develops, its application is becoming technically simpler; however, the method has serious drawbacks: it is difficult to guarantee that the information will not be discovered by a third party, and the prolonged use of identical concealment algorithms increases the likelihood that a malicious actor will learn not only to detect but also to decipher messages without being identified.

Fig.1. A sample of shorthand

Thirdly, there is the open transmission of an encrypted message. This method involves sending data over an open communication channel after its cryptographic transformation. The method provides protection based on the computational complexity of reversing the transformation without knowledge of the corresponding cryptographic keys. The study and development of such methods constitute the field of cryptography.

What is Cryptography?

Our next step is to answer the question: what is cryptography? The term originates from the fusion of two Greek words: kryptós "secret" and grapho "to write". Literally translated from Greek, the word "cryptography" means "secret writing," but in reality, this term implies the secure transmission of information.

Cryptography has been known since the times of ancient civilizations; people used early versions of cryptography, applying mathematical methods to protect secret information from disclosure. These codes, known as ciphers, could be as simple as taking a message and shifting each letter of the alphabet by a certain number of positions so that A became D, B became E, etc. (Fig. 2).

Fig. 2. An example of an ancient cipher

The art of encryption evolved over millennia, and today, in creating complex algorithms and ciphers that protect confidential data transmitted over digital channels, cryptography relies on advanced digital devices, statistical mathematics, and other engineering sciences. Next, we will deal with a modern achievement in the field of information encoding – quantum cryptography.

Quantum Cryptography: a Groundbreaking Discovery to Protect Your Data

Quantum cryptography is a set of methods that uses the rules of quantum mechanics to securely encrypt, transmit and decode information. Quantum cryptography employs quantum devices, such as sensors capable of recording individual particles of light (photons), to protect data from an adversarial attack. Although technically challenging, quantum cryptography promises advantages over classical, nonquantum cryptographic systems.

The key difference between the quantum world and the classical one lies in the procedure of measurement. In classical physics, you can measure an object without changing it (for example, measuring the length of a table with a ruler). In quantum mechanics, measurement is an active interaction that, in general, irreversibly changes the original state of the system. It is this fact – that one cannot "eavesdrop" on a quantum system without leaving traces in it – that is the fundamental principle upon which the security of quantum cryptography is built.

Your Data are Secure with QKD

Various specialized cryptographic algorithms are used to solve different information security tasks. One of the fundamental schemes involves using a secret key for both encryption and decryption. In this approach, information is encoded using a specific code, after which both the transformed data and the code itself (the secret key) are sent to the recipient. The main vulnerability of this method is that a malicious actor who gains access to the transmission can easily recover the original content.

Quantum Key Distribution (QKD) addresses one of the fundamental challenges in encryption: the secure transmission of a secret key. It relies on the use of quantum particles, such as photons, to generate a key. The advantage of this method is that two parties can generate a shared, completely random key over a standard, unsecured channel. The security of the protocol is guaranteed by the laws of quantum mechanics: if a third party attempts to intercept the key, they will inevitably leave traces of their activity.

So, how does QKD Work?

Let's take a closer look at how QKD works. Photons can have different polarizations – think of polarization as different "directions" characterizing the particle. We consider two types of polarization: (+) and (×), each with two possible states. Due to the uncertainty principle, no single device can identify all four possible states; instead, two different devices are needed – one for (+) states and another for (×) states (Fig. 3).

Fig. 3. Demonstration of bases

Now, we’ll ask Alice and Bob to help us with further explanations. Meet Alice, she will be the sender of information while Bob is going to be the receiver. They exchange messages in the following way.

Alice generates a random sequence of bits (0s and 1s) and sends them one by one to Bob via a fibre-optic cable, encoding each bit as a photon with a specific polarization — either (+) or (×). As she sends each photon, Alice records which polarization she used and which bit she sent. Upon receiving each photon, Bob randomly chooses how to "read" it — using either the (+) or (×) measurement basis (Fig. 4).

Fig. 4.  Key creation cycle

After all photons have been sent and measured, Alice and Bob compare their records over an open channel. They discard the instances where their measurement bases did not match — this step is called key sifting. If Alice and Bob used the same measurement basis for a particular photon and there was no interference, their records for that bit should match.

Ideally, after key sifting, Alice and Bob should be left with identical sequences. However, if errors are present or someone is eavesdropping, their sequences will not match. To check for interference, they publicly disclose approximately half of their remaining sequences. According to the laws of statistics, the error rate in this randomly selected portion is a true estimate of the overall error rate in the entire sequence. If the error rate is too high (each quantum protocol has its own critical error parameter; for BB-84, for instance, it is 11%), they abort the transmission as this indicates the presence of an interceptor. Otherwise, they proceed to create a shared secret key.

What if Eve decided to interfere?

An eavesdropper, often called Eve, might attempt to intercept the communication between Bob and Alice (Fig.5). How can she possibly do it? And is there any way she can be stopped?

Fig. 5. Eve intercepting messages

In case of the intercept-and-resend attack, Eve intercepts the photons, measures them, and sends new photons to Bob based on her measurements. Like Bob, she randomly chooses a measurement basis for each photon. However, according to the laws of quantum mechanics, she cannot clone the photon's state. If she chooses the wrong measurement basis, she will alter the photon's state, introducing errors that Alice and Bob can detect during key sifting. What is more, even if Eve occasionally guesses Alice's basis correctly, this doesn't solve the problem because Alice and Bob generate long keys (thousands of bits). With a large number of sent photons, Eve cannot consistently guess correctly; for just 50 photons, the chance of guessing all bases correctly is approximately 0.00000000000008%. Her "luck" would be random, and on average, she would still introduce enough errors to be detected.

In case of the Photon Number Splitting (PNS) attack, Eve "listens" passively. If Alice sends a pulse containing multiple photons (as creating perfect single-photon sources is very challenging in practice), Eve can split the pulse, keeping one photon for herself and allowing the others to pass through to Bob. Later, after the key sifting step, Eve can measure her stored photons to learn parts of the key. To counter this attack, the Decoy-State Protocol is used: Alice sends pulses of varying intensities, making it difficult for Eve to distinguish between "signal" pulses and "decoy" pulses. Her actions again lead to detectable anomalies in the channel.

Are these attacks the only problems that have to be dealt with? Actually, not.

Problems of Quantum Cryptography

As with quantum computers, there are a number of problems that hinder the widespread adoption of quantum cryptographic technologies. First and foremost, significant obstacles remain the high costs of building and maintaining specialized quantum communication networks.

Another problem in implementing encryption that uses the physical properties of quantum particles is the range of information transmission. The greater the distance over which the secret key needs to be transmitted, the higher the risk that not all photons will reach the recipient in their original state.

Finaly, quantum cryptography does not create security out of nothing. It solves the problem of key distribution, but for its operation, it relies on a trusted channel or pre-distributed keys obtained by classical methods.

To quantum-encrypt, or not to quantum-encrypt, that is the question…

Why deploy quantum cryptography if it requires: a) the creation of specialized fiber-optic quantum channels; b) the use of single-photon sources; c) the use of complex detector equipment? To put it simply, it will cost the cryptographer a fortune. Besides, up until the 1980s, information security tasks had been successfully addressed by means of classical cryptography. Why have these methods been discarded?

The fundamental difference in the current situation is that traditional public-key cryptosystems are based on the computational complexity of certain mathematical problems, such as the factorization of large numbers (that is the process of decomposing a composite number into factors) or the discrete logarithm problem, which form the basis of the RSA algorithm used in global data transmission protocols like HTTPS and SSH.

A pivotal shift occurred in 1994 when Peter Shor developed quantum algorithms that enable the efficient solving of these problems on quantum computers. This poses a serious threat to the entire paradigm of asymmetric encryption. Consequently, there is a pressing need to develop and implement new cryptographic protocols that are resistant to attacks utilizing any computational power, including quantum.

Last but not least, it must be acknowledged that the absolute security of quantum cryptosystems is, in practice, counterbalanced by vulnerabilities in their physical implementation. Thus, cryptanalysts face the task of eliminating these shortcomings and creating truly reliable and practical quantum key distribution systems.

List of References

1. What is cryptography? [Electronic source]. URL: https://www.kaspersky.ru/resource-center/definitions/what-is-cryptography. Accessed: 01.11.2025.
2. Quantum Cryptography: Simple Protocols and a Little Cryptanalysis // habr.com. [Electronic resource]. URL: https://habr.com/ru/articles/530362/. Accessed: 01.11.2025.
3. Quantum Cryptography: a tutorial / D. A. Kronberg, Yu. I. Ozhigov, A. Yu. Chernyavsky; Lomonosov Moscow State University, Faculty of Computational Mathematics and Cybernetics. — Moscow: MAKS Press, 2011. 111 pp.