Introduction to Quantum Cryptography

Quantum cryptography is a new technique of securing computer network communication channel. Existing standard crypto systems are using advanced algorithms to create key pairs which are extremely hard to inverse engineer. Quantum cryptography avoids any mathematical algorithm and uses principles of quantum physics.

Quantum crypto implements a new technique of generating and exchanging crypto keys which makes it impossible for third party entities to get those keys by snooping or to create man in the middle by snooping and sending copies of original key. Keys generated in this way will automatically destroy themselves if read by third-party interferer.

When generated between two sides, using quantum key distribution, secret keys will be used with standard and well known symmetric encryption. The key generation process is the only part which uses quantum principles to work, from there, using this “hyper-secure key” already existing symmetric encryption will be used to encrypt and decrypt data, which will be sent over standard, currently available, optic data networks.

Quantum cryptography usually uses photons to generate the key by sending pieces of data between two sides. It uses of course standard optic communication channel used in computer networks today.

So we see that the mechanism is not only transmitting the key using photon polarization, but the process of sending polarized photons is actually the process which generates the key.

The whole process of generating key, using quantum mechanics characteristics on a photon, is making the quantum cryptography work. The communication channel is actually not really quantum, it’s basically normal optical line. Photons as information qubits are only quantum pieces of this quantum crypto puzzle.

Quantum Key Distribution

In computer systems today, almost everything we have, and use, is based on Electrodynamics. Deeper understanding of the world of small, under-atom particles, is the next step for physicist of our time to explore. Huge steps are already taken and we today know so much about quantum mechanics. First ideas, emerged in the last 50 years, about exploiting quantum mechanics are starting to give results.

First Quantum Cryptology systems are already available today.

Quantum mechanics explores quantum size particles known as quanta. It further explores characteristics of quantum particles and their entanglement as special way of interaction at a distance. Technologies like cryptography are one of the first examples of quantum mechanics effects in use. In order to perform cryptographic tasks and enhance cryptographic systems, quantum effects are leveraged and we already have first commercial Quantum key distribution (QKD) products available.

Quantum key distribution is the most known and most studies example but is surely not the only one. Many more applications are emerging today and some of the examples include random number generators, delegated quantum computation and secure multiparty computation systems.

Quantum Cryptography takes the advantage from fairly negative set of rules from quantum physics. Quantum physics states that we are unable to measure the system without perturbing it. It is further impossible to accurately measure the position and the momentum of a particle simultaneously. It is also not possible to measure photon polarization in both vertical-horizontal and diagonal basis.

It is possible to measure only one of the basis of the photon and after the measurement we used that photon and cannot make another measurement. Quantum state in a system is impossible to duplicate.
It is hard to imagine that all this negative rules could be used to make something so simply functional like Quantum Key Distribution.

Quantum Cryptography is based on standard cryptography principles which are enhanced by usage of Quantum key distribution system. Quantum key distribution technique enables Quantum Cryptography by employing basic quantum mechanics principles listed above.

Quantum Cryptography today is trying to examine and possibly take advantage of other limitations from quantum mechanics. This includes the impossibility of quantum bit commitment, the issue with quantum rewinding and the definition of quantum security models for classical primitives.

Modern cryptology needs to be enhanced in a way that prevents future quantum computers or similar future calculation systems to break current crypto systems. This issue was shortly mentioned in the introductory article about cryptography.

As we will have the chance to see here, current crypto system are mostly based on prime integer factorization or similar mathematical problem with no known efficient solution. No known efficient solution is good enough for today’s systems as todays computer systems need huge number of calculation steps (CPU cycles) to calculate one prime number out of factorized two big primes.

Creation of quantum computers that can get to the result of that calculation in dramatically reduced number of steps will give those computers a chance to break current popular AES or similar crypto algorithms in the time shorter that the key life time.

Today, Cryptology’s most advanced part is surely Quantum Key Distribution. Quantum Key Distribution is making the enhancement to crypto key distribution system in a way that solves the issue regarding quantum computer brute force attack on the key.

Quantum Cryptology principle is based on quantum mechanics rule which defines that is not possible to take a measurement of a quantum system state without changing it.


Flowing text is part of a ongoing academic project I’m involved with for the last few years at University of Rijeka – Department of Informatics. It’s a part of academic paper written as a short overview of Quantum Key Distribution technique – QKD. Topics in this paper are edited and transformed in few articles here describing QKD:



  1. Johannes Weber December 6, 2017
    • Valter Popeskic December 7, 2017

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