Guest editorial for special issue on selected extended papers from QCrypt 2020

Cryptography is essential for security of communications. While the traditional public key cryptography still ensures confidentiality and authenticity of communicating parties for many applications, it is under threat from emerging quantum computing techniques. As an alternative to the traditional cryptography, in recent decades, quantum cryptography has been under active investigation and development. These efforts resulted in establishing a growing worldwide community working on these tasks, which is attracting researchers from other fields of theoretical and applied sciences.

This Special Issue of Selected Extended Papers from QCrypt 2020 is based on the research presented at QCrypt 2020 (10–14 August 2020), a conference for students and researchers who work on quantum cryptography, which welcomed research contributions on the possibilities and limitations of quantum methods for secure communications and computation. The conference enabled scientists, researchers and engineers to discuss and summarise the latest achievements in quantum cryptography and to publish their current theoretical and practical results, engineering innovations and other achievements, as well as to discuss some of the state-of-the-art approaches to the mentioned problems.

This Special Issue contains four papers presented at the conference covers areas relating to quantum and secure communications, such as the quantum key distribution (QKD) and generation, the secure random number generation and the fault-tolerant synchronization coding and decoding schemes.

In the article entitled ‘Using QKD in MACsec for secure Ethernet networks’, Joo Yeon Cho and Andrew Sergeev investigate a QKD-integrated Media Access Control security (MACsec) protocol for a quantum-secure Ethernet, assuming that a QKD infrastructure has been already deployed and is available for MACsec key rollover. The authors develop a new key exchange protocol based on the QKD that is applicable for such networks. Furthermore, an experiment is conducted that verifies that QKD can be integrated into MACsec without any performance degradation.

In the article entitled ‘Certification of the efficient random number generation technique based on single-photon detector arrays and time-to- digital converters’ by Andrea Stanco et al., a quantum random number generator (QRNG) capable of producing certified random numbers is analysed and tested. The combination of a complementary metal–oxide–semiconductor single-photon avalanche diode array, a high-resolution time-to-digital converter implemented on a field programmable gate array enables generation of true random bits with a high bitrate in a compact and easy-to-calibrate device. The QRNG proposed in this article uses environmental light as the photon source. According to the authors, the generated bitstring has passed all the National Institute of Standards and Technology suite tests showing feasibility of such high-performance QRNGs with simple setup and calibration and a compact form factor.

In the article entitled ‘Timing and synchronisation for high-loss free-space quantum communication with Hybrid de Bruijn Codes’ by Peide Zhang et al., a synchronisation method based on de Bruijn sequences suitable for timing and synchronisation over high-loss communication links is proposed for potential applications in the satellite-based global quantum secure communication networks, with a possible extension to non-quantum communications over terrestrial free space or fibre optic channels. In this study, the authors realise a representative synchronisation timing system and test it in laboratory conditions. They conclude that the proposed solution demonstrates high fault tolerance for the error-correction algorithm even under high channel loss and allows for the possibility of implementation on a real-time system-on-chip.

In the article entitled ‘Key Generation Schemes for Channel Authentication in QKD Protocol’, Mikhail Borodin, Andrey Zhilyaev and Alexey Urivskiy propose to use hybridisation of quantum keys and classical pre-shared keys to construct optimal key generation and distribution schemes for authenticating the classical channel in the QKD systems. They formulate a detailed adversary model that allows one to compare various key renewal schemes versus the probabilities of successful attacks. The authors conclude that the proposed hybrid key renewal scheme has superior security properties among the considered schemes and is recommended to be used in QKD systems.

The four papers published in this Special Issue deal with a few topics which are of elevated significance for the theory and application of quantum communications and quantum cryptography. Especially, we would like to highlight that, in all these works, both theoretical and practical aspects of the considered problems are given equal standing. Therefore, as the Guest Editors of this Special Issue, we believe that this selection of papers represents important trends in the QKD, QRNG, and signal synchronisation and coding techniques. We are looking forward to practical implementations of the concepts and solutions presented in these works and their applications in real telecommunication systems.