Revisiting security assumptions in satellite-based quantum communications

Researchers from the UK, Germany, Switzerland and Singapore have analysed the security of satellite-based quantum communications considering the presence of 'bypass channels'.

A recent paper published in PRX Quantum gives a boost to quantum secure communications via satellite. The international team including CQT’s Alexander Ling focused on the limitations that potential eavesdroppers may face in practice in collecting and transmitting quantum signals in satellite-to-ground links. The team conclude that satellite-based quantum systems could achieve higher secure key rates than was previously expected.

The work was led by researchers from the University of Leeds in the UK and also involved other researchers from the UK, Germany and Switzerland. Alexander, the only member of the team in Singapore, is a Principal Investigator at CQT and Associate Professor in the Department of Physics at NUS.

Quantum key distribution (QKD) is a mature quantum technology, which protects communications and other transactions through the quantum-secure distribution of cryptographic keys used to encrypt and decrypt data. QKD by satellite addresses the distance limitations inherent to this technology over terrestrial optical fibre, which result from signal loss and are a barrier to quantum-secure communications on continental, intercontinental and global scales.

Security assumptions

Conventionally, quantum communications security protocols characterize signal transmissions between two communicating parties (described as “Alice” and “Bob”) while considering the potential disruptive effect of an unwanted eavesdropper (“Eve”). The standard approach is to make minimal assumptions about the capabilities of this eavesdropper. In QKD jargon, Eve is considered to be in full control of the channel that connects Alice and Bob.

In the real world, an eavesdropper could hardly be this effective at intercepting a satellite link. Alexander first spoke to his UK coauthors about Eve’s challenges in space when he was planning for the 2019 launch of SpooQy-1 – a nanosatellite his group built to test technology for quantum communication. “I was looking at how a satellite orbits the ground station and how the signal comes to ground, and I realised that an eavesdropper cannot simply do whatever they wish. They are constrained by satellite orbital physics and diffraction of beams,” he said.

In the paper, the team detail how orbiting objects could be spotted by LIDAR surveillance if they got too big, and how the area around the telescope on the ground could be searched and protected.

Bypass channels

Lead researcher Mohsen Razavi from the University of Leeds explained “there has been little attention to classical characteristics, such as size, weight, or energy consumption, of Eve’s apparatus. Our work analyses the security of QKD systems without restricting Eve’s quantum processing power, but rather by assuming some size limitations on flying objects that Eve may need to employ in a line-of-sight link.” This implies limits on how much of the signal between the satellite and ground station Eve could receive. The result is that there exists so-termed “bypass transmission channels”, where at least part of the signal between Alice and Bob bypasses Eve.

The research presented in this paper considered such realistic restricted eavesdropping scenarios. The team’s analysis of achievable QKD key rates in the presence of bypass channels highlighted considerably improved performance for certain security protocols. Any performance improvements could contribute to commercial exploitation of satellite-based quantum communications technology and global-scale deployment of QKD systems.

The results open up new possibilities for future research directions in QKD security, especially when delivered via satellites, or when eavesdropping is naturally restricted. Mohsen commented: “This results in new security scenarios that we investigate in our work, and creates many others that are yet to be explored.”

“Usually when we try to make QKD practical we find eavesdropping mechanisms we need to guard against, this time the real world scenario helps us,” said Alexander.