quantum key distribution (QKD)

What is quantum key distribution (QKD)?

Quantum key distribution (QKD) is a secure communication method for exchanging encryption keys only known between shared parties. It uses properties found in quantum physics to exchange cryptographic keys in such a way that is provable and guarantees security.

QKD enables two parties to produce and share a key that is used to encrypt and decrypt messages. Specifically, QKD is the method of distributing the key between parties.

Key distribution on a conventional scale relies on public key cyphers that use complicated mathematical calculations requiring a prohibitive amount of processing power to break. The viability of public key ciphers, however, faces several issues, such as the constant implementation of new strategies used to attack these systems, weak random number generators and general advances in computing power. In addition, quantum computing will render most of today's public key encryption strategies unsafe.

QKD is different from conventional key distribution because it uses a quantum system that relies on basic and fundamental laws of nature to protect the data, rather than relying on mathematics. For example, the no-cloning theorem states that it is impossible to create identical copies of an unknown quantum state, which prevents attackers from simply copying the data in the same manner that they can copy network traffic today. Additionally, if an attacker disturbs or looks at the system, the system will change in such a way that the intended parties involved will know. This is a process that is not vulnerable to increased processing power.

How does QKD work?

QKD works by transmitting many light particles, or photons, over fiber optic cables between parties. Each photon has a random quantum state, and collectively, the photons sent make up a stream of ones and zeros. This stream of ones and zeros are called qubits -- the equivalent of bits in a binary system. When a photon reaches its receiving end, it travels through a beam splitter, which forces the photon to randomly take one path or another into a photon collector. The receiver then responds to the original sender with data regarding the sequence of the photons sent, and the sender then compares that with the emitter, which would have sent each photon.

Photons in the wrong beam collector are discarded; what's left is a specific sequence of bits. This bit sequence can then be used as a key to encrypt data. Any errors and data leakage are removed during a phase of error correction and other post-processing steps. Delayed privacy amplification is another post-processing step that removes any information an eavesdropper might have gained about the final secret key.

Types of QKD

There are many different types of QKD, but two main categories are prepare-and-measure protocols and entanglement-based protocols.

• Prepare-and-measure protocols focus on measuring unknown quantum states. They can be used to detect eavesdropping, as well as how much data was potentially intercepted.
• Entanglement-based protocols focus around quantum states in which two objects are linked together, forming a combined quantum state. The concept of entanglement means that measurement of one object thereby affects the other. If an eavesdropper accesses a previously trusted node and changes something, the other involved parties will know.

By implementing quantum entanglement or quantum superpositions, just the process of trying to observe the photons changes the system, making an intrusion detectable.

Other more specific types of QKD include discrete variable QKD (DV-QKD) and continuous variable QKD (CV-QKD).

• DV-QKD encodes quantum information in variables using a photon detector to measure quantum states. An example of a DV-QKD protocol is the BB84 protocol.
• CV-QKD encodes quantum information on the amplitude and phase quadrants of a laser, sending the light to a receiver. The Silberhorn protocol uses this method.

Some examples of QKD protocols are the following:

• BB84
• Silberhorn
• Decoy state
• KMB09
• E91

Challenges of QKD

QKD has the following three primary challenges:

1. The integration of QKD systems into current infrastructure;
2. The distance in which photons can travel; and
3. The use of QKD in the first place.

It is difficult to implement an ideal infrastructure for QKD. It is perfectly secure in theory, but in practice, imperfections in tools such as single photon detectors create security vulnerabilities. It is important to keep security analysis in mind.

Modern fiber optic cables are typically limited in how far they can carry a photon. The range is often upward of 100 km. Some groups and organizations have managed to increase this range for the implementation of QKD. The University of Geneva and Corning Inc. worked together, for example, to construct a system capable of carrying a photon 307 km under ideal conditions. Quantum Xchange launched Phio, a QKD network in the U.S. capable of delivering quantum keys, an apparent unlimited distance using a patent-pending, out-of-band delivery system called Phio Trusted Xchange.

Another challenge of QKD is it relies on having a classically authenticated channel of communications established. This means that one of the participating users already exchanged a symmetric key in the first place, creating a sufficient level of security. A system can already be made sufficiently secure without QKD through using another advanced encryption standard. As the use of quantum computers becomes more frequent, however, the possibility that an attacker could use quantum computing to crack into current encryption methods rises -- making QKD more relevant.

Implementation examples

QKD has been worked on and implemented for a relatively long period of time. Some examples are the following:

• In 2007, Los Alamos National Laboratory and the National Institute of Standards and Technology (NIST) used the BB84 protocol over a 148.7 km optical fiber.
• In 2005, University of Geneva and Corning Inc. used a fiber optic wire of 307 km.
• University of Cambridge and Toshiba collaborated in making a high-bit-rate QKD system using the BB84 protocol.
• Peking University and Beijing University of Posts and Telecommunications collaborated on creating a QKD system.
• In 2017, University of Science and Technology of China measured entangled photons over 1,203 km.
• China and the Institute for Quantum Optics and Quantum Information (IQOQI) in Vienna collaborated to create a quantum channel between the two locations.
• Launched in 2004 and operated until 2007, the Defense Advanced Research Projects Agency (DARPA) Quantum Network was a 10-node QKD network developed by a collaboration of multiple entities, such as Boston University, Harvard University and IBM Research.
• In 2018, Quantum Xchange launched the first quantum network in the U.S., offering 1,000 km of fiber optic cable and 19 colocation centers along the Boston-to-Washington, D.C., corridor and metro hubs.
• To make QKD systems easier to deploy, the European Telecommunications Standards Institute (ETSI) released a standard in February 2019 that provides a standard interface for devices and applications to receive quantum keys.

Commercial companies, such as ID Quantique, Toshiba, QuintessenceLabs and MagiQ Technologies Inc., have also started offering commercial QKD systems. In addition, Tokyo is testing its own QKD network.

QKD attack methods

Even though QKD is secure in theory, imperfect implementations of QKD have the potential to compromise security. Techniques for breaching QKD systems have been discovered in real-life applications. For example, even though the BB84 protocol should be secure, there is currently no way to perfectly implement it.

The phase remapping attack was devised to create a backdoor for eavesdroppers. The attack takes advantage of the fact that one party member must allow signals to enter and exit their device. This process takes advantage of methods used widely in many commercial QKD systems.

Another attack method is the photon number splitting attack. In an ideal setting, one user should be able to send one photon at a time to the other user. Most of the time, however, additional similar photons are sent. These photons could be intercepted without either party knowing. The photon number splitting attack takes advantage of this.

To combat this type of attack, an improvement to the BB84 protocol was implemented -- called decoy state QKD -- which uses a set of decoy signals mixed in with the intended BB84 signal while enabling both parties to detect if an eavesdropper is listening.

History of QKD

QKD got its start with the first proposal of quantum cryptography in the 1970s when Stephen Wiesner at Columbia University came up with the idea of quantum conjugate coding. Wiesner's paper was published in 1983. Charles H. Bennett later introduced a concept of secure communication, basing his ideas on Wiesner's work. Bennett also came up with BB84 -- the first quantum cryptography protocol -- which worked using nonorthogonal states. In 1990, Artur Ekert discovered another method to QKD, basing his idea around quantum entanglement.

Future of QKD

The Quantum-Safe Security Working Group (QSSWG) was formed by the Cloud Security Alliance (CSA) to promote the adoption of new technologies that help quantum computing be adopted at a steady pace. New technology is being worked on to improve high data rates and increase the overall effective distance of QKD. QKD is beginning to be used more widely in a commercial setting, with new networks and companies offering commercial QKD systems.