scope:

The Use Cases Document shall provide an overview of possible application scenarios in which Quantum Key Distribution (QKD) systems ([i.1]) can be used as building blocks for high security Information and communication technology (ICT) systems.

QKD systems are commercially available today - there are a handful of small enterprises producing and selling QKD systems. Even more QKD systems are being developed in research laboratories of big enterprises and at research centers and universities. All these systems have in common, that they consist of two units, usually for 19" rack mount, connected by a quantum channel of up to 100 km - either optical telecom fiber, or a free space channel through-the-air between two telescopes. They use quantum physical properties of light to generate and simultaneously output identical but random bit strings in the two units on both ends of the quantum channel.

The output of a QKD system can serve as a shared secret in any computer security system from which cryptographic key can be generated.

The laws of quantum physics ensure that it is virtually impossible to eavesdrop on this key distribution process on the quantum channel without the two stations immediately noticing it ([i.3] and [i.4]). More precisely, QKD systems never output insecure key. The net effect of eavesdropping is a decrease, or eventually, a stop in the key output. The degree of security of the keys is cryptographically denoted as "information-theoretical security". In broad terms this implies that the key is almost perfectly random, while the state of knowledge of the eavesdropper is almost zero. The deviations of these "ideal properties" are measurable and it is in the hand of the legitimate operators to make them arbitrarily small at the expense of a small reduction in the key generation rate.

The actual implementations of the QKD devices vary strongly and belong to a number of broad technological realization classes: discrete variable realizations, continuous variable realization, and distributed phase-reference realizations (for a detailed technical description of QKD, see [i.2], [i.12] and the documents referenced therein). However, the basic functionality of a QKD system as an information-theoretically secure key-distribution facility is universal. All these implementations have an optical subsystem with components used for the preparation and measurement of quantum information in photons of light, as well as complex computer systems for transforming measured results into digital data. These implementations are, like any security system, subject to several side channels through which information may eventually leak out of a secure boundary. Besides the showcase "use cases", the present document presents the specifications and mechanisms for driving development towards a security certification of QKD systems - an indispensable requirement for their qualified and dependable use.