For secure communication of classified information, researchers want to deploy the sender’s location, so a receiver can be sure that a message is, for instance, really coming from the White House. Classic location-based methods are shown to be unsafe but location-based quantum cryptography seemed to have a chance. In 2014 researchers demonstrated that this method was not safe for a group of cooperating attackers having an infinite amount of ‘entangled states’. However, the method could still be safe for realistic attacks. Florian Speelman, PhD student from Centrum Wiskunde & Informatica (CWI) in Amsterdam and QuSoft, studied quantum attacks with teleportation techniques and showed that some of the location-based protocols were also not safe for a realistic number of entangled states. Speelman defended his PhD thesis ‘Position-based Quantum Cryptography and Catalytic Computation’ on 16 November at the University of Amsterdam (UvA).
Quantum communication uses ‘entangled’ states of particles that may be located at a large distance from one another but which are nevertheless coupled to each other. Speelman: “With teleportation attacks, a group of cooperating attackers shares a quantum system with entangled states. I proved that a certain category of proposed protocols (ones that can be written as small quantum circuits) is not safe, not even for practical attacks. For example, we have made an effective attack on the ‘Interleaved Product Protocol for position verification’. The challenge is now to find what protocols will be safe.”
Besides his research on position-based quantum cryptography Florian Speelman investigated in an international team the possibilities of ‘catalytic memory’. Catalytic memory is a method of calculation that ‘uses memory without using it’. Here, the computer makes a calculation for which it uses memory that is already completely full, and it returns that memory to its original state after its use. The team showed in 2014 that this is possible and that it gives rise to a new way of thinking about memory usage with applications in cryptography. That using already-full memory gives extra computational power is counter intuitive and was thought to be impossible.
The PhD research of Florian Speelman was carried out at CWI in the Algorithms and Complexity research group, partly under the auspices of the QuSoft research center for quantum software in Amsterdam. His promotor is Prof. Harry Buhrman (CWI and UvA).
– the Algorithms and Complexity reseach group at CWI, headed by Prof. Harry Buhrman (CWI and UvA)