Position-based Quantum Cryptography, Impossibility and Constructions


Harry Buhrman, Serge Fehr and Christian Schaffner from the Centrum Wiskunde & Informatica (CWI) in Amsterdam, in collaboration with researchers from the University of California, Los Angeles (UCLA) and Microsoft Research India, presented their paper ‘Position-based Quantum Cryptography, Impossibility and Constructions’ at the 14th Workshop on Quantum Information Processing - QIP 2011 (Singapore 8 – 14 January 2011). QIP is the leading international congress on quantum information processing. The work of the researchers was one of three papers that were granted a plenary presentation at QIP 2011.

In their work, the researchers studied position-based cryptography in the quantum setting. The goal of position-based cryptography is to let the geographical position of a person act as its only credential for accessing secured data and services. This has the important advantage that no digital cryptographic keys need to be distributed and locally stored, which is often the bottleneck in standard cryptographic solutions and offers additional room for attacks.

The outcome of the investigation is twofold and opens up an interesting new line of research. The work demonstrates that the possibility of doing position-based cryptography depends on the opponents' capability of sharing entangled quantum states. The researchers show that if the opponents cannot share any entangled quantum state, then secure position-based cryptography is possible. On the other hand, the researchers also show that if the opponents are able to share a huge entangled quantum state, then any scheme can be broken and no position-based cryptography is possible at all. These results open up the interesting question whether secure position-based cryptography is possible in the realistic setting of bounded shared entanglement.

In models where secure position-based cryptography is possible, it has a number of interesting applications. For example, it enables secure communication over an insecure channel without having any pre-shared key, with the guarantee that only a party at a specific location can learn the content of the conversation. Also other position-based cryptographic schemes are possible, such as secure position-based authentication and secure position-based encryption.