Quantum players in constructive cryptography


This project started in 2017 and has been successfully completed.


Christopher Portmann (ETH Zurich)
Ueli Maurer (ETH Zurich)


Quantum mechanics is one of the most successful physical theories, and has been verified by numerous experiments. But what does this imply for cryptography? On one hand, adversaries may have abilities that are not captured by a “classical” adversary. On the other, the (honest) users may also use quantum technology to increase the security of their protocols. But before being able to formulate the risks and benefits of quantum players, one needs cryptographic models and security definitions that encompass such parties.

The goal of this project is to model quantum players in the constructive cryptography framework of Maurer and Renner. The first part of the project involves modifying the framework itself so that it has the power need to capture such quantum players. For example, quantum mechanics allows a message to be in a superposition of sent and not sent, or a superposition of sent to Alice and sent to Bob, which needs to fit in the underlying communication model used by the framework. Furthermore, one may consider various message scheduling models, e.g., sequential scheduling (the players are activated one after the other), time-based scheduling (the time it takes to send and receive messages is explicitly modeled, and used to determine the order in which messages are processed) and non-determiistic scheduling (one computes all possible orders of messages and looks at the worst case). This projects studies these different scheduling models in the quantum context.

The second part of the project consists in using the framework to model cryptographic security in various applications. For example, we wish to find the best way to model CPA and CCA attacks on schemes that encrypt quantum messages. Another example is to study device-independent cryptography, and model the reuse of devices in a composable framework. It is indeed well-known that current security proofs only hold for devices that are used just once.


Composable and Finite Computational Security of Quantum Message Transmission.
Fabio Banfi, Ueli Maurer, Christopher Portmann, Jiamin Zhu.
Theory of Cryptography – TCC 2019, LNCS, Springer, vol. 11891, pp. 282–311, 2019.

Quantum Authentication with Key Recycling.
Christopher Portmann
Advances in Cryptology – EUROCRYPT 2017 – Proceedings, Part III, pp. 339-368, 2017.