Composition in Quantum Cryptography

Dominique Unruh (University of Tartu)

 

In cryptography, composition refers to the process of combining several secure protocols in order to obtain a more complex one. Although one would intuitively expect it, the composition of secure protocols is not always itself secure. The same applies for quantum protocols. We present two striking examples where a naive but natural composition of secure quantum protocols leads to insecure protocols.

To address these challenges, we present the quantum universal composability framework. This framework allows us to get strong composability guarantees for quantum protocols. We illustrate the power of this framework by deriving a protocol for everlastingly secure multi-party computation based on signature cards (which is impossible classically).