The Quantum Decoherence Model: Everlasting Composable Secure Computation and More

Summary

Quantum cryptography allows to achieve security goals which are unobtainable using classical cryptography alone: it offers the promise of everlasting privacy. That is, an adversary trying to attack a protocol must succeed during the run of the protocol. After the protocol has ter- minated, security holds unconditionally. In this work, we initiate the study of a new model which we call the quantum decoherence model (QDM). In a nutshell, this model captures adversaries that are computationally bounded during the run of a proto- col (and some time after), but become computationally unbounded long after the protocol terminates. Importantly, once the adversary becomes computationally unbounded, he can only remember a bounded number of qubits from before the computational bound was lifted. We provide a variant of the Universal Composability framework which captures the new notion of quantum decoherence and augment it with quantum random oracles. As our main contribution, we construct a non- interactive commitment scheme achieving unconditional and statistical security against malicious senders and everlasting security against mali- cious receivers under our new security notion. Such commitments imply general secure multiparty computation with everlasting security.