Abstract

We make progress towards the impossibility of imperfectly complete quantum-computation, classical-communication (QCCC) key agreement by constructing the first unconditional attacks on quantum key agreement in the following restricted settings. In the two-message setting, we assume that Alice makes only classical queries to the oracle in the first round and that her message to Bob is classical, but otherwise both parties may perform arbitrary quantum computation, make quantum queries, and send a quantum state in the second round. In the round-independent setting, we show that the attack of Barak and Mahmoody (CRYPTO 2009; J. Cryptology 2017) can be extended to multiple rounds when Alice and Bob share classical communication and make only classical queries in all but the final round.