Quantum oblivious transfer for quantum messages

Oblivious transfer (OT) is an essential building block for secure multiparty computation. Up to now, one- and two-server OT protocols have been investigated. It has been proven that unconditionally secure one-server OT is impossible even with the quantum system. Recently, Hayashi and Song proposed the first two-server OT protocol for quantum messages [Adv. Quantum Technol. 7, 2300184 (2024)], which is the only one for two-server OT for quantum messages to our knowledge. This protocol has the assumption that two quantum servers do not communicate with each other. However, it is unrealistic to make such an assumption. In addition, an equation error occurs in the protocol description. To tackle these issues, we first point out the equation error existent in the protocol of Hayashi and Song and correct it. Then, we propose a two-server OT protocol for quantum messages by using entanglement swapping. Security analysis shows that the user privacy holds even if the two quantum servers communicate mutually.