Two intercept-and-resend attacks on a bidirectional quantum secure direct communication and its improvement

Quantum secure direct communication is an important branch of quantum cryptography. One of the main requirements of quantum secure direct communication is to ensure that no secret information can be stolen. Recently, a bidirectional quantum secure direct communication protocol [Quantum Information Processing 16, 147 (2017)] was proposed. It was believed that the intercept-and-resend attack and information leakage problem can be avoided via this protocol. However, in this paper, we point out that attackers can obtain useful information about the secret messages by constructing two intercept-and-resend attacks on the above protocol. Attackers can obtain Alice’s secret message exclusive OR Bob’s secret message by the first attack and both secret messages by the second attack. To resist the two constructed attacks, we design an improved bidirectional quantum secure direct communication protocol. Furthermore, we show that the designed protocol can resist the two constructed attacks and its efficiency has increased. It is interesting that the designed protocol can publish Alice’s result states, i.e., Bob’s initial states, without affecting its security. The designed protocol can prevent Alice (Bob) from obtaining Bob’s (Alice’s) secret message before Alice (Bob) sends her (his) secret message. This work can notice researchers to avoid similar security problems in constructing quantum cryptography protocols. © The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature 2023.