An interesting protocol for quantum identity authentication on a basis of the classic shared secret has been presented recently (Hong et al. in Quantum Inf Process 16(10): 236, 2017 ). It requires few resources and it is technologically feasible. Its seminal analysis focuses on average eavesdropper’s information gain per single protocol run—a parameter suitable for estimation of the security margin applicable to the protocols aiming at provision of confidentiality. However, the security requirements of the authentication process are very stringent—no single bit of the secret can be revealed, otherwise the eavesdropper can collect subsequent bits of the secret in successive protocol runs. The failure of the seminal version to meet this requirement is demonstrated. The sequential processing of qubits and the static nature of the compared data is the rationale behind the improvement. The introduced changes make that partial information gained in some authentication attempt gives the eavesdroppers no advantage in breaking the next ones. They are forced to consider each authentication transaction as a separate puzzle that can be solved according to all-or-nothing paradigm. The improvement retains conceptual simplicity and technological feasibility of the seminal version of the protocol.

Quantum secure direct communication protocols offer confidential transmission of classic information over quantum channel without prior key agreement. The ping-pong based protocols provide asymptotic security and detailed analysis of security level provided by each variant of the protocol is required. The paper presents a general method of calculation of the eavesdropped information as a function of the attack detection probability. The method is applied to the ping-pong protocol based on completely entangled pairs of qudits. The upper and lower bounds on the amount of the leaked information and eavesdropping detection probability are provided.

A security layer for the asymptotically secure ping-pong protocol is proposed and analyzed in the paper. The operation of the improvement exploits inevitable errors introduced by the eavesdropping in the control and message modes. Its role is similar to the privacy amplification algorithms known from the quantum key distribution schemes. Messages are processed in blocks which guarantees that an eavesdropper is faced with a computationally infeasible problem as long as the system parameters are within reasonable limits. The introduced additional information preprocessing does not require quantum memory registers and confidential communication is possible without prior key agreement or some shared secret.