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Possible schemes for sending N-bit secret message with established N EPR pairs and a classical channel between two parties, are considered. The corresponding N-particle quantum key-distribution for N-bit secret message communication is outlined.

We provide an asymmetrical quantum key distribution protocol based on the correlations of EPR pairs. It is a variant of the modified Lo–Chau protocol where the EPR pair is not in the state Φ⁺ but is in one of the four states {Φ^±, Ψ^±}. The two sides communicating with each other are not equal in the process of establishing the key. A public key system can be built on the protocol. It differs from classical public key system in that there are three keys: the public key, the secret key and the real key.

In 2002, Li presented an asymmetrical quantum key distribution protocol which utilizes the correlations of Einstein-Podolsky-Rosen (EPR) pairs to establish a session key between two participants. Li claimed that an eavesdropper Eve cannot prevent his existence from being found by participants. However, this study shows that the session key can be obtained by Eve without being detected.

This paper presents a deterministic BB84 (dBB84) protocol that not only inherits the unconditional security of the original BB84 protocol but also enables the receiver to deterministically measure and decode all qubits sent by the sender. The proposed dBB84 protocol is then extended to be a deterministic secure quantum communication (DSQC) protocol wherein the sender can securely transmit secret messages to the receiver via quantum channels and the receiver can read out the secret messages only after receiving an additional classical bit for each qubit from the sender. In contrast to the existing single-photon-based secure communication protocols, which require the sender to either prepare two-qubit photon states or to establish two-way quantum channels with the receiver, the newly proposed protocol requires the sender to prepare single-qubit photon states for message transmissions and only set up one-way quantum channels to the receiver. Therefore, the proposed protocol is very suitable and feasible in practical applications.

A three-party controlled deterministic secure quantum communication scheme through entanglement swapping is proposed firstly. In the scheme, the sender needs to prepare a class of Greenberger–Horne–Zeilinger (GHZ) states which are used as quantum channel. The two communicators may securely communicate under the control of the controller if the quantum channel is safe. The roles of the sender, the receiver, and the controller can be exchanged owing to the symmetry of the quantum channel. Different from other controlled quantum secure communication schemes, the scheme needs lesser additional classical information for transferring secret information. Finally, it is generalized to a multiparty controlled deterministic secure quantum communication scheme.

Recently, Zhou et al. proposed a quantum secret sharing (QSS) protocol, which provides only 25% qubit efficiency after the processes of the random sampling discussion and the secret derivation. This work proposes an enhancement of Zhou et al.'s protocol. With the technique of the Bell measurement, the improved QSS protocol has 50% qubit efficiency.

It has been said that quantum cryptography in general offers a secure solution to the problem of key enhancement. This means that two parties who already share a small secret key, can use quantum protocols to establish a new large secret key. This large secret key can be arbitrarily long and is unbreakable. Thus, to date, the main contribution of quantum cryptography has been believed to be quantum key enhancement. This paper shows that quantum cryptography can do significantly more. The quantum protocol described here distributes an unbreakable secret key to the two parties by relying on public information only. This is the first time that quantum cryptography is shown to be able to produce secret information using only public information. This contribution is also unique for cryptography in general, classical and quantum.

Access control in a hierarchy refers to a selective access to a database. A large number of users work with the same database. These users are organized in a hierarchical structure and therefore have different access rights to the data. This paper offers a solution to the problem of access control in a hierarchy based on quantum cryptography. Each user has two keys: a classical key and a quantum key. Our scheme offers several security advantages over the classical schemes to date. It protects users from identity theft and prevents collusion attacks. Most importantly though, our scheme adapts to dynamic changes of the user hierarchy: users may join, leave, or change position in the hierarchy, without affecting the rest of the user structure.

Using one-time pads is the only cryptographic solution that really ensures the secrecy of a transmitted message. The communicating parties have to meet in advance to establish the value of any one-time pad. This means that the two parties have to meet regularly to renew the batch of one-time pads they share. By contrast, we show that after developing a secret reading mask using any quantum key distribution algorithm, the two parties can communicate indefinitely by encrypting each of their messages with a one-time pad. The value of the one-time pad is quantum encrypted and transmitted prior to the transmission of the message itself.

Quantum secure direct communication allows one participant to transmit secret messages to another directly without generating a shared secret key first. In most of the existing schemes, quantum secure direct communication can be achieved only when the two participants have full quantum ability. In this paper, we propose two semi-quantum secure direct communication protocols to allow restricted semi-quantum or “classical” users to participate in quantum communication. A semi-quantum user is restricted to measure, prepare, reorder and reflect quantum qubits only in the classical basis $\{|0〉,|1〉\}$. Both protocols rely on quantum Alice to randomly prepare Bell states, perform Bell basis measurements and publish the initial Bell states, but the semi-quantum Bob only needs to measure the qubits in classical basis to obtain secret information without quantum memory. Security and qubit efficiency analysis have been given in this paper. The analysis results show that the two protocols can avoid some eavesdropping attacks and their qubit efficiency is higher than some current related quantum or semi-quantum protocols.

Quantum private comparison (QPC), whose security is based on some laws of quantum mechanics (e.g. quantum noncloning theorem and Heisenbergs uncertainty principle), allows $n(n\ge 2)$ parties who do not trust each other to judge whether their secret data are the same while maintaining data privacy. In this paper, we investigate the utility of the maximally entangled seven-qubit state for QPC, and we propose a new protocol which enables two parties to compare their secret data with each other for equality without disclosing their secret data. In our protocol, single particle measurements and Bell-basis measurements are employed, both of which can be implemented with current technologies. In addition to quantum measurements, our protocol does not use other quantum technologies such as entanglement swapping and unitary operations. A semi-honest third-party who assists two parties in implementing the protocol is assumed in our protocol. Furthermore, we use the entanglement correlations of the maximally entangled seven-qubit state and collaborative computing between parties for privacy protection, and we use quantum key distribution (QKD) to ensure the security of the cooperative computing when two parties are in different locations. What is more, we show that the security towards both outsider and insider attacks can be guaranteed.

It is found that the controlled quantum secure direct communication (CQSDC) based on a seven-qubit entangled state protocol [S. K. Wang, X. W. Zha and H. Wu, Int. J. Theor. Phys.57, 48 (2018)] can be passively attacked. That is to say, this protocol has the security loophole of information leakage. To be specific, the first bit of the sender’s two-bit message can be deduced by an attacker Eve according to the first and the third bits of Alice’s announcement and the first bit of the controller Charlie’s announcement. Then, a way to fix this loophole is discussed.

In 2017, Qin and Dai [Quantum Inf. Process.16, 64 (2017). https://doi.org/10.1007/s11128-017-1525-y], proposed a dynamic quantum secret sharing (DQSS) scheme based on the d-dimensional state. However, as shown in this study, a malicious participant can reveal the secret key of other participants without being detected. Furthermore, this study identifies a security issue in Qin and Dai’s DQSS protocol pertaining to the honesty of a revoked participant. Without considering these security issues, the DQSS protocol could fail at providing secret-sharing function. Therefore, two improvements are proposed to circumvent these problems.

In 2018, Song et al. [Quantum Inf. Process 17, 244 (2018)] proposed a dynamic quantum direct secret sharing (DQDSS) protocol based on generalized GHZ states. However, this protocol is vulnerable to a security problem, known as the collusion attack, using which the dishonest agents can collude to reveal the master key without collaborating with other agents. This security problem, if unaddressed, can prevent the protocol from providing the secret sharing function. Therefore, this study proposes an improved DQDSS protocol to address the security problem in Song et al.’s DQDSS protocol. The proposed DQDSS protocol is resistant to the collusion attack and various other common attacks. Furthermore, the advantages of Song et al.’s DQDSS protocol are retained in the proposed scheme.

As a fundamental cryptographic primitive, bit commitment has lots of important and practical applications in modern cryptography. All previously proposed non-relativistic quantum bit commitment protocols cannot evade the Lo–Chau and Mayers attacks. Furthermore, relativistic quantum bit commitment protocols require rigorous spacetime constraints. In this paper, we present a simple, feasible but practically secure quantum bit commitment protocol without any spacetime constraint. The security of the proposed protocol is based on non-relativistic quantum mechanics, but it can resist all known attacks, including the Lo–Chau and Mayers attacks in practice.

As one of the most important branches of quantum cryptography, quantum secure direct communication (QSDC) is used to transmit the secret message directly rather than distribute a random key. Quantum homomorphic encryption (QHE) enables arbitrary quantum transformation on encrypted data without decrypting the data. To date, the previously proposed QSDC schemes are mainly based on different quantum states. The research of the QSDC scheme based on QHE is still blank. In this paper, a QSDC scheme by taking advantage of the properties of QHE is proposed. The proposed protocol has applied QHE and decoy photons to prevent various types of attacks. The proposed scheme only utilizes the rotation operation to encode the secret message which is easy to implement with the current technologies. Moreover, the communication efficiency and the qubit-utilization ratio are analyzed in this paper, which shows that this protocol has good performance in the qubit-utilization ratio, and the qubit efficiency of the QSDC scheme has improved.

Recently, Cao et al. [Mod. Phys. Lett. A 34, 1950332 (2019)] presented a two-party quantum key agreement scheme using the five-qubit Brown states and the decoy protocol. They also extended their scheme to the multiparty case, and it was compared with existing works. The authors also introduced the security analysis that proves the security of their protocol against outsider and insider attacks. However, this work shows that the quantum Cao et al. [Mod. Phys. Lett. A 34, 1950332 (2019)] protocol fails to fulfill one of the most significant conditions of the quantum key agreement, which is fairness. The computed final key by the users does not have the same level of security, and the protection of users’ private data is not maintained equally. Finally, an improved version of Cao et al.’s protocol has been suggested.

Recently, Cui et al. [Sci. China Phys. 62, 110311 (2019)] proposed a measurement-device-independent quantum key distribution (MDI-QKD) scheme based on high-dimensional encoding to increase the channel capacity of each individual photon. A secret key is shared among two legitimate parties via the help of a dishonest third-party (TP). They claimed that the proposed protocol can improve the drawback of the low-key generation rate that existed in the past MDI-QKD schemes. However, this study pointed out a loophole during the security check of TP. That is, the two legitimate participants merely use the corresponding qubit pairs that are both generated in the diagonal basis for the discussion, which leads to a potential attack that not only allows TP to avoid the security check but also creates partial inconsistency of key bits for both communicating sides. Fortunately, this problem can be fixed by a minor correction of the protocol.

The current theoretical analysis model for high-dimensional quantum key distribution (QKD) was developed without carefully considering the dead time and after-pulse of the single-photon detectors. Here, we propose a new model that accounts for these two effects. With this model, we analyze the effects of various properties of the single-photon detector on the performance of the high-dimensional quantum key distribution and show numerically that the high-dimensional quantum key distribution can be achieved up to 180 km with InGaAs/InP-type single-photon detectors. This work can improve our understanding of the factors that determine the performance of high-dimensional quantum key distribution and will help to promote the application of single-photon detectors based on avalanche photodiodes in high-dimensional quantum key distribution.

Currently, quantum signature protocols assume that all participants in the protocol (signers, verifiers, and arbitrators) should exhibit quantum capabilities. If quantum devices are lacking, the quantum signature protocol cannot be implemented. However, quantum devices are not widely used, and not all users can own quantum devices. Boyer et al. first proposed the concept of the semi-quantum environment. Zhao et al. proposed a semi-quantum signature protocol with double signers based on the W state. Yang et al. discovered that in the semi-quantum signature protocol with double signers developed by Zhao et al., an internal attacker could launch an impersonation attack during the verification and signature stages to capture the private key and use it to impersonate the user’s identity. In addition, the eavesdropper can obtain all the secret information of the signers by performing a man-in-the-middle attack. All three attack modes can pass the eavesdropping check. Hence, if these security problems are not solved, the signature of the semi-quantum signature protocol with double signers proposed by Zhao et al. cannot be completed safely. Therefore, based on Bell states, a new semi-quantum signature protocol of double signers was developed in this study. The proposed protocol not only resists impersonation attacks and man-in-the-middle attacks but also resists other common attack modes.