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Quantum cryptography allows for unconditionally secure communication against an eavesdropper endowed with unlimited computational power and perfect technologies, who is only constrained by the laws of physics. We review recent results showing that, under the assumption that the eavesdropper can store quantum information only for a limited time, it is possible to enhance the performance of quantum key distribution in both a quantitative and qualitative fashion. We consider quantum data locking as a cryptographic primitive and discuss secure communication and key distribution protocols. For the case of a lossy optical channel, this yields the theoretical possibility of generating secret key at a constant rate of 1 bit per mode at arbitrarily long communication distances. [ABSTRACT FROM AUTHOR]

In this paper, we first present a five-party scheme for sharing a single-qutrit state by using GHZ states as the quantum channel. Any one of the agents has the access to reconstruct the original state if other controlling agents cooperate with him. We also sketch the generation of five-party scheme to the case of multi-qudit states and multiple participants by a composite channel composed of generalized Bell states and GHZ states. In our scheme, the physical operations, especially for the controllers and the final receiver, are considerably reduced. It also demonstrates a high degree of symmetry and provides a useful inspiration for implementing hierarchical quantum information splitting. [ABSTRACT FROM AUTHOR]

Recently, Yang et al. (Quantum Inf Process 12(1):109, ) proposed an enhanced quantum blind signature based on two-taste vector formalism. The protocol can prevent signatory Bob from deriving Alice's message with invisible photon eavesdropping attack or fake photon attack. In this paper, we show that the enhanced protocol also has a loophole that Alice can utilize an entanglement swapping attack to obtain Bob's secret key and forge Bob's valid signature at will later. Then, we reanalyze two existing protocols and try to find some further methods to fix them. [ABSTRACT FROM AUTHOR]

In this work, we respond to a comment by A. Abidin and J.-Å. Larsson on our previous paper, Int. J. Quant. Inf.3 (2005) 225. [ABSTRACT FROM AUTHOR]

We show that quantum entanglement is helpful in improving the security of arbitrated quantum signature (AQS). Using the properties of quantum entanglement, a potential forgery which might occur in arbitrated quantum signature schemes without using them can be avoided. The fact is illustrated by relevant schemes. [ABSTRACT FROM AUTHOR]

We present a three-stage quantum cryptographic protocol based on public key cryptography in which each party uses its own secret key. Unlike the BB84 protocol, where the qubits are transmitted in only one direction and classical information exchanged thereafter, the communication in the proposed protocol remains quantum in each stage. A related system of key distribution is also described. [ABSTRACT FROM AUTHOR]

The Fuchs–Peres–Brandt (FPB) probe realizes the most powerful individual attack on Bennett-Brassard 1984 quantum key distribution by means of a single controlled-NOT gate in which Alice’s transmitted qubit becomes the control-qubit input, Bob’s received qubit is the control-qubit output, and Eve supplies the target-qubit input and measures the target-qubit output. The FPB probe uses the minimum-error-probability projective measurement for discriminating between the target-qubit output states that are perfectly correlated with Bob’s sifted bit value when that bit is correctly received. This paper analyzes a recently proposed modification of the FPB attack in which Eve’s projective measurement is replaced by a probability operator-valued measurement chosen to unambiguously discriminate between the same two target-qubit output states. [ABSTRACT FROM AUTHOR]

If an eavesdropper succeeds in compromising the quantum as well as the classical channels and mimics the receiver "Bob" for the sender "Alice" and vice versa, one defence strategy is the successive, temporally interlocked partial transmission of the entire encrypted message. [ABSTRACT FROM AUTHOR]

The quantum circuit and design are presented for an optimized entangling probe attacking the BB84 Protocol of quantum key distribution (QKD) and yielding maximum information to the probe. Probe photon polarization states become optimally entangled with the signal states on their way between the legitimate transmitter and receiver. Although standard von-Neumann projective measurements of the probe yield maximum information on the pre-privacy amplified key if instead the probe measurements are performed with a certain positive operator valued measure (POVM), then the measurement results are unambiguous, at least some of the time. It follows that the BB84 (Bennett-Brassard 1984) protocol of quantum key distribution has a vulnerability similar to the well-known vulnerability of the B92 (Bennett 1992) protocol. [ABSTRACT FROM AUTHOR]

We analyze some of the critical factors that confine the transmission length for a quantum key distribution system under a certain Quantum Bit Error Rate (QBER). Emphasis was placed on the relationship between transmission length and QBER. Relationships between the minimum QBER and transmission length in theory for ideal M–Z interferometers and for the real case in theory according to the parameters of our system are given. The relationship between them as obtained by experiment was also given. The results showed that our system is very close to the performance of an ideal experimental system. [ABSTRACT FROM AUTHOR]

Sealing information means making it publicly available, but with the possibility of knowing if it has been read. Commenting on Ref. 1, we will show that perfect quantum sealing is not possible for perfectly retrievable information, due to the possibility of performing a perfect measurement without disturbance, even on unknown states. The measurement is a collective one, and this makes the protocol of quantum sealing very interesting as the only example of the power of collective measurements in breaking security. [ABSTRACT FROM AUTHOR]

Many quantum key distribution QKD analyses examine the link security in a subset of the full Hilbert space that is available to describe the system. In reality, information about the photon state can be embedded in correlations between the polarization space and other dimensions of the full Hilbert space in such a way that Eve can determine the polarization of a photon without affecting it. This paper uses the concept of suitability Hockneyet al. “Suitability versus Fidelity for Rating Single Photon Guns” to quantify the available information for Eve to exploit, and demonstrate how it is possible for Alice and Bob to fool themselves into thinking they have a highly secure link. [ABSTRACT FROM AUTHOR]

Recent work has shown how to use the laws of quantum mechanics to keep classical and quantum bits secret in a number of different circumstances. Among the examples are private quantum channels, quantum secret sharing and quantum data hiding. In this paper we show that a method for keeping two classical bits hidden in any such scenario can be used to construct a method for keeping one quantum bit hidden, and vice–versa. In the realm of quantum data hiding, this allows us to construct bipartite and multipartite hiding schemes for qubits from the previously known constructions for hiding bits. [ABSTRACT FROM AUTHOR]