We suggest that the randomness of the choices of measurement basis by Alice and Bob provides an additional important resource for quantum cryptography. As a specific application, we present a novel protocol for quantum key distribution (QKD) which enhances the BB84 scheme by encrypting the information sent over the classical channel during key sifting. We show that, in the limit of long keys, this process prevents an eavesdropper from reproducing the sifting process carried out by the legitimate users. The inability of the eavesdropper to sift the information gathered by tapping the quantum channel reduces the amount of information that an eavesdropper can gain on the sifted key. We further show that the protocol proposed is self sustaining, and thus allows the growing of a secret key.
We suggest that the randomness of the choices of measurement basis by Alice and Bob provides an additional important resource for quantum cryptography. As a specific application, we present a novel protocol for quantum key distribution (QKD) which enhances the BB84 scheme by encrypting the information sent over the classical channel during key sifting. We show that, in the limit of long keys, this process prevents an eavesdropper from reproducing the sifting process carried out by the legitimate users. The inability of the eavesdropper to sift the information gathered by tapping the quantum channel reduces the amount of information that an eavesdropper can gain on the sifted key. We further show that the protocol proposed is self sustaining, and thus allows the growing of a secret key.
This paper suggests an improvement to the BB84 scheme in Quantum key distribution. The original scheme has its weakness in letting quantifiably more information gain to an eavesdropper during public announcement of unencrypted bases lists. The security of the secret key comes at the expense of the final key length. We aim at exploiting the randomness of preparation (measurement) basis and the bit values encoded (observed), so as to randomize the bases lists before they are communicated over the public channel. A proof of security is given for our scheme and proven that our protocol results in lesser information gain by Eve in comparison with BB84 and its other extensions. Moreover, an analysis is made on the feasibility of our proposal as such and to support entanglement based QKD. The performance of our protocol is compared in terms of the upper and lower bounds on the tolerable bit error rate. We also quantify the information gain (by Eve) mathematically using the familiar approach of the concept of Shannon entropy. The paper models the attack by Eve in terms of interference in a multi-access quantum channel. Besides, this paper also hints at the invalidation of a separate privacy amplification step in the "prepare-and-measure" protocols in general.
In this paper, we introduce a novel two-way quantum key distribution (QKD) protocol in which the sender (Alice) and receiver (Bob) employ one qubit of a maximally entangled Bell state as the quantum channel for key exchange. The protocol incorporates a new security mechanism based on a scrambling operator. Unlike conventional two-way QKD protocols, all sifting operations and eavesdropper detection procedures are postponed until the completion of the quantum communication stage and are performed exclusively by Bob. Since the control mode is never publicly announced, attacks that rely on mode-dependent adaptations or attempt to remain hidden within the control mode are inherently prevented. Furthermore, the traveling qubit does not directly encode key information, substantially limiting the information that can be extracted from attacks targeting the quantum channel alone. An additional distinctive feature of the protocol is that rounds that would ordinarily be discarded can instead be utilized to detect the presence of an eavesdropper. We analyze a broad class of ancilla-based attacks, in which an eavesdropper couples an ancillary system to the transmitted qubit in an attempt to gain information about the key, and show that such attacks are detectable in their most general form.
Abstract Several quantum key distribution (QKD) protocols employ iterative sifting. After each quantum transmission round, Alice and Bob disclose part of their setting information (including their basis choices) for the detected signals. This quantum phase then ends when the basis dependent termination conditions are met, i.e., the numbers of detected signals per basis exceed certain pre-agreed threshold values. Recently, however, Pfister et al (2016 New J. Phys. 18 053001) showed that the basis dependent termination condition makes QKD insecure, especially in the finite key regime, and they suggested to disclose all the setting information after finishing the quantum phase. However, this protocol has two main drawbacks: it requires that Alice possesses a large memory, and she also needs to have some a priori knowledge about the transmission rate of the quantum channel. Here we solve these two problems by introducing a basis-independent termination condition to the iterative sifting in the finite key regime. The use of this condition, in combination with Azuma’s inequality, provides a precise estimation on the amount of privacy amplification that needs to be applied, thus leading to the security of QKD protocols, including the loss-tolerant protocol (Tamaki et al 2014 Phys. Rev. A 90 052314), with iterative sifting. Our analysis indicates that to announce the basis information after each quantum transmission round does not compromise the key generation rate of the loss-tolerant protocol. Our result allows the implementation of wider classes of classical post-processing techniques in QKD with quantified security.
The publication of the BB84 protocol by Bennett and Brassard in 1984 [10] marks the beginning of quantum key distribution. Since then, many other protocols have been invented. Yet, BB84 keeps a privileged place in the list of existing protocols: it is the one most analyzed and most often implemented, including those used in commercial products, e.g., [91, 113]. In this chapter, I define the BB84 protocol, although I already described it informally in Section 1.1. The physical implementation is then investigated. Finally, I analyze the eavesdropping strategies against BB84 and deduce the secret key rate. Description Alice chooses binary key elements randomly and independently, denoted by the random variable X ∈ χ = {0, 1}. In this protocol, there are two encoding rules, numbered by i ∈ {1, 2}. Alice randomly and independently chooses which encoding rule she uses for each key element. In case 1, Alice prepares a qubit from the basis {|0〉, |1〉} as In case 2, Alice prepares a qubit from the basis {|+〉, |−〉} as On his side, Bob measures either Z or X , yielding the result Y Z or Y X , choosing at random which observable he measures. After sending a predefined number of qubits, Alice reveals to Bob the encoding rule for each of them. They proceed with sifting, that is, they discard the key elements for which Alice used case 1 (or case 2) and Bob measured Z (or X ). For the remaining (sifted) key elements, we denote Bob's sifted measurements by Y .
[[abstract]]An enhanced version of the quantum key distribution (QKD) protocol in quantum cryptography is proposed. We consider that realistic detectors and imperfect devices may introduce noise into the transmission thereby causing a high error rate. The proposed scheme is based on an error rate analysis and a random sampling procedure in statistics to produce a probabilistic bound on the error estimation. Confidence intervals are used to interpret the error estimate with a specified confidence level. Based on the information obtained, Alice and Bob judge what caused the errors and establish confidence levels for the error rate of the remaining untested bits. Hence, they can determine the maximum number of times reconciliation needs to be performed to remove all errors from what remains of a raw key to produce an error-free secret key. As a result, the tedious reconciliation process can be reduced and a neglectful probability of not detecting the existence of remaining errors can be avoided as well. The privacy amplification is then applied: (1) to extract a secret key from a partially secret key, and (2) to enlarge the length of the secret key into the final key. The proposed scheme can work when dealing with errors caused by random noise and eavesdropping.
The prepare-and-measure quantum key distribution (QKD) has the merits of fast speed, high key generation rate, and easy implementation. However, the detector side channel attacks greatly undermine the security of the key bits. The eavesdropper, Eve, exploits the flaws of the detectors to obtain illegal information without violating quantum principles. It means that she can intervene in the communication without being detected. A prepare-and-measure Bell test protocol will be proposed. By randomly carrying out Bell test at the side of the information receiver, Bob, Eve's illegal information gain within the detector side channel attack can be well bounded. This protocol does not require any improvement on the detectors used in available prepare-and-measure QKD. Though we only illustrate its application in the BB84 protocol, it is applicable for any prepare-and-measure QKD.