Quantum key distribution (QKD) relies on quantum and classical procedures in orderto achieve the growing of a secret random string --the key-- known only to the twoparties executing the protocol. L...
Quantum cryptography is essentially the quantum key distribution (QKD). In the context of QKD, one from two partners (Alice) generates and sends a sequence of qubits through a private quantum channel to another partner (Bob) and Bob receives the sequence and measures the state of each qubit. After the quantum transmission stage, Alice and Bob have almost identical qubit sequences. The erros are due to physical imperfections in the channel and presence of an eavesdropper. The next stage in QKD is key reconciliation (i.e. finding and correcting discrepancies between Alice's string and that of Bob). This reconciliation can be done by public discussion. Let us suppose there is a secret quantum channel between Alice and Bob through which Alice transmits a n-bit string A=(A1, A2,...,An)ε{0,1}n. Then Bob receives a n-bit string B=(B1, B2,...,Bn)ε{0,1)n. The string B differs from A due to the presence of noise and eavesdropper in the channel. One can estimate the bit error probability in the channel. For example, Bob can choose a random subset from his string and send it to Alice in public. Then Alice compares the received string with her corresponding subset and calculates the total number of protocol steps. The cascade scheme uses the interaction over the public channel to correct the secret strings by dividing them into the blocks of a fixed length. The length is determined from the bit error probability. A simple interactive routine is applied in each of these blocks. An error found in some block results in some action with other blocks. It is important to optimize the error-finding routines in standalone blocks as well as to organize the effective constrution of blocks with the object of protocol benchmark, information leakage and number of interactions between partners.
Quantum key distribution (QKD) relies on quantum and classical procedures in order to achieve the growing of a secret random string -the key- known only to the two parties executing the protocol. Limited intrinsic efficiency of the protocol, imperfect devices and eavesdropping produce errors and information leakage from which the set of measured signals -the raw key- must be stripped in order to distill a final, information theoretically secure, key. The key distillation process is a classical one in which basis reconciliation, error correction and privacy amplification protocols are applied to the raw key. This cleaning process is known as information reconciliation and must be done in a fast and efficient way to avoid cramping the performance of the QKD system. Brassard and Salvail proposed a very simple and elegant protocol to reconcile keys in the secret-key agreement context, known as Cascade, that has become the de-facto standard for all QKD practical implementations. However, it is highly interactive, requiring many communications between the legitimate parties and its efficiency is not optimal, imposing an early limit to the maximum tolerable error rate. In this paper we describe a low-density parity-check reconciliation protocol that improves significantly on these problems. The protocol exhibits better efficiency and limits the number of uses of the communications channel. It is also able to adapt to different error rates while remaining efficient, thus reaching longer distances or higher secure key rate for a given QKD system.
Quantum key distribution (QKD) relies on quantum and classical procedures in order to achieve the growing of a secret random string -the key- known only to the two parties executing the protocol. Limited intrinsic efficiency of the protocol, imperfect devices and eavesdropping produce errors and information leakage from which the set of measured signals -the raw key- must be stripped in order to distill a final, information theoretically secure, key. The key distillation process is a classical one in which basis reconciliation, error correction and privacy amplification protocols are applied to the raw key. This cleaning process is known as information reconciliation and must be done in a fast and efficient way to avoid cramping the performance of the QKD system. Brassard and Salvail proposed a very simple and elegant protocol to reconcile keys in the secret-key agreement context, known as Cascade, that has become the de-facto standard for all QKD practical implementations. However, it is highly interactive, requiring many communications between the legitimate parties and its efficiency is not optimal, imposing an early limit to the maximum tolerable error rate. In this paper we describe a low-density parity-check reconciliation protocol that improves significantly on these problems. The protocol exhibits better efficiency and limits the number of uses of the communications channel. It is also able to adapt to different error rates while remaining efficient, thus reaching longer distances or higher secure key rate for a given QKD system.
Cryptography is an unexpected revolution in information security in the recent decades, where remarkable improvements have been created to provide confidentiality and integrity. Quantum cryptography is one such improvement that has grown rapidly since the first announced protocol. Quantum cryptography contains substantial elements that must be addressed to ensure secure communication between legitimate parties. Quantum key distribution (QKD), a technique for creating a secret key, is one of the most interesting areas in quantum cryptography. This paper reviews some well-known quantum key distribution techniques that have been demonstrated in the past three decades. Furthermore, this paper discusses the process of creating a secret key using quantum mechanics and cryptography methods. Moreover, it explains the relationships between many basic aspects of QKD protocols and suggests some improvements in the cryptosystem. An accurate quantitative comparison between the QKD protocols is presented, especially the runtime execution for each QKD protocol. In addition, the paper will demonstrate a general model of each considered QKD protocol based on security principles.
In this section, I wish to put quantum key distribution (QKD) in the wider context of a cryptosystem. I shall discuss informally some aspects that are considered important. The questions I wish to answer here are: “What are the ingredients needed to make a QKD-based cryptosystem work? What services does it bring? What are its limitations?” As I shall detail below, QKD may be used to provide the users with confidential communications. This can be achieved when we combine QKD and the one-time pad. For the quantum modulation, QKD needs a source of truly random numbers. Also, QKD requires a classical authenticated channel to work, so authentication plays an essential role. As a consequence, QKD must start with a secret key, making it a secret-key encryption scheme. I will also discuss what happens if classical cryptography is introduced in the system. Finally, I will describe the implementation of a simple cryptosystem on top of QKD. A key distribution scheme The first function of QKD is to distribute a secret key between two parties. The use of this key is outside the scope of this first section – the need for a secret key is omnipresent in cryptography. As depicted in Fig. 5.1, QKD relies on a classical authenticated channel for sifting and secret-key distillation and on random bits for the modulation of quantum states. The key produced by QKD can be intended for encryption purposes – this will be discussed in Section 5.2 – but is also required by authentication. A part of the distributed key is used for authentication. When QKD is run for the first time, however, an initial secret key must be used instead.