In 1991 scientists have for the first time demonstrated practical quantum cryptography (QC) and given a proof of its unconditional security against two most obvious attacking strategies. In the next decade several groups of scientists have made more stringent security proofs and it was soon widely accepted that quantum cryptographic protocol BB84 and its variants are the only known protocols that allow unconditionally secure key growth. Moreover, QC was able to offer unique possibility that any eavesdropping activity could be detected by legitimate parties. In 2004 Swiss spin-off IdQuantique has shown the world's first commercial quantum key distribution system Clavis, followed soon by the QPN system of American start-up Magiq. For almost 7 years nobody suspected that there might be something terribly wrong with these systems. Then, at the end of 2010, a group of Norwegian scientists experimenting with attacks on photon detectors constructed tailored attacks to both commercial systems which were so successful that the plaintext was recovered in full (100%) and moreover the legitimate parties could not detect any suspicious activity ! This raised the question: where does it leave us with quantum cryptography, is it secure or not? What about the security proofs, were they wrong? Our conclusion is that security proofs were correct except that they proved security of something else - not the machines that were cracked. At present state of the art it is not clear how or is it even possible to build a provable QC machine.
In 1991 scientists have for the first time demonstrated practical quantum cryptography (QC) and given a proof of its unconditional security against two most obvious attacking strategies. In the next decade several groups of scientists have made more stringent security proofs and it was soon widely accepted that quantum cryptographic protocol BB84 and its variants are the only known protocols that allow unconditionally secure key growth. Moreover, QC was able to offer unique possibility that any eavesdropping activity could be detected by legitimate parties. In 2004 Swiss spin-off IdQuantique has shown the world's first commercial quantum key distribution system Clavis, followed soon by the QPN system of American start-up Magiq. For almost 7 years nobody suspected that there might be something terribly wrong with these systems. Then, at the end of 2010, a group of Norwegian scientists experimenting with attacks on photon detectors constructed tailored attacks to both commercial systems which were so successful that the plaintext was recovered in full (100%) and moreover the legitimate parties could not detect any suspicious activity ! This raised the question: where does it leave us with quantum cryptography, is it secure or not? What about the security proofs, were they wrong? Our conclusion is that security proofs were correct except that they proved security of something else - not the machines that were cracked. At present state of the art it is not clear how or is it even possible to build a provable QC machine.
The peculiar properties of quantum mechanics enable possibilities not allowed by classical physics. In particular, two parties can generate a random, secret key at a distance, even though an eavesdropper can do anything permitted by the laws of physics. Measuring the quantum properties of the signals generating the key, would ultimately change them, and thus reveal the eavesdropper’s presence. This exchange of a random, secret key is known as quantum cryptography.\n\nQuantum cryptography can be, and has been proven unconditionally secure using perfect devices. However, when quantum cryptography is implemented, one must use components available with current technology. These are usually imperfect. Although the security of quantum cryptography has been proven for components with certain imperfections, the question remains: can quantum cryptography be implemented in a provable, unconditionally secure way, using components available with current technology? This thesis contains both a theoretical, and an experimental contribution to the answer of this question. On the experimental side, components used in, and complete quantum cryptography systems have been carefully examined for security loopholes. In particular, it turned out that two commercial quantum cryptography systems contained loopholes, which would allow an eavesdropper to capture the full secret key, without exposing her presence. Furthermore, this detector control attack could be implemented with current technology. The attack is applicable against a variety of quantum cryptography implementations and protocols.\n\nThe theoretical contribution consists of security proofs for quantum cryptography in a very general setting. Precisely, the security is proven with arbitrary individual imperfections in the source and detectors. These proofs should make it possible to use a wide array of imperfect devices in implementations of quantum cryptography.\n\nFinally, a secure detection scheme is proposed, immune to the detector control attack and compatible with those security proofs. Therefore, if this scheme is implemented correctly, it offers provable security.
Quantum key distribution (QKD) protocols make it possible for two parties to generate a secret shared key. One of the most important QKD protocols, BB84, was suggested by Bennett and Brassard in 1984. Various proofs of unconditional security for BB84 have been suggested, but the first security proofs were not composable. Here we improve a security proof of BB84 given by [Biham, Boyer, Boykin, Mor, and Roychowdhury, J. Cryptology, 19(4):381-439, 2006] to be composable and match the state-of-the-art results for BB84, and we extend it to prove unconditional security of several variants of the BB84 protocol. Our composable security proof for BB84 and its variants is mostly self-contained, algebraic, and relatively simple, and it gives tight finite-key bounds.
A common desire in today's world is that of security. Whether it is keeping your e-mail private or stopping the government from hacking into your computer, the idea behind cryptography is to communicate between two parties in different locations, and to secure this information from outsiders. During the last half century there have been numerous advances in encryption schemes and also in the machines that process such information. Modern encryption algorithms have become increasingly more complex with advances in computers and technology, and encryption algorithms such as RSA and DES have been presented as algorithms that have remained secure for decades. These recent advances in encryption schemes will be examined in the first part of this paper. On the other hand, because the security of classical ciphers relies on the secrecy of a key, advances in research and computing may begin to compromise the security of these cryptosystems, as quantum computers would be capable of mathematical calculations that could break many modern encryption algorithms. Unlike classical cryptosystems, quantum cryptography obeys the laws of quantum physics, resulting in a much stronger, provable security. Many great advances have come in recent decades, and the latter part of this paper deals with these advances as well as the phenomena of quantum physics, the evolution of quantum computing, and the study of quantum cryptography.
Leveraging quantum mechanics, cryptographers have devised provably secure key sharing protocols. Despite proving the security in theory, real-world application falls short of the ideal. Last year, cryptanalysts completed an experiment demonstrating a successful eavesdropping attack on commercial quantum key distribution (QKD) systems. This attack exploits a weakness in the typical real-world implementation of quantum cryptosystems. Cryptanalysts have successfully attacked several protocols. In this paper, we examine the Kak quantum cryptography protocol and how it may perform under such attacks.
Quantum Cryptography is a novel approach to make the information and network world more secure. Unlike classical cryptography, quantum cryptography ensures the security of communication because it is based on the laws of Quantum Mechanics; a branch of physics that deals with the fundamental behavior of matter and their energies to the levels of subatomic particles. These particles and their intrinsic properties such as photon polarization and entanglement allow us to develop an unbreakable system of cryptographic information. Conventionally, the security has been primarily concerned with the secured distribution of secret key carried over an insecure channel. However, in Quantum Cryptographic systems the transmission of secret key is done using aforementioned laws of nature through a secured quantum channel highly susceptible to intrusion detection. Consequently, the Quantum Key Distribution (QKD), in conjunction with protocols such as BB84 protocol has emerged as an ultimate modern secured communication system. This article concentrates on briefly reviewing quantum cryptography, some recent developments and reports of its vulnerabilities.
Quantum cryptography, or, more precisely, quantum key distribution (QKD) allows two distant parties, Alice and Bob, to expand a previously shared secret key by sending quantum states through a quantum channel. The goal here is for Alice and Bob to communicate securely in the presence of an eavesdropper, Eve. The most well-known QKD protocol is the Bennett-Brassard 1984 (BB84) protocol, which has been proved unconditionally secure against any attacks allowed by quantum mechanics. The main advantage of QKD over conventional cryptography is its absolute security. Thus, it is imperative to evaluate the security and performance of QKD protocols and realistic QKD systems. In this thesis, we study the security and performance aspects of QKD in four parts. In the first part, we study the Scarani-Acin-Ribordy-Gisin 2004 (SARG04) protocol. This protocol differs from the BB84 protocol only in the classical post-processing step and uses the same four quantum states as the BB84 protocol. We compared these two protocols and showed that in typical situations, the classical post-processing software for BB84 should be used in order to have a higher performance. In the second part, we prove the security of a three-state protocol. This protocol is similar to the BB84 protocol in that they share the same three qubit states. In the third part, we consider a practical Trojan horse attack on practical QKD systems implementing the BB84 protocol. The attack works by taking advantage of the finite response time of practical phase modulators to remap the states sent out by the legitimate user, Alice. We showed that if Alice and Bob are unaware of our attack, the final key shared between them can be compromised in some situations. In the fourth part, we consider another type of Trojan horse attack, which is based on mismatch between detection efficiencies of detectors. Practical detectors in QKD systems usually do not have identical efficiencies and an eavesdropper Eve may be able to exploit this efficiency mismatch to steal information about the final secret key. We analyze the security of the attack and show that it is possible for Eve to acquire some information on the final secret key if Alice and Bob are unaware of this attack.