Dissertations / Theses on the topic 'Key Distribution'

ABSTRACT BY Anuja A Sonalker on Asymmetric Key Distribution. (Under the direction of Dr. Gregory T. Byrd) Currently, in Threshold Public Key Systems key shares are generated uniformly and distributed in the same manner to every participant. We propose a new scheme, Asymmetric Key Distribution (AKD), in which one share server is provided with a larger, unequal chunk of the original secret key. Asymmetric Key Distribution is a unique scheme for generating and distributing unequal shares via a Trusted Dealer to all the registered peers in the system such that without the combination of the single compulsory share from the Special Server no transaction can be completed. This application is aimed for circumstances where a single party needs to co-exist within a group of semi-trusted peers, or in a coalition where every entity should have a choice to participate and one of the entities needs to be privileged with more powers. This thesis presents the algorithm and security model for Asymmetric Key Distribution, along with all the assumptions and dependencies within the boundaries of which this algorithm is guaranteed to be secure. Its robustness lies in its simplicity and in its distributed nature. We address all security concerns related to the model including compromised share servers and cryptanalytic attacks. A variation, called the Dual Threshold Scheme, is created to reduce the vulnerability in the algorithm, namely, the compromise of the Special Server and its secret share. In this scheme, a combination of another threshold number of Distributed Special Servers must combine to collectively generate a share equivalent to the Special Server?s share. This flexibility allows us to adjust our threshold scheme for the environment. We describe a Java-based implementation of the AKD algorithm, using Remote Method Invocation (RMI) for communication among share servers. A typical scenario of a Trusted Dealer, a Special Server and a number of Share Servers was created, where timed asymmetric key generation and distribution was carried out after which the servers initiated and carried out certificate signing transactions in the appropriated manner. As an interesting exercise, the share servers were corrupted so that they would try to exclude the Special Server in the transactions and try to form its share themselves, to observe the consequence. All their efforts were futile. Another interesting aspect was the key generation timing. Key generation is known to be a very time-extensive process but the key share reuse concept used in this implementation reduced the time for key generation by 66-90% of the classical key generation time.

Cryptography has begun its journey into the field of quantum information theory. Classical cryptography has shown weaknesses, which may be exploited in the future, either by development in mathematics, or by quantum computers. Quantum key distribution (QKD) is a promising path for cryptography to enable secure communication in the future. Although the theory of QKD promises absolute security, the reality is that current quantum crypto systems have flaws in them, as perfect devices have proven impossible to build. However, this can be taken into account in security proofs to ensure security, even with flaws. Security loopholes in QKD systems are being discovered as development progresses. Nevertheless, the system being built at NTNU is intended to address them all, creating a totally secure system. During this thesis, work was continued assembling the interferometer which is the basis for encoding qubits. It was fully connected on an optical table, and interference was obtained. Concerning theoretical work, calculations for a photon source specific parameter was carried out. It consisted of expanding previous framework and applying the results in both an established security proof, and a recent generalization of this proof. Two source effects were in focus, the lasers random phase and its fluctuating pulse intensity. Where analytical derivation was no longer possible, Matlab was used for numerical calculations. Under the conditions of the framework and proofs this thesis lies on, randomized phase turned out to have a negligible improvement over the case of non-random phase. Fluctuating amplitude showed a larger effect, reducing system performance. The input parameters were extreme, thus in a realistic situation it should not affect system performance significantly. However, these fluctuations must be taken into account when proving system security.

La distribution quantique de clés est une technique cryptographique permettant l'échange de clés secrètes dont la confidentialité est garantie par les lois de la mécanique quantique. Le comportement particulier des particules élémentaires est exploité. En effet, en mécanique quantique, toute mesure sur l'état d'une particule modifie irrémédiablement cet état. En jouant sur cette propriété, deux parties, souvent appelées Alice et Bob, peuvent encoder une clé secrète dans des porteurs quantiques tels que des photons uniques. Toute tentative d'espionnage demande à l'espion, Eve, une mesure de l'état du photon qui transmet un bit de clé et donc se traduit par une perturbation de l'état. Alice et Bob peuvent alors se rendre compte de la présence d'Eve par un nombre inhabituel d'erreurs de transmission.

L'information échangée par la distribution quantique n'est pas directement utilisable mais doit être d'abord traitée. Les erreurs de transmissions, qu'elles soient dues à un espion ou simplement à du bruit dans le canal de communication, doivent être corrigées grâce à une technique appelée réconciliation. Ensuite, la connaissance partielle d'un espion qui n'aurait perturbé qu'une partie des porteurs doit être supprimée de la clé finale grâce à une technique dite d'amplification de confidentialité.

Cette thèse s'inscrit dans le contexte de la distribution quantique de clé où les porteurs sont des états continus de la lumière. En particulier, une partie importante de ce travail est consacrée au traitement de l'information continue échangée par un protocole particulier de distribution quantique de clés, où les porteurs sont des états cohérents de la lumière. La nature continue de cette information implique des aménagements particuliers des techniques de réconciliation, qui ont surtout été développées pour traiter l'information binaire. Nous proposons une technique dite de réconciliation en tranches qui permet de traiter efficacement l'information continue. L'ensemble des techniques développées a été utilisé en collaboration avec l'Institut d'Optique à Orsay, France, pour produire la première expérience de distribution quantique de clés au moyen d'états cohérents de la lumière modulés continuement.

D'autres aspects importants sont également traités dans cette thèse, tels que la mise en perspective de la distribution quantique de clés dans un contexte cryptographique, la spécification d'un protocole complet, la création de nouvelles techniques d'amplification de confidentialité plus rapides à mettre en œuvre ou l'étude théorique et pratique d'algorithmes alternatifs de réconciliation.

Enfin, nous étudions la sécurité du protocole à états cohérents en établissant son équivalence à un protocole de purification d'intrication. Sans entrer dans les détails, cette équivalence, formelle, permet de valider la robustesse du protocole contre tout type d'espionnage, même le plus compliqué possible, permis par les lois de la mécanique quantique. En particulier, nous généralisons l'algorithme de réconciliation en tranches pour le transformer en un protocole de purification et nous établissons ainsi un protocole de distribution quantique sûr contre toute stratégie d'espionnage.

Quantum key distribution is a cryptographic technique, which allows to exchange secret keys whose confidentiality is guaranteed by the laws of quantum mechanics. The strange behavior of elementary particles is exploited. In quantum mechnics, any measurement of the state of a particle irreversibly modifies this state. By taking advantage of this property, two parties, often called Alice and bob, can encode a secret key into quatum information carriers such as single photons. Any attempt at eavesdropping requires the spy, Eve, to measure the state of the photon and thus to perturb this state. Alice and Bob can then be aware of Eve's presence by a unusually high number of transmission errors.

The information exchanged by quantum key distribution is not directly usable but must first be processed. Transmission errors, whether they are caused by an eavesdropper or simply by noise in the transmission channel, must be corrected with a technique called reconciliation. Then, the partial knowledge of an eavesdropper, who would perturb only a fraction of the carriers, must be wiped out from the final key thanks to a technique called privacy amplification.

The context of this thesis is the quantum key distribution with continuous states of light as carriers. An important part of this work deals with the processing of continuous information exchanged by a particular protocol, where the carriers are coherent states of light. The continuous nature of information in this case implies peculiar changes to the reconciliation techniques, which have mostly been developed to process binary information. We propose a technique called sliced error correction, which allows to efficiently process continuous information. The set of the developed techniques was used in collaboration with the Institut d'Optique, Orsay, France, to set up the first experiment of quantum key distribution with continuously-modulated coherent states of light.

Other important aspects are also treated in this thesis, such as placing quantum key distribution in the context of a cryptosystem, the specification of a complete protocol, the creation of new techniques for faster privacy amplification or the theoretical and practical study of alternate reconciliation algorithms.

Finally, we study the security of the coherent state protocol by analyzing its equivalence with an entanglement purification protocol. Without going into the details, this formal equivalence allows to validate the robustness of the protocol against any kind of eavesdropping, even the most intricate one allowed by the laws of quantum mechanics. In particular, we generalize the sliced error correction algorithm so as to transform it into a purification protocol and we thus establish a quantum key distribution protocol secure against any eavesdropping strategy.