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- Abidin, Aysajan, 1983-
(author)
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Authentication in Quantum Key Distribution : Security Proof and Universal Hash Functions
- 2013
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Doctoral thesis (other academic/artistic)abstract
- Quantum Key Distribution (QKD) is a secret key agreement technique that consists of two parts: quantum transmission and measurement on a quantum channel, and classical post-processing on a public communication channel. It enjoys provable unconditional security provided that the public communication channel is immutable. Otherwise, QKD is vulnerable to a man-in-the-middle attack. Immutable public communication channels, however, do not exist in practice. So we need to use authentication that implements the properties of an immutable channel as well as possible. One scheme that serves this purpose well is the Wegman-Carter authentication (WCA), which is built upon Almost Strongly Universal2 (ASU2) hashing. This scheme uses a new key in each authentication attempt to select a hash function from an ASU2 family, which is then used to generate the authentication tag for a message.The main focus of this dissertation is on authentication in the context of QKD. We study ASU2 hash functions, security of QKD that employs a computationally secure authentication, and also security of authentication with a partially known key. Specifically, we study the following.First, Universal hash functions and their constructions are reviewed, and as well as a new construction of ASU2 hash functions is presented. Second, security of QKD that employs a specific computationally secure authentication is studied. We present detailed attacks on various practical implementations of QKD that employs this authentication. We also provide countermeasures and prove necessary and sufficient conditions for upgrading the security of the authentication to the level of unconditional security. Third, Universal hash function based multiple authentication is studied. This uses a fixed ASU2 hash function followed by one-time pad encryption, to keep the hash function secret. We show that the one-time pad is necessary in every round for the authentication to be unconditionally secure. Lastly, we study security of the WCA scheme, in the case of a partially known authentication key. Here we prove tight information-theoretic security bounds and also analyse security using witness indistinguishability as used in the Universal Composability framework.
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| 2. |
- Berthold, Stefan, 1982-
(author)
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Towards Inter-temporal Privacy Metrics
- 2011
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Licentiate thesis (other academic/artistic)abstract
- Informational privacy of individuals has significantly gained importance after information technology has become widely deployed. Data, once digitalised, can be copied and distributed at negligible costs. This has dramatic consequences for individuals that leave traces in form of personal data whenever they interact with information technology. The right of individuals for informational privacy, in particular to control the flow and use of their personal data, is easily undermined by those controlling the information technology. The objective of this thesis is the measurement of informational privacy with a particular focus on scenarios where an individual discloses personal data to a second party, the data controller, which uses this data for re-identifying the individual within a set of others, the population. Several instances of this scenario are discussed in the appended papers, most notably one which adds a time dimension to the scenario for modelling the effects of the time passed between data disclosure and usage. This extended scenario leads to a new framework for inter-temporal privacy metrics. The common dilemma of all privacy metrics is their dependence on the information available to the data controller. The same information may or may not be available to the individual and, as a consequence, the individual may be misguided in his decisions due to his limited access to the data controller’s information when using privacy metrics. The goal of this thesis is thus not only the specification of new privacy metrics, but also the contribution of ideas for mitigating this dilemma. However a solution will rather be a combination of technological, economical and legal means than a purely technical solution.
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| 3. |
- Abidin, Aysajan
(author)
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Weaknesses of Authentication in Quantum Cryptography and Strongly Universal Hash Functions
- 2010
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Licentiate thesis (other academic/artistic)abstract
- Authentication is an indispensable part of Quantum Cryptography, which is an unconditionally secure key distribution technique based on the laws of nature. Without proper authentication, Quantum Cryptography is vulnerable to “man-in-the-middle” attacks. Therefore, to guarantee unconditional security of any Quantum Cryptographic protocols, the authentication used must also be unconditionally secure. The standard in Quantum Cryptography is to use theWegman-Carter authentication, which is unconditionally secure and is based on the idea of universal hashing.In this thesis, we first investigate properties of a Strongly Universal hash function family to facilitate understanding the properties of (classical) authentication used in Quantum Cryptography. Then, we study vulnerabilities of a recently proposed authentication protocol intended to rule out a "man-in-the-middle" attack on Quantum Cryptography. Here, we point out that the proposed authentication primitive is not secure when used in a generic Quantum Cryptographic protocol. Lastly, we estimate the lifetime of authentication using encrypted tags when the encryption key is partially known. Under simplifying assumptions, we derive that the lifetime is linearly dependent on the length of the authentication key. Experimental results that support the theoretical results are also presented.
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