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Automated runtime security adaptation has great potential in providing timely and fine grained security control. In this thesis we study the practical utility of a runtime security-performance trade off for the pervasive Secure Socket Layer (SSL/TLS) protocol. To that end we address a number of research challenges. We develop an Adaptive Security methodology to extend non-adaptive legacy security systems with adaptive features. We also create a design of such an extended system to support the methodology. The design aids in identifying additional key components necessary for the creation of an adaptive security system. We furthermore apply our methodology to the Secure Socket Layer (SSL) protocol to create a design and implementation of a practical Adaptive SSL (ASSL) solution that supports runtime security adaptation in response to cross-cutting environmental concerns. The solution effectively adapts security at runtime, only reducing maximum server load by 15% or more depending on adaptation decision complexity. Next we address the security-performance trade off research challenge. Following our methodology we conduct an offline study of factors affecting server performance when security is adapted. These insights allow for the creation of policies that can trade off security and performance by taking into account the expected future state of the system under adaptation. In so doing we found that client SSL session duration, requested file size and current security algorithm play roles predicting future system state. Notably, performance deviation is smaller when sessions are longer and files are smaller and vice versa. A complete Adaptive Security solution which successfully demonstrates our methodology is implemented with trade-off policies and ASSL as key components. We show that the solution effectively utilises available processing resources to increase security whilst still respecting performance guarantees.

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The main aim of this thesis is to construct efficient protocols to help preserve privacy of users in today's digital world. We are interested in not only theoretically sound constructions but also those which could be deployed in practice. In particular, we are interested in constructions whose security could be based on standard assumptions rather than idealized ones which are hard to realize in real life. We look at different types of protocols which include: proof systems, blind signatures, group signatures and their applications. We investigate the security of such schemes and present new efficient constructions. We investigate the Groth-Sahai proof system and extend its applicability to new settings which were not possible before. We then provide some efficient implementations of different proof systems in different models and compare and contrast their efficiency. For such implementations, we present optimization techniques which make the proofs more efficient and hence more suitable for being deployed in practice. For instance, we provide details of how to efficiently batch verify such proofs which would significantly speed up the verification process. In particular, we look at the case of using proof systems for the problem of circuit satisfiability. We also look at another application of proof systems and consider the case of proving set membership with as little interaction as possible. We then turn our attention to different variants of signature schemes where we present a new efficient blind signature scheme whose security is proved under standard assumptions. We also look at group signatures and related primitives where we formally investigate the security of group blind signatures for which we present a formalized security model for the first time. This would promote more rigorous security proofs. We then present an efficient construction which has a number of desirable properties and yet its security does not rely on any non-standard assumptions.

Anonymity means that a user can authenticate herself or use some service without leaking her identity. This thesis formally studies and analyzes cryptographic protocols with anonymity preserving properties. Some of these anonymity protocols are theoretically interesting and some are practically relevant. First of all , we investigate the Fiat- Shamir transform which is a popular tool for transforming identification protocols to sigl1ature schemes. We present a generic construction which yields group signature schemes from group identification schemes via the Fiat- Shamir transform. We then provide necessary and sufficient conditions for a group identification scheme to yield a secure group signature scheme (via the Fiat- Shamir transform). We then turn our attention to the enhanced privacy-CA solution (ePCAS) which was intended to withstand chosen compromised TPM attack against the TGC privacy-CA solution (PCAS) but had never been formally analyzed. We formalize three security properties desired from the PCAS and ePCAS protocols. We then prove that the ePCAS protocol indeed meets the security requirements that we formalize. Finally, we investigate the level of anonymity provided by the UNITS/LTE protocol. We quantify the limited form of anonymity via a formal security model. We consider two forms of anonymity for UNITS/LTE: one where the mobile stations under attack are statically selected before the execution, and a second one where the adversary selects these stations adaptively. We prove that the UMTS/LTE protocol meets both of the security definitions.