Construction of Multiplicative Monotone Span Program

Multiplicative monotone span program is one of the important tools to realize secure multiparty computation. It is essential to construct multiplicative monotone span programs for secure multiparty computations. For any access structure, Cramer et al. gave a method to construct multiplicative monotone span programs, but its row size became double, and the column size also increased. In this paper, we propose a new construction which can get a multiplicative monotone span program with the row size less than double without changing the column size

Adaptive Microarchitectural Optimizations to Improve Performance and Security of Multi-Core Architectures

With the current technological barriers, microarchitectural optimizations are increasingly important to ensure performance scalability of computing systems. The shift to multi-core architectures increases the demands on the memory system, and amplifies the role of microarchitectural optimizations in performance improvement. In a multi-core system, microarchitectural resources are usually shared, such as the cache, to maximize utilization but sharing can also lead to contention and lower performance. This can be mitigated through partitioning of shared caches.However, microarchitectural optimizations which were assumed to be fundamentally secure for a long time, can be used in side-channel attacks to exploit secrets, as cryptographic keys. Timing-based side-channels exploit predictable timing variations due to the interaction with microarchitectural optimizations during program execution. Going forward, there is a strong need to be able to leverage microarchitectural optimizations for performance without compromising security. This thesis contributes with three adaptive microarchitectural resource management optimizations to improve security and/or\ua0performance\ua0of multi-core architectures\ua0and a systematization-of-knowledge of timing-based side-channel attacks.\ua0We observe that to achieve high-performance cache partitioning in a multi-core system\ua0three requirements need to be met: i) fine-granularity of partitions, ii) locality-aware placement and iii) frequent changes. These requirements lead to\ua0high overheads for current centralized partitioning solutions, especially as the number of cores in the\ua0system increases. To address this problem, we present an adaptive and scalable cache partitioning solution (DELTA) using a distributed and asynchronous allocation algorithm. The\ua0allocations occur through core-to-core challenges, where applications with larger performance benefit will gain cache capacity. The\ua0solution is implementable in hardware, due to low computational complexity, and can scale to large core counts.According to our analysis, better performance can be achieved by coordination of multiple optimizations for different resources, e.g., off-chip bandwidth and cache, but is challenging due to the increased number of possible allocations which need to be evaluated.\ua0Based on these observations, we present a solution (CBP) for coordinated management of the optimizations: cache partitioning, bandwidth partitioning and prefetching.\ua0Efficient allocations, considering the inter-resource interactions and trade-offs, are achieved using local resource managers to limit the solution space.The continuously growing number of\ua0side-channel attacks leveraging\ua0microarchitectural optimizations prompts us to review attacks and defenses to understand the vulnerabilities of different microarchitectural optimizations. We identify the four root causes of timing-based side-channel attacks: determinism, sharing, access violation\ua0and information flow.\ua0Our key insight is that eliminating any of the exploited root causes, in any of the attack steps, is enough to provide protection.\ua0Based on our framework, we present a systematization of the attacks and defenses on a wide range of microarchitectural optimizations, which highlights their key similarities.\ua0Shared caches are an attractive attack surface for side-channel attacks, while defenses need to be efficient since the cache is crucial for performance.\ua0To address this issue, we present an adaptive and scalable cache partitioning solution (SCALE) for protection against cache side-channel attacks. The solution leverages randomness,\ua0and provides quantifiable and information theoretic security guarantees using differential privacy. The solution closes the performance gap to a state-of-the-art non-secure allocation policy for a mix of secure and non-secure applications

Blackbox secret sharing revisited: A coding-theoretic approach with application to expansionless near-threshold schemes

A blackbox secret sharing (BBSS) scheme works in exactly the same way for all finite Abelian groups G; it can be instantiated for any such group G and only black-box access to its group operations and to random group elements is required. A secret is a single group element and each of the n players’ shares is a vector of such elements. Share-computation and secret-reconstruction is by integer linear combinations. These do not depend on G, and neither do the privacy and reconstruction parameters t, r. This classical, fundamental primitive was introduced by Desmedt and Frankel (CRYPTO 1989) in their context of “threshold cryptography.” The expansion factor is the total number of group elements in a full sharing divided by n. For threshold BBSS with t-privacy (Formula presented)-reconstruction and arbitrary n, constructions with minimal expansion (Formula presented) exist (CRYPTO 2002, 2005). These results are firmly rooted in number theory; each makes (different) judicious choices of orders in number fields admitting a vector of elements of very large length (in the number field degree) whose corresponding Vandermonde-determinant is sufficiently controlled so as to enable BBSS by a suitable adaptation of Shamir’s scheme. Alternative approaches generally lead to very large expansion. The state of the art of BBSS has not changed for the last 17 years. Our contributions are two-fold. (1) We introduce a novel, nontrivial, effective construction of BBSS based on coding theory instead of number theory. For threshold-BBSS we also achieve minimal expansion factor O(log n).(2) Our method is more versatile. Namely, we show, for the first time, BBSS that is near-threshold, i.e., r-t is an arbitrarily small constant fraction of n, and that has expansion factor O(1), i.e., individual share-vectors of constant length (“asymptotically expansionless”). Threshold can be concentrated essentially freely across full range. We also show expansion is minimal for near-threshold and that such BBSS cannot be attained by previous methods. Our general construction is based on a well-known mathematical principle, the local-global principle. More precisely, we first construct BBSS over local rings through either Reed-Solomon or algebraic geometry codes. We then “glue” these schemes together in a dedicated manner to obtain a global secret sharing scheme, i.e., defined over the integers, which, as we finally prove using novel insights, has the desired BBSS properties. Though our main purpose here is advancing BBSS for its own sake, we also briefly address possible protocol applications

Authentication and Identity Management for the EPOS Project

The increase in the number of online services emphasizes the value of authentication and identity management that we, even without realizing, depend on. In EPOS this authentication and identity management are also crucial, by dealing and being responsible for large amounts of heterogeneous data in multiple formats and from various providers, that can be public or private. Controlling and identify the access to this data is the key. For this purpose, it is necessary to create a system capable of authenticating, authorizing, and account the usage of these services. While services in a development phase can have authentication and authorization modules directly implemented in them, this is not an option for legacy services that cannot be modified. This thesis regards the issue of providing secure and interoperable authentication and authorization framework, associated with correct identity management and an accounting module, stating the difficulties faced and how to be addressed. These issues are approached by implementing the proposed methods in one of the GNSS Data and Products TCS services, that will serve as a study case. While authentication mechanisms have improved constantly over the years, with the addition of multiple authentication factors, there is still not a clear and defined way of how authentication should be done. New security threats are always showing up, and authentication systems need to adapt and improve while maintaining a balance between security and usability. Our goal is, therefore, to propose a system that can provide a good user experience allied to security, which can be used in the TCS services or other web services facing similar problems.A importância da autenticação e gestão de identidades, de que dependemos inconscientemente, aumenta com o crescimento do número de serviços online ao nosso dispor. No EPOS, devido à disponibilização e gestão de dados heterogéneos de várias entidades, que podem ser públicas ou privadas, a existência de um sistema de autenticação e gestão de identidades é também crucial, em que o controlo e identificação do acesso a estes dados é a chave. Numa fase de desenvolvimento dos serviços, estes módulos de autenticação e autorização podem ser diretamente implementados e é possível existir uma adaptação do software aos mesmos. No entanto, há serviços já existentes, cujas alterações implicam mudanças de grande escala e uma reformulação de todo o sistema, e como tal não é exequível fazer alterações diretas aos mesmos. Esta dissertação aborda o desenvolvimento de um sistema de autenticação e autorização seguro e interoperável, associado a uma correta gestão de identidades e um módulo de controlo, identificando os problemas encontrados e propondo soluções para os mesmos. Este desenvolvimento é aplicado num dos serviços do TCS GNSS Data and Products e servirá como caso de estudo. Embora os mecanismos de autenticação tenham melhorado continuamente ao longo dos anos, com a adição de vários fatores de autenticação, ainda não existe um método único e claro de como a autenticação deve ser feita. Novas ameaças estão sempre a surgir e os sistemas atuais precisam de se adaptar e melhorar, mantendo um equilíbrio entre segurança e usabilidade. O nosso objetivo é propor um sistema que possa aliar a segurança a uma boa experiência para o utilizador, e que possa ser utilizado não só nos serviços do TCS, mas também em outros serviços web que enfrentem problemas semelhantes

Oblivious Pseudo-Random Functions via Garbled Circuits

An Oblivious Pseudo-Random Function (OPRF) is a protocol that allows two parties – a server and a user – to jointly compute the output of a Pseudo-Random Function (PRF). The server holds the key for the PRF and the user holds an input on which the function shall be evaluated. The user learns the correct output while the inputs of both parties remain private. If the server can additionally prove to the user that several executions of the protocol were performed with the same key, we call the OPRF verifiable. One way to construct an OPRF protocol is by using generic tools from multi-party computation, like Yao’s seminal garbled circuits protocol. Garbled circuits allow two parties to evaluate any boolean circuit, while the input that each party provides to the circuit remains hidden from the respective other party. An approach to realizing OPRFs based on garbled circuits was e.g. mentioned by Pinkas et al. (ASIACRYPT ’09). But OPRFs are used as a building block in various cryptographic protocols. This frequent usage in conjunction with other building blocks calls for a security analysis that takes composition, i.e., the usage in a bigger context into account. In this work, we give the first construction of a garbled-circuit-based OPRF that is secure in the universal composability model by Canetti (FOCS ’01). This means the security of our protocol holds even if the protocol is used in arbitrary execution environments, even under parallel composition. We achieve a passively secure protocol that relies on authenticated channels, the random oracle model, and the security of oblivious transfer. We use a technique from Albrecht et al. (PKC ’21) to extend the protocol to a verifiable OPRF by employing a commitment scheme. The two parties compute a circuit that only outputs a PRF value if a commitment opens to the right server-key. Further, we implemented our construction and compared the concrete efficiency with two other OPRFs. We found that our construction is over a hundred times faster than a recent lattice-based construction by Albrecht et al. (PKC ’21), but not as efficient as the state-of-the-art protocol from Jarecki et al. (EUROCRYPT ’18), based on the hardness of the discrete logarithm problem in certain groups. Our efficiency-benchmark results imply that – under certain circumstances – generic techniques as garbled circuits can achieve substantially better performance in practice than some protocols specifically designed for the problem. Büscher et al. (ACNS ’20) showed that garbled circuits are secure in the presence of adversaries using quantum computers. This fact combined with our results indicates that garbled-circuit-based OPRFs are a promising way towards efficient OPRFs that are secure against those quantum adversaries