General two-dimensional linear flows of particle suspensions

This thesis investigates the flow of suspensions of solid spheres in a viscous fluid. We look at a monolayer of particles in an unbounded fluid, and carry out numerical simulations of its behaviour under a variety of linear flows. In chapter 1 we review the field and discuss the different approaches to simulating a suspension of solid spheres in a viscous fluid. We outline the case for the method of Stokesian Dynamics, and explain its derivation. In chapter 2 we introduce the concept of a spatially periodic lattice which self-replicates in time under flow. We then go on to derive a suitable periodic box for each possible two dimensional linear flow, from pure strain to pure rotation, through simple shear and flows of intermediate type. Using the numerical method of Stokesian Dynamics, in chapter 3 we proceed to investigate the macroscopic properties of our two-dimensional suspension in the various flows. The viscosity and normal stress difference are probed at both short and long times. We find evidence of crystallisation, and our major discovery is that crystallisation sets in earlier (in terms of increasing concentration) for flows that are closer to shear flow than those with a larger component of rotation or of strain. We also present results on the duration of transients in start up flow. In chapter 4 we consider the effects of surface roughness on viscosity. Two different models for roughness are considered, the usual hard contact and a new soft contact model first proposed by Wilson in [65]. A comparison of the results of the two models is undertaken and we discuss about the effects of lower viscosity occurring at low concentrations due to surface roughness. In Appendix A we consider the method of Ewald summation which can be used to properly account for far-field interactions in a lattice-periodic system, and derive the relevant forms for a system which is periodic in only two dimensions. Unfortunately we discover a problem with the Hankel transform but the real space relations are still valid. This will have application both to monolayer systems such as the one we have studied, and to confined suspensions in a variety of applications where the relevant geometry has a large aspect ratio

Ideal hierarchical secret sharing schemes

Hierarchical secret sharing is among the most natural generalizations of threshold secret sharing, and it has attracted a lot of attention from the invention of secret sharing until nowadays. Several constructions of ideal hierarchical secret sharing schemes have been proposed, but it was not known what access structures admit such a scheme. We solve this problem by providing a natural definition for the family of the hierarchical access structures and, more importantly, by presenting a complete characterization of the ideal hierarchical access structures, that is, the ones admitting an ideal secret sharing scheme. Our characterization deals with the properties of the hierarchically minimal sets of the access structure, which are the minimal qualified sets whose participants are in the lowest possible levels in the hierarchy. By using our characterization, it can be efficiently checked whether any given hierarchical access structure that is defined by its hierarchically minimal sets is ideal. We use the well known connection between ideal secret sharing and matroids and, in particular, the fact that every ideal access structure is a matroid port. In addition, we use recent results on ideal multipartite access structures and the connection between multipartite matroids and integer polymatroids. We prove that every ideal hierarchical access structure is the port of a representable matroid and, more specifically, we prove that every ideal structure in this family admits ideal linear secret sharing schemes over fields of all characteristics. In addition, methods to construct such ideal schemes can be derived from the results in this paper and the aforementioned ones on ideal multipartite secret sharing. Finally, we use our results to find a new proof for the characterization of the ideal weighted threshold access structures that is simpler than the existing one.Peer ReviewedPostprint (author's final draft

Trollthrottle -- Raising the Cost of Astroturfing

Astroturfing, i.e., the fabrication of public discourse by private or state-controlled sponsors via the creation of fake online accounts, has become incredibly widespread in recent years. It gives a disproportionally strong voice to wealthy and technology-savvy actors, permits targeted attacks on public forums and could in the long run harm the trust users have in the internet as a communication platform. Countering these efforts without deanonymising the participants has not yet proven effective; however, we can raise the cost of astroturfing. Following the principle `one person, one voice', we introduce Trollthrottle, a protocol that limits the number of comments a single person can post on participating websites. Using direct anonymous attestation and a public ledger, the user is free to choose any nickname, but the number of comments is aggregated over all posts on all websites, no matter which nickname was used. We demonstrate the deployability of Trollthrottle by retrofitting it to the popular news aggregator website Reddit and by evaluating the cost of deployment for the scenario of a national newspaper (168k comments per day), an international newspaper (268k c/d) and Reddit itself (4.9M c/d)

Verifiable Sealed-Bid Auction on the Ethereum Blockchain

The success of the Ethereum blockchain as a decentralized application platform with a distributed consensus protocol has made many organizations start to invest into running their business on top of it. Technically, the most impressive feature behind the success of Ethereum is its support for a Turing complete language.On the other hand, the inherent transparency and, consequently, the lack of privacy poses a great challenge for many financial applications. In this paper, we tackle this challenge and present a smart contract for a verifiable sealed-bid auction on the Ethereum blockchain. In a nutshell, initially, the bidders submit homomorphic commitments to their sealed-bids on the contract. Subsequently, they reveal their commitments secretly to the auctioneer via a public key encryption scheme. Then, according to the auction rules, the auctioneer determines and claims the winner of the auction. Finally, we utilize interactive zero-knowledge proof protocols between the smart contract and the auctioneer to verify the correctness of such a claim. The underlying protocol of the proposed smart contract is partially privacy-preserving. To be precise, no information about the losing bids is leaked to the bidders. We provide an analysis of the proposed protocol and the smart contract design, in addition to the estimated gas costs associated with the different transactions

Combinatorial Bounds and Characterizations of Splitting Authentication Codes

We present several generalizations of results for splitting authentication codes by studying the aspect of multi-fold security. As the two primary results, we prove a combinatorial lower bound on the number of encoding rules and a combinatorial characterization of optimal splitting authentication codes that are multi-fold secure against spoofing attacks. The characterization is based on a new type of combinatorial designs, which we introduce and for which basic necessary conditions are given regarding their existence.Comment: 13 pages; to appear in "Cryptography and Communications

Some remarks on fair exchange protocol

Abstract. Fair exchange turns out to be an increasingly importanttopic due to the rapid growth of electronic commerce. An exchange is deemed to be fair if at the end of exchange, either each party receives the expected item or neither party receives any useful information about the other's item. Several protocols for fair exchange have been proposed in recent years. In this paper, we rst examine a newly published fair exchange protocol and point out its aws and weaknesses. We then put forward a more e cient and secure protocol and give an informal analysis

Secure multiparty PageRank algorithm for collaborative fraud detection

Collaboration between financial institutions helps to improve detection of fraud. However, exchange of relevant data between these institutions is often not possible due to privacy constraints and data confidentiality. An important example of relevant data for fraud detection is given by a transaction graph, where the nodes represent bank accounts and the links consist of the transactions between these accounts. Previous works show that features derived from such graphs, like PageRank, can be used to improve fraud detection. However, each institution can only see a part of the whole transaction graph, corresponding to the accounts of its own customers. In this research a new method is described, making use of secure multiparty computation (MPC) techniques, allowing multiple parties to jointly compute the PageRank values of their combined transaction graphs securely, while guaranteeing that each party only learns the PageRank values of its own accounts and nothing about the other transaction graphs. In our experiments this method is applied to graphs containing up to tens of thousands of nodes. The execution time scales linearly with the number of nodes, and the method is highly parallelizable. Secure multiparty PageRank is feasible in a realistic setting with millions of nodes per party by extrapolating the results from our experiments

Solving Problems of Practice in Education

The authors identify and discuss the many complexities involved in the translation of scientific information in the social sciences into forms usable for solving problems of practice in education. As a means of appropriately handling these complexities and the issues that arise, they prescribe a series of stages to be followed from the advent of a practitioner's situational problem to the design of a response to it. They assert that unless the process of translation is conducted with the prescribed level of understanding, appreciation, and rigor, the application of knowledge will be inaccurate.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/68934/2/10.1177_107554708400600103.pd

Fast multi-computations with integer similarity strategy

Abstract. Multi-computations in finite groups, such as multiexponentiations and multi-scalar multiplications, are very important in ElGamallike public key cryptosystems. Algorithms to improve multi-computations can be classified into two main categories: precomputing methods and recoding methods. The first one uses a table to store the precomputed values, and the second one finds a better binary signed-digit (BSD) representation. In this article, we propose a new integer similarity strategy for multi-computations. The proposed strategy can aid with precomputing methods or recoding methods to further improve the performance of multi-computations. Based on the integer similarity strategy, we propose two efficient algorithms to improve the performance for BSD sparse forms. The performance factor can be improved from 1.556 to 1.444 and to 1.407, respectively

Migrant women, place and identity in contemporary women's writing

While recent scholarship on migration has reflected growing attention to gender, and to the intersectionality of race, gender and sexuality, there has been little focus on women's emotional and bodily responses to migration in the context of larger structures of sexism, racism, and the legacies of colonialism. In this paper I examine some literary portrayals of how migrant women's relationships with specific places of origin and settlement, both steeped in structural relationships of unequal power and experienced on an immediate, psychological and bodily plane, are fundamental to migrant women's changing sense of belonging and identity. Jamaica Kincaid in her novel Lucy, Tsitsi Dangarembga in her novel Nervous Conditions, and Dionne Brand in the opening poems of her volume No Language is Neutral evoke some of the complex ways in which migration can affect women's lives and identities, thus both complementing and critiquing some contemporary theorisations of migration and migrant identities