Alexei Vernitski

Optimized hash for network path encoding with minimized false positives

The Bloom filter is a space efficient randomized data structure for representing a set and supporting membership queries. Bloom filters intrinsically allow false positives. However, the space savings they offer outweigh the disadvantage if the false positive rates are kept sufficiently low. Inspired by the recent application of the Bloom filter in a novel multicast forwarding fabric, this paper proposes a variant of the Bloom filter, the optihash. The optihash introduces an optimization for the false positive rate at the stage of Bloom filter formation using the same amount of space at the cost of slightly more processing than the classic Bloom filter. Often Bloom filters are used in situations where a fixed amount of space is a primary constraint. We present the optihash as a good alternative to Bloom filters since the amount of space is the same and the improvements in false positives can justify the additional processing. Specifically, we show via simulations and numerical analysis that using the optihash the false positives occurrences can be reduced and controlled at a cost of small additional processing. The simulations are carried out for in-packet forwarding. In this framework, the Bloom filter is used as a compact link/route identifier and it is placed in the packet header to encode the route. At each node, the Bloom filter is queried for membership in order to make forwarding decisions. A false positive in the forwarding decision is translated into packets forwarded along an unintended outgoing link. By using the optihash, false positives can be reduced. The optimization processing is carried out in an entity termed the Topology Manger which is part of the control plane of the multicast forwarding fabric. This processing is only carried out on a per session basis, not for every packet. The aim of this paper is to present the optihash and evaluate its false positive performances via simulations in order to measure the influence of different parameters on the false positive rate. The false positive rate for the optihash is then compared with the false positive probability of the classic Bloom filter.

Filters in (Quasiordered) Semigroups and Lattices of Filters

A filter in a semigroup is a subsemigroup whose complement is an ideal. (Alternatively, in a quasiordered semigroup, a slightly more general definition can be given.) We prove a number of results related to filters in a semigroup and the lattice of filters of a semigroup. For instance, we prove that every complete algebraic lattice can be the lattice of filters of a semigroup. We prove that every finite semigroup is a homomorphic image of a finite semigroup whose lattice of filters is boolean and which belongs to the pseudovariety generated by the original semigroup. We describe filter lattices of some well-known semigroups such as full transformation semigroups of finite sets (which are three-element chains) and free semigroups (which are boolean).

Ordered and J-trivial semigroups as divisors of semigroups of languages

A semigroup of languages is a set of languages considered with (and closed under) the operation of catenation. In other words, semigroups of languages are subsemigroups of power-semigroups of free semigroups. We prove that a (finite) semigroup is positively ordered if and only if it is a homomorphic image, under an order-preserving homomorphism, of a (finite) semigroup of languages. Hence it follows that a finite semigroup is -trivial if and only if it is a homomorphic image of a finite semigroup of languages.

Finite quasivarieties and self-referential conditions

In this paper, we concentrate on finite quasivarieties (i.e. classes of finite algebras defined by quasi-identities). We present a motivation for studying finite quasivarieties. We introduce a new type of conditions that is well suited for defining finite quasivarieties and compare these new conditions with quasi-identities.

Too Taxing on the Mind! Authentication Grids are not for Everyone

The security and usability issues associated with passwords have encouraged the development of a plethora of alternative authentication schemes. These aim to provide stronger and/or more usable authentication, but it is hard for the developers to anticipate how users will perform with and react to such schemes. We present a case study of a one-time password entry method called the Vernitski Authentication Grid VAG, which requires users to enter their password in pairs of characters by finding where the row and the column containing the characters intersect and entering the character from this intersection. We conducted a laboratory user evaluation ni¾ź=i¾ź36 and found that authentication took 88.6i¾źs on average, with login times decreasing with practice. Participants were faster authenticating on a tablet than on a PC. Overall, participants found using the grid complex and time-consuming. Their stated willingness to use it depended on the context of use, with most participants considering it suitable for accessing infrequently used and high-stakes accounts and systems. While using the grid, 31 out of 36 participants pointed at the characters, rows and columns with their fingers or mouse, which undermines the shoulder-surfing protection that the VAG is meant to offer. Our results demonstrate there cannot be a one-size-fits-all replacement for passwords --- usability and security can only be achieved through schemes designed to fit a specific context of use.

An approximate dynamic programming approach for improving accuracy of lossy data compression by Bloom filters

Bloom filters are a data structure for storing data in a compressed form. They offer excellent space and time efficiency at the cost of some loss of accuracy (so-called lossy compression). This work presents a yes–no Bloom filter, which as a data structure consisting of two parts: the yes-filter which is a standard Bloom filter and the no-filter which is another Bloom filter whose purpose is to represent those objects that were recognized incorrectly by the yes-filter (that is, to recognize the false positives of the yes-filter). By querying the no-filter after an object has been recognized by the yes-filter, we get a chance of rejecting it, which improves the accuracy of data recognition in comparison with the standard Bloom filter of the same total length. A further increase in accuracy is possible if one chooses objects to include in the no-filter so that the no-filter recognizes as many as possible false positives but no true positives, thus producing the most accurate yes–no Bloom filter among all yes–no Bloom filters. This paper studies how optimization techniques can be used to maximize the number of false positives recognized by the no-filter, with the constraint being that it should recognize no true positives. To achieve this aim, an Integer Linear Program (ILP) is proposed for the optimal selection of false positives. In practice the problem size is normally large leading to intractable optimal solution. Considering the similarity of the ILP with the Multidimensional Knapsack Problem, an Approximate Dynamic Programming (ADP) model is developed making use of a reduced ILP for the value function approximation. Numerical results show the ADP model works best comparing with a number of heuristics as well as the CPLEX built-in solver (B&B), and this is what can be recommended for use in yes–no Bloom filters. In a wider context of the study of lossy compression algorithms, our research is an example showing how the arsenal of optimization methods can be applied to improving the accuracy of compressed data.

Automated Reasoning for Knot Semigroups and \pi π -orbifold Groups of Knots.

The paper continues the first author’s research which shows that automatic reasoning is an effective tool for establishing properties of algebraic constructions associated with knot diagrams. Previous research considered involutory quandles (also known as keis) and quandles. This paper applies automated reasoning to knot semigroups, recently introduced and studied by the second author, and \(\pi \)-orbifold groups of knots. We test two conjectures concerning knot semigroups (specifically, conjectures aiming to describe knot semigroups of diagrams of the trivial knot and knot semigroups of 4-plat knot diagrams) on a large number of examples. These experiments enable us to formulate one new conjecture. We discuss applications of our results to a classical problem of the knot theory, determining whether a knot diagram represents the trivial knot.

Conjugacy and Other Properties of One-Relator Groups

In this note, we prove that certain one-relator groups are residually finite and have solvable (power) conjugacy problem, by an examination of the co-primeness of the exponent sum of some of the generators appearing in the relator.

Routing in hexagonal computer networks: How to present paths by Bloom filters without false positives

In this study it is introduced that the structure behind a random data structure called Bloom filter is applied to a routing scheme in a hexagonal grid in two dimensional case. The Bloom filter is a method to store the data in a very space efficiency and processing simplicity. We construct the Bloom filters for a complicated routing structure in a hexagonal mesh. It is aimed to obtain a fastest scheme without errors. A coding structure for the edges is developed to increase the efficiency use of network resources.

On Using the Join Operation to Define Classes of Algebras

We call a class of algebras a finitary prevariety if the class is closed under the formation of subalgebras and finitary direct products, and contains the one-element algebra. The join of two finitary prevarieties and a concept of a join-irreducible finitary prevariety may be introduced naturally. We develop techniques for proving that a finitary prevariety of semigroups is join-irreducible, and find many examples of join-irreducible finitary prevarieties of semigroups. For example, we prove that if a class of finite semigroups is defined by ω-identities and contains the class J, then it is a join-irreducible finitary prevariety.