513 research outputs found
Twistor Strings with Flavour
We explore the tree-level description of a class of N=2 UV-finite SYM
theories with fundamental flavour within a topological B-model twistor string
framework. In particular, we identify the twistor dual of the Sp(N) gauge
theory with one antisymmetric and four fundamental hypermultiplets, as well as
that of the SU(N) theory with 2N hypermultiplets. This is achieved by suitably
orientifolding/orbifolding the original N=4 setup of Witten and adding a
certain number of new topological 'flavour'-branes at the orientifold/orbifold
fixed planes to provide the fundamental matter. We further comment on the
appearance of these objects in the B-model on CP(3|4). An interesting aspect of
our construction is that, unlike the IIB description of these theories in terms
of D3 and D7-branes, on the twistor side part of the global flavour symmetry is
realised geometrically. We provide evidence for this correspondence by
calculating and matching amplitudes on both sides.Comment: 38+12 pages; uses axodraw.sty. v2: References added, minor
clarification
Weakly supervised deep learning for the detection of domain generation algorithms
Domain generation algorithms (DGAs) have become commonplace in malware that seeks to establish command and control communication between an infected machine and the botmaster. DGAs dynamically and consistently generate large volumes of malicious domain names, only a few of which are registered by the botmaster, within a short time window around their generation time, and subsequently resolved when the malware on the infected machine tries to access them. Deep neural networks that can classify domain names as benign or malicious are of great interest in the real-time defense against DGAs. In contrast with traditional machine learning models, deep networks do not rely on human engineered features. Instead, they can learn features automatically from data, provided that they are supplied with sufficiently large amounts of suitable training data. Obtaining cleanly labeled ground truth data is difficult and time consuming. Heuristically labeled data could potentially provide a source of training data for weakly supervised training of DGA detectors. We propose a set of heuristics for automatically labeling domain names monitored in real traffic, and then train and evaluate classifiers with the proposed heuristically labeled dataset. We show through experiments on a dataset with 50 million domain names that such heuristically labeled data is very useful in practice to improve the predictive accuracy of deep learning-based DGA classifiers, and that these deep neural networks significantly outperform a random forest classifier with human engineered features
Can we Construct Unbounded Time-Stamping Schemes from Collision-Free Hash Functions?
KĂ€esolevas töös uurime piiranguteta ajatempliskeemi jaoks turvaliste rĂ€sifunktsioonide konstrueerimise vĂ”imalusi kollisioonivabadest rĂ€sifunktsioonidest. Kasutades Harberi ja Stornetta poolt loodud ajatembeldusskeemi ning Buldase ja Saarepera poolt selle jaoks konstrueeritud turvatingimust uurime nn. musta kasti konstruktsioonide vĂ”imatuse tĂ”estuse vĂ”imalikkust. Kuna vĂ”imatuse tĂ”estuse lihtsaim variant on oraakliga eraldus, keskendumegi just ĂŒhe selle eralduse jaoks sobivana tunduva oraakli omaduste ja vĂ”imaluste uurimisele.
Me eeldame, et oraakel konstrueerib rÀsipuu, vÀljastab puu juurvÀÀrtuse ning annab seejÀrel sellest puust lÀhtuvalt ajatemplisertifikaate. Me tÔestame, et kui oraakli argumendiks olev musta kasti meetodil koostatud rÀsifunktsioon ainult algse rÀsifunktsiooni kollisioonipaare kontrollib vÔi nn. suurem-kui predikaati kasutab, ei saa seda oraaklit kasutada kollisioonide leidmiseks . Töö tulemused annavad lootust, et nimetatud oraakel on tÔepoolest eralduseks sobiv ja lubavad oletada, et sarnaste oraaklite edasine uurimine vÔib lÔpuks probleemi lahenduseni viia.It has been known for quite some time that collision-resistance of hash functions does not seem to give any actual security guarantees for unbounded hash-tree time-stamping, where the size of the hash-tree created by the time-stamping service is not explicitly restricted. We focus on the possibility of showing that there exist no black-box reductions of unbounded time-stamping schemes to collision-free hash functions. We propose an oracle that is probably suitable for such a separation and give strong evidence in support of that. However, the existence of a separation still remains open. We introduce the problem and give a construction of the oracle relative to which there seem to be no secure time-stamping schemes but there still exist collision-free hash function families. Although we rule out many useful collision-finding strategies (relative to the oracle) and the conjecture seems quite probable after that, there still remains a possibility that the oracle can be abused by some very smartly constructed wrappers. We also argue why it is probably very hard to give a correct proof for our conjecture
Laminations and groups of homeomorphisms of the circle
If M is an atoroidal 3-manifold with a taut foliation, Thurston showed that
pi_1(M) acts on a circle. Here, we show that some other classes of essential
laminations also give rise to actions on circles. In particular, we show this
for tight essential laminations with solid torus guts. We also show that
pseudo-Anosov flows induce actions on circles. In all cases, these actions can
be made into faithful ones, so pi_1(M) is isomorphic to a subgroup of
Homeo(S^1). In addition, we show that the fundamental group of the Weeks
manifold has no faithful action on S^1. As a corollary, the Weeks manifold does
not admit a tight essential lamination, a pseudo-Anosov flow, or a taut
foliation. Finally, we give a proof of Thurston's universal circle theorem for
taut foliations based on a new, purely topological, proof of the Leaf Pocket
Theorem.Comment: 50 pages, 12 figures. Ver 2: minor improvement
The Convex Hull Problem in Practice : Improving the Running Time of the Double Description Method
The double description method is a simple but widely used algorithm for computation of extreme points in polyhedral sets. One key aspect of its implementation is the question of how to efficiently test extreme points for adjacency. In this dissertation, two significant contributions related to adjacency testing are presented. First, the currently used data structures are revisited and various optimizations are proposed. Empirical evidence is provided to demonstrate their competitiveness. Second, a new adjacency test is introduced. It is a refinement of the well known algebraic test featuring a technique for avoiding redundant computations. Its correctness is formally proven. Its superiority in multiple degenerate scenarios is demonstrated through experimental results. Parallel computation is one further aspect of the double description method covered in this work. A recently introduced divide-and-conquer technique is revisited and considerable practical limitations are demonstrated
SymbolâRelation Grammars: A Formalism for Graphical Languages
AbstractA common approach to the formal description of pictorial and visual languages makes use of formal grammars and rewriting mechanisms. The present paper is concerned with the formalism of SymbolâRelation Grammars (SR grammars, for short). Each sentence in an SR language is composed of a set of symbol occurrences representing visual elementary objects, which are related through a set of binary relational items. The main feature of SR grammars is the uniform way they use context-free productions to rewrite symbol occurrences as well as relation items. The clearness and uniformity of the derivation process for SR grammars allow the extension of well-established techniques of syntactic and semantic analysis to the case of SR grammars. The paper provides an accurate analysis of the derivation mechanism and the expressive power of the SR formalism. This is necessary to fully exploit the capabilities of the model. The most meaningful features of SR grammars as well as their generative power are compared with those of well-known graph grammar families. In spite of their structural simplicity, variations of SR grammars have a generative power comparable with that of expressive classes of graph grammars, such as the edNCE and the N-edNCE classes
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