2,870 research outputs found
Kronecker Graphs: An Approach to Modeling Networks
How can we model networks with a mathematically tractable model that allows
for rigorous analysis of network properties? Networks exhibit a long list of
surprising properties: heavy tails for the degree distribution; small
diameters; and densification and shrinking diameters over time. Most present
network models either fail to match several of the above properties, are
complicated to analyze mathematically, or both. In this paper we propose a
generative model for networks that is both mathematically tractable and can
generate networks that have the above mentioned properties. Our main idea is to
use the Kronecker product to generate graphs that we refer to as "Kronecker
graphs".
First, we prove that Kronecker graphs naturally obey common network
properties. We also provide empirical evidence showing that Kronecker graphs
can effectively model the structure of real networks.
We then present KronFit, a fast and scalable algorithm for fitting the
Kronecker graph generation model to large real networks. A naive approach to
fitting would take super- exponential time. In contrast, KronFit takes linear
time, by exploiting the structure of Kronecker matrix multiplication and by
using statistical simulation techniques.
Experiments on large real and synthetic networks show that KronFit finds
accurate parameters that indeed very well mimic the properties of target
networks. Once fitted, the model parameters can be used to gain insights about
the network structure, and the resulting synthetic graphs can be used for null-
models, anonymization, extrapolations, and graph summarization
MV3: A new word based stream cipher using rapid mixing and revolving buffers
MV3 is a new word based stream cipher for encrypting long streams of data. A
direct adaptation of a byte based cipher such as RC4 into a 32- or 64-bit word
version will obviously need vast amounts of memory. This scaling issue
necessitates a look for new components and principles, as well as mathematical
analysis to justify their use. Our approach, like RC4's, is based on rapidly
mixing random walks on directed graphs (that is, walks which reach a random
state quickly, from any starting point). We begin with some well understood
walks, and then introduce nonlinearity in their steps in order to improve
security and show long term statistical correlations are negligible. To
minimize the short term correlations, as well as to deter attacks using
equations involving successive outputs, we provide a method for sequencing the
outputs derived from the walk using three revolving buffers. The cipher is fast
-- it runs at a speed of less than 5 cycles per byte on a Pentium IV processor.
A word based cipher needs to output more bits per step, which exposes more
correlations for attacks. Moreover we seek simplicity of construction and
transparent analysis. To meet these requirements, we use a larger state and
claim security corresponding to only a fraction of it. Our design is for an
adequately secure word-based cipher; our very preliminary estimate puts the
security close to exhaustive search for keys of size < 256 bits.Comment: 27 pages, shortened version will appear in "Topics in Cryptology -
CT-RSA 2007
Exploring Behaviours of RESTful APIs in an Industrial Setting
A common way of exposing functionality in contemporary systems is by
providing a Web-API based on the REST API architectural guidelines. To describe
REST APIs, the industry standard is currently OpenAPI-specifications. Test
generation and fuzzing methods targeting OpenAPI-described REST APIs have been
a very active research area in recent years. An open research challenge is to
aid users in better understanding their API, in addition to finding faults and
to cover all the code. In this paper, we address this challenge by proposing a
set of behavioural properties, common to REST APIs, which are used to generate
examples of behaviours that these APIs exhibit. These examples can be used both
(i) to further the understanding of the API and (ii) as a source of automatic
test cases. Our evaluation shows that our approach can generate examples deemed
relevant for understanding the system and for a source of test generation by
practitioners. In addition, we show that basing test generation on behavioural
properties provides tests that are less dependent on the state of the system,
while at the same time yielding a similar code coverage as state-of-the-art
methods in REST API fuzzing in a given time limit
Quantile-Based Spectral Analysis in an Object-Oriented Framework and a Reference Implementation in R: The quantspec Package
Quantile-based approaches to the spectral analysis of time series have
recently attracted a lot of attention. Despite a growing literature that
contains various estimation proposals, no systematic methods for computing the
new estimators are available to date. This paper contains two main
contributions. First, an extensible framework for quantile-based spectral
analysis of time series is developed and documented using object-oriented
models. A comprehensive, open source, reference implementation of this
framework, the R package quantspec, was recently contributed to CRAN by the
author of this paper. The second contribution of the present paper is to
provide a detailed tutorial, with worked examples, to this R package. A reader
who is already familiar with quantile-based spectral analysis and whose primary
interest is not the design of the quantspec package, but how to use it, can
read the tutorial and worked examples (Sections 3 and 4) independently.Comment: 27 pages, 11 figures, R package available via CRAN
(http://cran.r-project.org/web/packages/quantspec) or GitHub
(https://github.com/tobiaskley/quantspec
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