Computer Sciences Commons^™

Privacy-Enhanced Electronic Mail, Matt Bishop

Computer Science Technical Reports

The security of electronic mail sent through the Internet may be described in exactly three words: there is none. The Privacy and Security Research Group has recommended implementing mechanisms designed to provide security enhancements. The first set of mechanisms provides a protocol to provide privacy, integrity, and authentication for electronic mail; the second provides a certificate-based key management infrastructure to support key distribution throughout the internet, to support the first set of mechanisms. This paper describes these mechanisms, as well as the reasons behind their selection and how these mechanisms can be used to provide some measure of securtiy in …

Implementation Notes On Bdes(1), Matt Bishop

Computer Science Technical Reports

This note describes the implementation of bdes, the file encryption program being distributed in the 4.4 release of the Berkeley Software Distribution. It implements all modes of the Data Encryption Standard program.

Connected Components In O(Lg3/2|V|) Parallel Time For The Crew Pram, Donald B. Johnson, Panagiotis Metaxas

Computer Science Technical Reports

Computing the connected components of an undirected graph G = (V,E) on |V| = n vertices and |E| = m edges is a fundamental computational problem. The best known parallel algorithm for the CREW PRAM model runs on O(lg2n) time using n2/lg2n processors [CLC82,HCS79]. For the CRCW PRAM model in which concurrent writing is permitted, the best known algorithm runs in O(lg n) time using almost (n+m)/lg n processors [SV82,CV86,AS87]. Unfortunately, simulating this algorithm on the weaker CREW model increases its running time to O(lg2n) [CDR86, KR90,Vis83]. We present here an efficient and simple algorithm that runs in O(lg 3/2n) …

Optimal Parallel And Sequential Algorithms For The Vertex Updating Problem Of A Minimum Spanning Tree, Donald B. Johnson, Panagiotis Metaxas

Computer Science Technical Reports

We present a set of rules that can be used to give optimal solutions to the vertex updating problem for a minimum spanning tree: Update a given MST when a new vertex z is introducted, along with weighted edges that connect z with the vertices of the graph. These rules lead to simple parallel algorithms that run in O(lg n) parallel time using n/lg n EREW PRAMs. They can also be used to derive simple linear-time sequential algorithms for the same problem. Furthermore, we show how our solution can be used to solve the multiple vertex updating problem.

Multipacket Routing On Rings, Fillia Makedon, Adonios Simvonis

Computer Science Technical Reports

We study multipacket routing problems. We divide the multipacket routing problem into two classes, namely, distance limited and bisection limited routing problems. Then, we concentrate on rings of processors. Having a full understanding of the multipacket routing problem on rings is essential before trying to attack the problem for the more general case of r-dimensional meshes and tori. We prove a new lower bound of 2n/3 routing steps for the case of distance limited routing problems. We also give an algorithm that tightens this lower bound. For bisection limited problems, we present an algorithm that completes the routing in near …

A Parallel Algorithm For The Minimum Spanning Tree, Donald B. Johnson, Panagiotis Metaxas

Computer Science Technical Reports

No abstract provided.

A Security Analysis Of Version 2 Of The Network Time Protocol Ntp: A Report To The Privacy And Security Research Group, Matt Bishop

Computer Science Technical Reports

The Network Time Protocol is being used throughout the Internet to provide an accurate time service. This paper examines the security requirements of such a service, analyzes version 2 of the NTP protocol to determine how well it meets these requirements, and suggests improvements where appropriate.

An Overview Of Computer Viruses In A Research Environment, Matt Bishop

Computer Science Technical Reports

The threat of attack by computer viruses is in reality a very small part of a much more general threat, specifically attacks aimed at subverting computer security. This paper examines computer viruses as malicious logic in a research and development environment, relates them to various models of security and integrity, and examines current research techniques aimed at controlling the threats viruses in particular, and malicious logic in gerneral, pose to computer systems. Finally, a brief examination of the vulnerabilities of research and development systems that malicious logic and computer viruses may exploit is undertaken.

A Tight Upper Bound On The Benefits Of Replication And Consistency Control Protocols, Donald B. Johnson, Larry Raab

Computer Science Technical Reports

We present an upper bound on the performance provided by a protocol guaranteeing mutually exclusive access to a replicated resource in a network subject to component failure and subsequent partitioning. The bound is presented in terms of the performance of a single resource in the same network. The bound is tight and is the first such bound known to us. Since mutual exclusion is one of the requirements for maintaining the consistency of a database object, this bound provides an upper limit on the availability provided by any database consistency control protocol, including those employing dynamic data relocation and replication. …

Robot Pedagogics: The Adaptation, Analysis, And Computer Control Of A Model Manipulator, Edward T. Hammerand, Chung You Ho

Computer Science Technical Reports

The subject of robotics is addressed by many different fields, among them computer science, electrical engineering, and mechanical engineering. This work is an attempt to bring together all of these aspects from the perspective of a computer science background. Different techniques are considered and reconciled with one another in the analytical area, while detail and explanation are added in all areas that were not previously available. In addition, geometrical interpretations arc presented for concepts that have heretofore been presented only in the form of equations.

A Direct Access Method Using A Neural Network Model, John William Meyer, George Winston Zobrist

Computer Science Technical Reports

One of the concerns in computer science involves optimizing usage of machines to make them more efficient and cost effective. One item of particular concern is the use of secondary storage devices, devices that store data other than in the main memory of the computer to which it is attached. The times for searching for data on these devices consistently proves to be a contributing factor in inefficient computer usage.

One data access method that avoids searching when possible is the hashing method. A function is defined to return the record number of a record based on its key field. …

Input Data Pattern Encoding For Neural Net Algorithms, Hyeoncheol Kim, George Winston Zobrist

Computer Science Technical Reports

First, a brief overview of neural networks and their applications are described, including the BAM (Bidirectional Associative Memory) model.

A bucket-weight-matrix scheme is proposed, which is a data pattern encoding method that is necessary to transform a set of real-world numbers into neural network state numbers without losing the pattern property the set has. The scheme is designed as a neural net so that it can be combined with other data processing neural nets. The net itself can be used as a bucket-sorting net also. This shows that traditional data structure problems can be an area that neural networks may …

Algorithms And Probabilistic Bounds For The Chromatic Number Of Random Composite Graphs, Jack L. Oakes, Billy E. Gillett

Computer Science Technical Reports

The composite graph coloring problem (CGCP) is a generalization of the standard graph coloring problem (SGCP). Associated with each vertex is a positive integer called its chromaticity. The chromaticity of a vertex specifies the number of consecutive colors which must be assigned to it.

An exact algorithm for solving the CGCP is presented. The algorithm is a generalization of the vertex-sequential with dynamic reordering approach for the SGCP. It is shown that the method is as effective on composite graphs as its counterpart is on standard graphs. Let X̅(CG_np) and X̅(SG_np) denote, respectively, the mean chromatic …

A Bound Of Data Availability When Networks Partition, Michael Goldweber, Donald B. Johnson

Computer Science Technical Reports

Many consistency or replication control schemes that increase data availability in distributed systems exist, and the search for improvements continues, though there have been no good nontrivial upper bound demonstrating how much improvement is possible. We present a new upper bound for data availability under replication for general networks. In addition we also describe a new technique that yields near optimal levels of data availability with respect to this bound.

A Fast O(K) Multicast Message Routing Algorithm, Thomas J. Sager, Bruce M. Mcmillin

Computer Science Technical Reports

In many multicomputer applications is it necessary for one node to send an identical message to many nodes. One to many communications are called multicasts. Although broadcasts (one to all) and unicasts (one to one) have been widely implemented, multicasts, in spite of their importance to efficient use of multicomputer systems, have not received much attention.

Minimal traffic multicasting is equivalent to the minimal Steiner tree problem and is known to be NP-complete. Therefore, a heuristic polynomial time approximation must be used; but, to take advantage of advances in second generation multicomputer hardware such as wormhole routing, a distributed multicast …

Computational Complexity Of Geometric Symmetry Detection In Graphs, Joseph Manning

Computer Science Technical Reports

Constructing a visually informative drawing of an abstract graph is a problem of considerable practical importance, and has recently been the focus of much investigation. Displaying symmetry has emerged as one of the foremost criteria for achieving good drawings. Linear-time algorithms are already known for the detection and display of symmetry in trees, outerplanar graphs, and embedded planar graphs. The central results of this paper show that for general graphs, however, detecting the presence of even a single axial or rotational symmetry is NP-complete. A number of related results are also established, including the #P-completeness of counting the axial or …

Planar Graphs And Sparse Graphs From Efficient Motion Planning In The Plane, L Paul Chew

Computer Science Technical Reports

Given a source, a destination, and a number of obstacles in the plane, the Motion Planning Program is to determine the best path to move an object (a robot) from the source to the destination without colliding with any of the obstacles. For us, motion is restricted to the plane, the robot is represented by a point, and the obstacles are represented by a set of polygons with a total of n vertices among all the polygonal obstacles.

Building Voronoi Diagrams For Convex Polygons In Linear Expected Time, L Paul Chew

Computer Science Technical Reports

Let P be a list of points in the plane such that the points of P taken in order form the vertices of a convex polygon. We introduce a simple, linear expected-time algorithm for finding the Voronoi diagram of the points in P. Unlike previous results on expected-time algorithms for Voronoi diagrams, this method does not require any assumptions about the distribution of points. With minor modifications, this method can be used to design fast algorithms for certain problems involving unrestricted sets of points. For example, fast expected-time algorithms can be designed to delete a point from a Voronoi diagram, …

Term Reduction Using Directed Congruence Closure, L Paul Chew

Computer Science Technical Reports

Many problems in computer science can be described in terms of reduction rules that tell how to transform terms. Problems that can be handled in this way include interpreting programs, implementing abstract data types, and proving certain kinds of theorems. A terms is said to have a normal form if it can be transformed, using the reduction rules, into a term to which no further reduction rules apply. In this paper, we extend the Congruence Closure Algorithm, an algorithm for finding the consequences of a finite set of equations, to develop Directed Congruence Closure, a technique for finding the normal …

Applying The Take-Grant Protection Model, Matt Bishop

Computer Science Technical Reports

The Take-Grant Protection Model has in the past been used to model multilevel security hierarchies and simple protection systems. The models are extended to include theft of rights and sharing of information, and additional security policies are examined. The analysis suggests that in some cases the basic rules of the Take-Grant Protection Model should be augmented to represent the policy properly; when appropriate, such modifications are made and their effects with respect to the policy and its Take-Grant representations are discussed.

Administrator's Guide To The Digital Signature Facility "Rover", Matt Bishop

Computer Science Technical Reports

This document describes the installation and maintenance of the rover utility, which provides a digital signature capability for internet messages.

Effects Of Replication On Data Availability, Donald B. Johnson, Larry Raab

Computer Science Technical Reports

In this paper we examine the effects of replication on the availability of data in a large network. This analysis differs from previous analyses in that it compares the performance of a dynamic consistency control protocol not only to that of other consistency control protocols, but also to the performance of non-replication and to an upper bound on data availability. This analysis also differes in that we gather extensive simulations on large networks subject to partitions at realistically high component reliabilities. We examine the dynamic consistency protocol presented by Jajodia and Mutchler [9, 12] and by Long and Paris[18] along …

Finding Optimal Quorum Assigments For Distributed Databases, Donald B. Johnson, Larry Raab

Computer Science Technical Reports

Replication has been studied as a method of increasing the availability of a data item in a distributed database subject to component failures and consequent partitioning. The potential for partitioning requires that a protocol be employed which guarantees that any access to a data item is aware of the most recent update to that data item. By minimizing the number of access requests denied due to this constraint, we maximize availability. In the event that all access requests are reads, placing one copy of the data item at each site clearly leads to maximum availability. The other extreme, all access …

A Proactive Password Checker, Matt Bishop

Computer Science Technical Reports

Password selection has long been a difficult issue; traditionally, passwords are either assigned by the computer or chosen by the user. When the computer does the assignments, the passwords are often hard to remember; when the User makes the selection, the passwords are often easy to guess. This paper describes a technique, and a mechanism, to allow users to select passwords which to them are easy to remember but to others would be very difficult to guess. The technique is site, user, and group configurable, and allows rapid changing of constraints impossed upon the passwords. Although experience with this technique …

Development Of An Expert System To Convert Knowledge-Based Geological Engineering Systems Into Fortran, Jill J. Cress, Ralph W. Wilkerson

Computer Science Technical Reports

A knowledge-based geographic information system (KBGIS) for geological engineering map (GEM) production was developed in GoldWorks, an expert system development shell. GoldWorks allows the geological engineer to develop a rule base for a GEM application. Implementation of the resultant rule base produced a valid GEM, but took too much time. This proved that knowledge-based GEM production was possible but in GoldWorks implementation failed as a practical production system. To solve this problem, a Conversion Expert System was developed which accepted, as input, a KBGIS and produced, as output, the equivalent Fortran code. This allowed the engineer to utilize GoldWorks for …

The Directed Steiner Problem On Graphs: A Simulated Annealing Approach, Lawrence Joseph Osborne, Billy E. Gillett

Computer Science Technical Reports

The well-known Steiner Problem on Graphs is an NP-complete problem for which there are many heuristic and exact algorithms that are deterministic. In this dissertation a new approach to the directed version of this problem is made by applying the ideas of statistical mechanics through the use of the method of simulated annealing. A version of annealing is developed for the Directed Steiner Problem and compared with one of the best general annealing schemes. Then a comparison is made between simulated annealing and the traditional branch and bound technique. The dual ascent algorithm of Richard T. Wong is used to …

A Comparison Of Consistency Control Protocols, Michael Goldweber, Donald B. Johnson, Larry Raab

Computer Science Technical Reports

In this paper we analyze three protocols for maintaining the mutual consistency of replicated objects in a distributed computing environment and compare their performance with that of an oracle protocol whose performance is optimal. We examine these protocols, two dynamic protocols and the majority consensus protocol, via simulations using two measures of availability. The analysis shows that the dynamic protocols, under realistic assumptions, do not perform significantly better than the static voting scheme. Finally we demonstrate that none of these approaches perform as well as our oracle protocol which is shown to be an upper bound on availability.

Automated Translation Of Digital Logic Equations Into Optimized Vhdl Code, John Evan Stark, George Winston Zobrist

Computer Science Technical Reports

It was desired to develop an algorithm for the automated translation of finite slate machines from state table form to optimized VHDL form. To do this, algorithms arc needed for reducing the state machine to simplest form, making state assignments, producing minimal logic equations to represent the state machine, and producing VHDL code which describes the intended circuit. Various such algorithms were examined and a prototype program written to perform this translation.

An Improved Exact Graph Coloring Algorithm, Thomas J. Sager, Shi-Jen Lin

Computer Science Technical Reports

We present two algorithms for exact graph coloring of the vertex sequential with dynamic reordering of vertices variety. The first, W-DEG, is a straight-forward improvement on Korman’s original algorithm. The second, SWAP2, is a not so straight forward improvement on Korman’s algorithm and appears to offer the best performance of known exact graph coloring algorithms.

A Color-Exchange Algorithm For Exact Graph Coloring, Thomas J. Sager, Shi-Jen Lin

Computer Science Technical Reports

DEXCH, a color-exchange exact graph coloring algorithm is presented. On many classes of graphs, DEXCH can, in the mean, find the chromatic number of a graph considerably faster than the DSATUR algorithm. The improvement over DSATUR stems from the ability to reorganize the subset of colored vertices and to detect in certain instances the existence of a complete subgraph of cardinality equal to the number of colors used in the best coloring found so far. The mean improvement over DSATUR is greatest on high edge-density graphs attaining the value of 42% on random graphs of edge-density 0.7 on 64 vertices.