Linda Morales

Combinatorial Optimization of Group Key Management

Given the growing number of group applications in many existing and evolving domains recent attention has been focused on secure multicasting over the Internet. When such systems are required to manage large groups that undergo frequent fluctuations in group membership, the need for efficient encryption key management becomes critical. This paper presents a new key management framework based on a combinatorial formulation of the group multicast key management problem that is applicable to the general problem of managing keys for any type of trusted group communication, regardless of the underlying transmission method between group participants. Specifically, we describe Exclusion Basis Systems and show exactly when they exist. In addition, the framework separates key management from encrypted message transmission, resulting in a more efficient implementation of key management.

An (18/11)n upper bound for sorting by prefix reversals

The pancake problem asks for the minimum number of prefix reversals sufficient for sorting any permutation of length n. We improve the upper bound for the pancake problem to (18/11)n+O(1)?(1.6363)n.

Combinatorial optimization of multicast key management

There are numerous applications that require secure group communication. Much recent attention has been focused on secure multicasting over the Internet. When such systems are required to manage large groups which undergo frequent fluctuations in group membership, the need for efficient encryption key management becomes critical. This paper presents a combinatorial formulation of the multicast key management problem that is applicable not only to the specific problem of multicast key management, but also to the general problem of managing keys for any type of trusted group communication, regardless of the underlying transmission method between group participants. Specifically, we describe exclusion basis systems, show exactly when they exist, and demonstrate that such systems represent improvements over the current binary tree-based key management systems and other related systems.

Efficient Algorithms for Batch Re-Keying Operations in Secure Multicast

Secure multicast has a variety of applications in e-commerce, e-banking, command and control, video-on-demand, and other internet-based services. In this paper, we present algorithms to improve on the number of re-keying messages (overhead) needed to add and delete sets of users to a secure multicast group. We present upper and lower bounds on the number of re-keying bits to add or delete groups of users from secure multicast groups. We also show how to amortize the cost of group re-keying over time to avoid long periods of system overhead that can potentially block the transmission of desired data at times when large numbers of users simultaneously join or leave multicast sessions.

Comparing star and pancake networks

Low dilation embedding are used to compare the computational capabilities of star and pancake networks.

Computing Cross Associations for Attack Graphs and Other Applications

Applications in information security, data mining, e-commerce, information retrieval and network management require the analysis of large graphs in order to discover homogeneous groupings of rows and columns, called cross associations. We show that finding an optimal cross association is NP-complete. Furthermore, we give a heuristic algorithm with an O(n4) running time for finding good cross associations

A faster and simpler 2-approximation algorithm for block sorting

Block sorting is used in connection with optical character recognition (OCR). Recent work has focused on finding good strategies which perform well in practice. Block sorting is

Calibrating embedded protocols on asynchronous systems

\mathcal{NP}

A quadratic time 2-approximation algorithm for block sorting

-hard and all of the previously known heuristics lack proof of any approximation ratio. We present here an approximation algorithm for the block sorting problem with approximation ratio of 2 and run time O(n2). The approximation algorithm is based on finding an optimal sequence of absolute block deletions.

Information Security Education and Foundational Research

Embedding is a method of projecting one topology into another. In one-to-one node embedding, paths in the target topology correspond to links in the original topology. A protocol running on the original topology can be modified to be executed on a target topology by means of embedding. However, if the protocol is tolerant to a number of faults - faults that affect the data but not the code of a distributed protocol executed by the nodes in a distributed systems - then the adapted protocol will not have the fault tolerance property preserved, due to the fact that links in the original topology can be embedded into paths of length greater than one: faults at the intermediate nodes on such paths are not accounted for in the protocol. We propose a communication protocol in the target topology that preserves the fault tolerance characteristics of any protocol designed for the original topology, namely by our mechanism the modification preserves fault tolerance.