Research Interests

I am primarily interested in all aspects of security in distributed systems. I have been working on applying cryptographic techniques in building secure protocols to solve various problems in distributed systems. I am not a cryptographer, but I would like to be one when I grow up. I have found that a common theme in my recent work is "balance." It manifests in various ways in the design of multi-party protocols: I am also interested in electronic commerce, mobile computing, operating systems, social implications of technology, fairness in societies in general, and history. My expertise in these areas rapidly diminishes from left to right. Surprisingly, my interest and enthusiasm seems to increase in the same direction!

Recent research topics

Optimistic fair exchange

Exchanges are a basic building block of many commercial processes. Examples include exchanging payment for receipt, certified mail, exchanging signatures on a contract text. In a two-party exchange, each player sends an item and expects to receive the other player's item in return. Fair exchanges are exchanges where each honest player is guaranteed that if he does not get the item he expects, then the other player(s) do not get the item he sent either. Optimistic fair exchanges rely on the use of a third party. But the third party needs to be involved only if something goes wrong. In the common case where players behave correctly, the third party need not be involved. Our first paper described detailed protocols for optimistic fair exchange. Our second paper improved the protocol so that it works on asynchronous networks as well. A companion paper employed the notion of verifiable encryption to build non-invasive fair exchange protocols (which do not dictate the format of the items exchanged) which guarantee a strong degree of fairness. Chapter 2 of my thesis is based on our first two papers. Chapter 3 of my thesis discusses the fair exchange of so called "generatable" items and how verifiable encryption can be used to make items generatable.

Electronic payment systems

A number of electronic payment systems have been proposed, implemented, and deployed in the 1990s. We carried out a survey of the state of the art in electronic payment systems. This survey turned out to be rather popular and was translated into Japanese. An updated version will appear in volume 50 of the "Advances in Computers" series.

The multitude of payment systems complicates the task of the application programmer. We developed a generic payment service framework within which an application can make use of any payment system (provided that the payment system has also been "adapted" into the framework).

Another important issue in electronic commerce is the techincal infrastructure to handle and resolve any disputes. This problem is rather complicated. We introduced and elaborated the problem in a paper presented at the 1998 Usenix e-commerce workshop. This remains an open problem.

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Pot-pourri

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List of publications

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Key Agreement in Ad-hoc Networks (with Philip Ginzboorg)
Authenticating Public Terminals (with Michael Steiner, Hervé Debar, and Michael Waidner)
Towards a Framework for Handling Disputes in Payment Systems (with Michael Steiner and Els van Herreweghen)
Protecting the Computation Results of Free-roaming Agents (with Ceki Gülcü and Günter Karjoth)
Fairness in Electronic Commerce
More on Optimistic Fair Exchange (with Victor Shoup and Michael Waidner)
Design of a Generic Payment Service (with Jose Abad-Peiro, Michael Steiner, and Michael Waidner)
Electronic Payment Systems (with Phil Janson, Michael Steiner, and Michael Waidner)
Optimistic Protocols for Fair Exchange (with Matthias Schunter and Michael Waidner)
Server-Supported Signatures (with Gene Tsudik and Michael Waidner)
Untraceability in Mobile Networks (with Didier Samfat and Refik Molva)
Security Issues in Mobile Computing
Anonymity in a Mobile Computing Environment
Some of the above are in the security-bib database.

In a previous life...

Parallel Algorithms for Constructing the Convex Hull in 2-Dimensions.
A Neural Network Simulator for the Connection Machine. (with Ravi Shankar et al)
A Parallel Free-text Search System with Indexing. (with Ophir Frieder and Sanjay Ranka)
A Parallel Implementation of the Hough Transform Method. (with Ravi Shankar)