PhD Thesis Defense

                    Efficient Electronic Cash:
                    New Notions and Techniques

                        Yiannis Tsiounis
                    Northeastern University

		     206, New Egan building
			 
                   10:00 am, Friday, 12/13/96


The importance of electronic payment methods has been widely
recognized, triggered by the proliferation of electronic services,
such as services available over the Internet or cellular systems.
Electronic cash, introduced as a concept by David Chaum in 1982, is
among the most important of these methods due to its guarantee of user
anonymity; hence the parallelism with physical cash. Privacy of
transactions is progressively viewed to be a highly desirable feature
in financial services. We discuss our results in efficient and secure
e-cash; our systems can be implemented in current smart cards and are
provably secure.

[Providing exact payments]
Efficiency for practical payment systems requires the ability to
conduct payments of exact amounts. We investigate two viable
approaches, each optimal for different system requirements:
1) Using electronic coins that can be divided to allow for exact
payments (divisible e-cash). We improve existing systems by three
orders of magnitude, without sacrificing security.
2) Keeping a multitude of coins in users' "electronic wallets": we
construct an optimal algorithm for withdrawing coins that suffice for
a fixed number of exact payments.

[Controlling user anonymity] 
The anonymity of e-cash could be used for criminal activities such as
money laundering, committing perfect (anonymous) crimes or making
illegal purchases. We propose to solve this problem by allowing an
escrow agent (Trustee/Judge) to remove anonymity of users when
necessary.  Earlier similar proposals required involvement of
Trustee(s) on a per-coin basis, preventing the systems from being used
in practice. Our system allows Trustee(s) to be Off-Line, and ranks
among the most efficient e-cash systems to date while remaining
provably secure.

We will conclude with future directions in research.