Monetary Forgery in the Digital Age: Will Physical-Digital Cash Be a Solution?1 N ICOLAS CHRISTIN2, ALESSANDRO ACQUISTI3, BRYAN

Despite cryptographic breakthroughs in the area of digital cash and the rapid advance of information technology, physical cash remains the dominant currency: it is easy to use and its exchanges are largely independent of computing devices. However, physical cash is vulnerable to rising threats such as high quality, government-mandated forgeries. Can a hybrid of physical and digital cash protect more effectively against these threats? We discuss the rise of high-quality counterfeits and review technological solutions to thwart such threats . Specifically, we study mechanisms to combine physical cash with digital cash to remove their respective shortcomings and obtain their combined advantages. The mechanisms range from cryptographic signatures embedded in 2-D barcodes to online verification systems assisted by physical one-way functions. Notably, we compare these different proposals by looking at them through the prism of economics, and examining their cost and benefit trade-offs . 1 A preliminary, abbreviated version appeared in the Proceedings of the Twelfth International Conference on Financial Cryptography and Data Security (FC'oS). Cozumel, Mexico. January 2S-31, 200S. 2 Carnegie Mellon University, Information Networking Institute 3 Carnegie Mellon University, Heinz School of Public Policy and Management 4 Carnegie Mellon University, Department of Electrical and Computer Engineering & Microsoft Research 5 Carnegie Mellon University, Department of Electrical and Computer Engineering & Carnegie Mellon University, Department of Engineering and Public Policy 172 I/S: A JOURNAL OF LAW AND POLICY [Vol. 7:2