In the past ten years, the dramatic growth of the Internet has had a profound and lasting impact on the way in which organizations and individuals communicate and conduct their public and private affairs. Tax forms are available online, students may submit their exams electronically to a (possibly remote) campus network, and companies may use the Internet as a public channel for linking up internal computing facilities or processes. For example, an employee may dial into a company's intranet from a hotel room or her home via a public Internet service provider. Since the Internet protocol does not provide sufficient mechanisms for ensuring the privacy, authenticity, integrity, and (if desired) anonymity of data that are processed through a usually dynamically determined chain of computers, there is a need for tools that guarantee the confidentiality and authenticity of data and of their communication sources and targets. Cautious consumers of mobile or foreign code prefer to verify that downloaded programs (e.g., Java applets) abide by a formal set of safety rules, possibly defined by the individual consumer. These needs appear to be even more pressing in the recent evolution of electronic commerce, where the act of selecting and purchasing a product occurs online. Although online companies are still waiting to reap their first real profits, it is evident that companies in general need to offer this mode of business in order to survive in a new economy that is global and at the same time strengthens regional identity.