Cryptographic Protocols to Prevent Spam

r example, spammers may add spaces or other special characters between letters, or inject intentional, minor spelling errors (Viiagrra). Spamming wastes considerable machine and human resources - most notably, the recipient's time. Indeed, spamming is reducing the usefulness of email as a communication mechanism these days. Many users reduce or avoid the use of email, most users limit the distribution of their email address, and many desirable messages are lost by aggressive (human and automated) filtering. As a result, there are many proposals and mechanisms trying to control and prevent spam. Spam can be a problem in any open, e#cient, low-cost messaging system, and indeed spam is used e.g. in instant messaging; most of our discussion is relevant to spam on any messaging technology. However, the problem is most acute, at least so far, for email, and therefore we will discuss also some email specific issues. We begin this manuscript by discussing the basic architecture and relevan