Cryptographic Protocols to Prevent Spam

Email's main (and initial) use is professional and personal communication. Email is very efficient, convenient and low-cost, especially when automatically sent by programs to many recipients ('bulk email'). Therefore, email is also used to distribute other messages: from unsolicited offerings and ads, to malicious content such as viruses and scams. Users are rarely interested in these messages; the result it that the vast majority of email messages are undesirable and discarded. These unwanted messages are often referred to as spam. Spam is very annoying to users, and wastes considerable time and resources of email users and service providers. There are few conventions for content labels, used to identify advertising or other potentially undesired messages. For example, some anti-spam legislation requires advertisers to use special prefixes such as 'ADV:' in the subject line [42, 43]. Messages that present correct content labels are not problematic, since they can be efficiently discarded by (most) users, who are not interested in them; therefore, we will exclude them from our definition of spam. Unfortunately, most messages containing potentially-undesired content do not contain appropriate content label, i.e. are spam. We therefore prefer to use the term spam to refer to undesirable messages without appropriate content label. Spam senders (spammers) not only avoid putting correct 'warning labels' on their messages, but in fact often use different evasive techniques to make it hard to distinguish between their spam messages and desirable professional/personal communication. For 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, efficient, 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 relevant properties of the email system, in Section 1, and the …