Controlling Spam through Lightweight Currency

Spam is an ongoing problem on the Internet today, and an increasing body of literature from research papers to the popular press addresses the problem. The solutions generally fall into the categories of paymentbased, legislative, and filter-based. In this paper, we present a payment-based solution to the spam problem that can reduce the level of unsolicited emails to reasonable levels without imposing legal restrictions on free speech, and without the use of email filters. Specifically, we propose that mail transfer agents (MTAs) make payments with the Lightweight Currency Protocol (LCP) when sending email into other mail domains. Mail service providers specify payment requirements that discourage spam, but encourage the free flow of other email. Thus, our proposal makes it possible to control spam without end user involvement or modification to user agents. The advantages of using the LCP for mail delivery include its conceptual simplicity, security, scalability, flexibility and low cost. With LCP, mail domains have the ability to issue their own currency or use currency issued by either other mail domains or by other LCPbased service providers outside of the email system. The ability for domains to issue their own currency ensures the system will scale to meet demand, and enables domains to operate spam-reduced mail ser∗The support of the Associated Students Incorporated at CSUSB and the National Science Foundation under award 9810708 are gratefully acknowledged. vices without paying an outside authority for needed currency.