Controlling spam by secure Internet content selection

Unsolicited and undesirable e-mail (spam) is a growing problem for Internet users and service providers. We present the Secure Internet Content Selection (SICS) protocol, an efficient cryptographic mechanism for spam-control, based on allocation of responsibility (liability). With SICS, e-mail is sent with a content label, and a cryptographic protocol ensures labels are authentic and penalizes falsely labeled e-mail (spam). The protocol supports trusted senders (penalized by loss of trust) and unknown senders (penalized financially). The recipient can determine the compensation amount for falsely labeled e-mail (spam). SICS is practical, with negligible overhead, gradual adoption path, and use of existing relationships; it is also flexible and appropriate for most scenarios, including deployment by end users and/or ISPs and support for privacy (including encrypted e-mail) and legitimate, properly labeled commercial e-mail. SICS improves on other crypto-based proposals for spam controls, and complements non-cryptographic spam controls.