I'm posting this for those who might find it interesting.
This paper is by respected cryptographer Amir Herzberg
http://www.cs.biu.ac.il/~herzbea/Papers/ecommerce/Spam.htm
Here is the abstract:
Controlling Spam by Secure Internet Content Selection
Amir Herzberg, Computer Science Dept., Bar-Ilan University, Israel
Abstract
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 and legitimate, properly labeled commercial e-mail. SICS improves on other crypto-based proposals for spam controls, and complements non-cryptographic spam controls.
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