> Too bad nobody has ever thought of it > before; we could really use the outcome > of that research while researchers has not thought about global PKI, their are research which focus on spam elimination. this is the work all about (yesterday's seminar in a MIT group) " If I don't know you, and you want your e-mail to appear in my inbox, then you must attach to your message an easily verified "proof of computational effort", just for me and just for this message. If the proof of effort requires, say, 10 seconds to compute, then the economics of sending spam are radically altered, as a single machine can send only 8,000 messages per day. The recent proliferation of spam has lead to a renewed interest in these ideas. This work is about both the choice of functions that can be used to yield easily verifiable proofs of computational effort, and architectures for implementing the proof of effort approach. Filtering and/or forcing senders to pay in other currencies, such as human attention and money, will be covered as time permits" for more details http://research.microsoft.com/research/sv/PennyBlack