You can do it a bit easier without needing to have a large central message repository. Start with a secure distributed write-only database (imagine something l

You can do it a bit easier without needing to have a large central message repository. Start with a secure distributed write-only database (imagine something like a big collection of random people running a variant of couchdb that included some crypto primitives) and then mix in a bit of private information retrieval[ * ] and you are most of the way there. There are some tricky bits to this general idea, like notifications and indexing the mailboxes, but the principle is fairly sound. The problem with these sort of ideas is that if you try to implement them in the real world you soon discover that your addressable userbase is really, really small, most of them are people you probably don't want to help out very much, and the resource provider/parasite ratio is hard to keep sustainable. [ * ] PIR in a nutshell: Servers A & B contain messages 1, 2, 3, 4 & 5. You ask server A to XOR messages 1 & 4 together and send it to you and you ask server B to XOR messages 1, 2 & 4 together and send them to you. You XOR the two streams and get message 2, but neither A nor B know what you asked them for. Obviously there are additional steps that make the system more secure, but that is the basic gist of how it works.