I wonder if this would be an interesting application for Homomorphic Encryption. True FHE is still wildly inefficient, but there are some interesting applicatio

I wonder if this would be an interesting application for Homomorphic Encryption. True FHE is still wildly inefficient, but there are some interesting applications like CryptDB where sort-of-Homomorphic-Encryption is feasible for certain restricted operations (keyword search being one). In a system like that, maybe you could send your encrypted message along with some encrypted keywords that you consider to be spammy to some centralised service. That would, at least, avoid some of the client-side-filtering-is-too-hard problem. As far as reputation, this might be one of the rare times where a Web of Trust seems like a good idea. Generating lots of false positives and negatives would be a lot less powerful if the value of those reports was filtered by how much you trust the account that made them. With email you already have an implicit source of trust, in that anyone you mutually email with is unlikely to be a spammer. Seems like a really interesting problem space to be involved in.