I've been working on a system that implements this, called "bitnet". The anti-spam system is basically what you said: you spend a small amount of bitcoin, and

I've been working on a system that implements this, called "bitnet". The anti-spam system is basically what you said: you spend a small amount of bitcoin, and the server grants you some large number of "tokens". These tokens can be used to perform actions which use resources on the server, either storing messages, or getting messages. The exact price is configurable, and currently set low [1]. There is a second mechanism that I use, which is intended to lower costs for the average user, but still prevent spam. Essentially, in each bitcoin transaction, a small "transaction fee" is required, which goes to the miner. It is possible to prove, via a cryptographic signature, that you were the originator of a transaction, and therefore the person who spent that transaction fee. The bitnet system grants tokens to people who can prove they spent money on transaction fees [2]. The idea being, a typical bitcoin user will accumulate transaction fees anyways, but a spammer will have to go out of his way to send bulk messages. [1] https://github.com/ortutay/bitnet/blob/master/bitnet.go#L50 https://github.com/ortutay/bitnet/blob/master/bitnet.go#L50 [2] https://github.com/ortutay/bitnet/blob/master/bitnet.go#L38 https://github.com/ortutay/bitnet/blob/master/bitnet.go#L38