I'm supprised no one has mentioned the fair exchange problem: if A, B engage in an exchange protocol, say A sends data to B while B paying to A, how to achieve

I'm supprised no one has mentioned the fair exchange problem: if A, B engage in an exchange protocol, say A sends data to B while B paying to A, how to achieve fairness using an asynchornous protocol. Without a third party, either one of the two parties has to be in a position of advantage where it can choose to walk off without fulfilling his part of the contract. The problem still exists under "micro-transaction" scheme, but in a less exploitable sense: because each transaction only involves a small amount of money or data, it's probably okay if you encountered with a cheater with one or two transactions. You can just block them after catching them cheating. But the cheaters can still exploit this problem for their own gain: they just repeat this scheme with all other users on the network, until they received the whoe file without spending anything. I don't see this protocol preventing such exploitable scenario. In fact it's a very difficult problem to solve efficiently and without a traditional trusted third party. It's even been proved [1] that strong fairness is impossible without a trusted third party. However relying on blockchain as a thrid party, it's possible to achieve this, and there are implementations [2] out there, but the challenge is throughput. Most of these solutions can only handle kilobytes/s of throughput. The computational barrier here is the vast amount of modular exponentiations. [1] https://www.cs.utexas.edu/~shmat/courses/cs395t_fall04/pagnia.pdf https://www.cs.utexas.edu/~shmat/courses/cs395t_fall04/pagni... [2] https://github.com/sec-bit/zkPoD-node https://github.com/sec-bit/zkPoD-node