Funny, I like the efficiency added by the ticket server, but this solution is actually more centralised the one that came before it -- hashcash: https://en.m.wi

Funny, I like the efficiency added by the ticket server, but this solution is actually more centralised the one that came before it -- hashcash: https://en.m.wikipedia.org/wiki/Hashcash https://en.m.wikipedia.org/wiki/Hashcash Hashcash has the features you described above, but no mechanism for "returning" PoW to a user.