As an alternative but similar idea, Proof of Work was originally developed to solve the email spam problem. The idea was that you would hash the email body + ti

As an alternative but similar idea, Proof of Work was originally developed to solve the email spam problem. The idea was that you would hash the email body + timestamp + nonce producing a hash with some number of leading zeros. It would be fast for individual devices to compute and fast for email servers to verify en mass but expensive if you needed to send lots of emails. The obvious downside is that since the system is fundamentally just cryptography it doesn't discriminate between spam and legitimate bulk mail (password resets, newsletters, mailing lists, etc.)