[CodesInChaos]

Using hashcash and other proof-of-work schemes to limit spam and similar abuse sounds appealing, but I don't see how you can make it work in practice.

1. Defenders use standard PCs and mobile phones. Attackers use GPUs, FPGAs and ASIC and run them in places where electricity is cheap.

For traditional hash algorithms this gives the attacker a thousandfold or so advantage. There has been some work on closing the gap in the context of crypto currencies, but I don't know how close they got.

2. It takes a phone (or even a PC) a long time to burn through $2.

3. Attackers have large botnets and don't pay for the electricity consumed by these.