Proof of work

Pricing functions, Hashcash, and difficulty retargeting.

Pricing via processing: making junk mail expensive (Dwork and Naor 1992)

Pricing via processing: making junk mail expensive (Dwork and Naor 1992)

Hashcash: proof of work you can verify in one hash (Back 1997)

Hashcash: proof of work you can verify in one hash (Back 1997)

Difficulty retargeting: keeping ten-minute blocks (Bitcoin 2009)

Difficulty retargeting: keeping ten-minute blocks (Bitcoin 2009)