The “hashcash CPU cost-function” encapsulates a Proof-of-Work (PoW) in order to legitimize a user’s participation. By making it very costly to spam or take over a network, like Bitcoin, the hashcash mechanism incentivizes participants to support the network rather than attack it.

Adam Back came up with the idea for hashcash in 1997, and describes how it works in his 2002 article, “Hashcash - A Denial of Service Counter-Measure.” Cypherpunk and British cryptographer Adam Back co-founded Blockstream in 2014, and currently serves as its CEO. Back was one of the first two recipients of an email from Satoshi Nakamoto.

Back clarifies that he did not invent PoW but rather “re-invented” or repurposed its use case by suggesting to implement it for mining coins.