Adam Back is the author of the Hashcash paper cited by Satoshi Nakamoto in Bitcoin’s white paper. That documentary connection gives us a clear starting point for understanding his place in Bitcoin’s intellectual history. Bitcoin’s references and proof-of-work section.
A cost for sending, a cheap check for receiving
Back’s 2002 paper describes Hashcash as a way to make certain kinds of abuse more expensive. A sender performs computational work to produce a token; a recipient can check that token comparatively cheaply. The paper identifies email and anonymous remailers among the intended applications.
Back dates the original proposal to May 1997. The linked paper itself is dated August 1, 2002. These refer to the proposal’s origin and a later written account, not competing dates for the same publication. The original paper.
The connection to Bitcoin
Bitcoin’s white paper explicitly invokes Hashcash when explaining proof of work. It places computational work into a chain of blocks so that changing an earlier part of the record requires competing with subsequent work. Nakamoto’s design.
A mechanism for attaching computational cost to a message is not, by itself, the whole Bitcoin system. The ledger, transaction rules, incentives, and method of choosing a history also matter. That distinction gives a predecessor its proper credit without treating it as an already-complete Bitcoin implementation.
Read the work directly
Start with the Hashcash paper’s introduction and cost-function discussion, then compare section 4 of the Bitcoin white paper. The useful question is how the same general asymmetry—work to produce, less work to verify—serves different purposes.
Continue with mining and proof of work or Hal Finney’s reusable proof-of-work work.