All concepts
Concept
Precursor
Hashcash
First appeared Mar 1997
Why it matters
Hashcash is the exact mechanism Bitcoin mining is built on — Satoshi chained it across blocks to make rewriting history cost real energy.
What it is
Adam Back's 1997 proof-of-work system that required senders to compute a partial hash collision before sending email or a message, making spam and denial-of-service expensive. It is reference [6] in the Bitcoin white paper.