Hal Finney launches RPOW (Reusable Proofs of Work), a system that allowed proof-of-work tokens to be reused as a form of digital cash. RPOW was the first working implementation of a transferable proof-of-work system.
Finney, a legendary cypherpunk and key PGP developer, would later become the recipient of the first Bitcoin transaction from Satoshi Nakamoto. RPOW demonstrated many concepts that Bitcoin would refine, including using proof of work as a basis for digital value.
RPOW archived at Nakamoto Institute