19 years ago today, Hal Finney officially released Reusable Proof of Work (RPoW)

Like the title states, 19 years ago on Sunday August 15, Hal Finney released his Reusable Proof of Work. This was intended as a prototype for a digital cash based on Nick Szabo's "Theory of Collectibles."

To launch RPOW, Hal sent this email to a list of cypherpunks.

https://preview.redd.it/76l75eybv9ib1.png?width=566&format=png&auto=webp&s=693e85805a9bcd70a963d381522fd22a157b61f8

On the RPOW website, Hal explained,

The RPOW system provides for proof of work (POW) tokens to be reused. A POW token is something that takes a relatively long time to compute but which can be checked quickly. RPOW uses hashcash, which are values whose SHA-1 hashes have many high bits of zeros.

Normally POW tokens can't be reused because that would allow them to be double-spent. But RPOW allows for a limited form of reuse: sequential reuse. This lets a POW token be used once, then exchanged for a new one, which can again be used once, then once more exchanged, etc. This approach makes POW tokens more practical for many purposes and allows the effective cost of a POW token to be raised while still allowing systems to use them effectively.

This was a novel digital cash that combined proof-of-work with an inflation resistant money. Though RPOW ultimately failed, it laid critical groundwork for Bitcoin.

Thank you, Hal, for all of your contributions!