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.
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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!
