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From a Time Before Bitcoin -- Hal Finney and Reusable Proofs of Work

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.

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This is useful functionality, but the unique feature of the RPOW system is its approach to security. RPOW is the first public implementation of a server designed to allow users throughout the world to verify its correctness and integrity in real time.

Based on principles similar to those proposed for so-called "Trusted Computing", RPOW allows third parties to dynamically and remotely verify what program is running on the RPOW server. The RPOW server is implemented on a high-quality secure processor, the IBM 4758 PCI Cryptographic Coprocessor, which has been validated to the highest level of security publicly available, FIPS-140 level 4. The 4758 is a self-contained single-board computer which has its own device key, generated on-board, which never leaves the card. That key can issue cryptographically signed attestations which describe the software configuration running on the card, including the SHA-1 hash of the application program.

The source code to the RPOW server is available from the download page. Using publicly available tools, anyone can build from this source code a memory image identical to that running on the RPOW server. If the SHA-1 hash of...

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