Summary
The original March 28, 1997 announcement of Hashcash, posted by Adam Back to the Cypherpunks mailing list (cypherpunks@toad.com). Subject line: “[ANNOUNCE] hash cash postage implementation.” This is the primary document confirming both the 1997 date and the invention of the proof-of-work mechanism that Bitcoin’s mining system would later adapt. Back was posting from aba@dcs.ex.ac.uk — University of Exeter.
Key Points
- Date: Friday, March 28, 1997. Posted to cypherpunks@toad.com.
- Author email: aba@dcs.ex.ac.uk (University of Exeter, UK) — confirms Back was based in England at the time.
- Core mechanism: “A partial hash collision based postage scheme.” The key asymmetry: collisions “can be made arbitrarily expensive to compute (by choosing the desired number of bits of collision), and yet can be verified instantly.” This asymmetry — expensive to produce, cheap to verify — is exactly what Bitcoin’s mining inherits.
- Primary application: Anti-spam postage. Computing 1,000,000 × 20-bit hashes = “100 MIP years which is going to be more compute than they’ve got” — making mass spam economically unviable without centralizing identity or payment.
- Why this matters for Bitcoin: Satoshi explicitly cited Hashcash in Section 4 of the Bitcoin whitepaper as the inspiration for Bitcoin’s proof-of-work mechanism. This post is the origin document of that citation.
- Cypherpunk context: The announcement goes to cypherpunks@toad.com — the same mailing list where Bitcoin’s intellectual precursors (b-money, bit gold, reusable proof-of-work) were developed and debated. Back was posting to the exact same community from which Satoshi would later emerge.
- Anti-surveillance framing: Back positioned Hashcash as keeping “net culture of free discourse” — a Cypherpunk value framing, not just a technical framing.
- Back’s post includes working code (compile instructions, usage examples), SHA1 implementation, double-spending protection database — not just a concept paper.
Newsletter Angles
- This post is proof that in March 1997, Adam Back was an active contributor to the exact intellectual community from which Bitcoin emerged — sending working code to the same mailing list where Satoshi later announced the whitepaper.
- The “expensive to compute, instantly verifiable” asymmetry Back described in 1997 is the essential Bitcoin mining mechanism. The conceptual bridge from Hashcash to Bitcoin is not circumstantial — it’s a direct citation with working code.
- Back’s University of Exeter email confirms his British identity at the time — consistent with Satoshi’s documented British spelling patterns (“bloody,” “favour,” etc.) noted in the NYT investigation.
Entities Mentioned
- Adam Back — author; inventor of Hashcash; posting from University of Exeter; member of Cypherpunks mailing list
- Satoshi Nakamoto — not present in this source, but cited Hashcash in the Bitcoin whitepaper 11 years later
- Bitcoin — the later system that built on Hashcash’s proof-of-work mechanism
Concepts Mentioned
- Cypherpunk Movement — the mailing list this was posted to; the intellectual community connecting Hashcash to Bitcoin
- Bitcoin Origin Mystery — this post is evidence that Back was active and technically capable in the exact community and period relevant to Satoshi’s identity
Quotes
“A partial hash collision based postage scheme… they can be made arbitrarily expensive to compute (by choosing the desired number of bits of collision), and yet can be verified instantly.” — Adam Back, March 28, 1997
Notes
Primary source — the original post. Hosted permanently at hashcash.org. Published 11 years before the Bitcoin whitepaper. The date (March 28, 1997) is embedded in the email header. Back’s email suffix (@dcs.ex.ac.uk) places him at the University of Exeter’s Department of Computer Science in 1997. This is the definitive citation for “In 1997 he invented Hashcash” — not a retrospective account but the contemporaneous announcement.