Back proposes a computational postage stamp to deter spam, and invents Bitcoin's engine

MAR 28 1997 · 12:00

Video: Bitcoin: The Future of Money? Date: 1997-03-28 · Type: primitive · Tags: adam-back, hashcash, proof-of-work, cypherpunks, 1997, bitcoin-precursor, anti-spam, blockstream The list barely engaged. Eleven years later, Satoshi emailed him first. Adam Back was a 26-year-old British postdoc at the University of Exeter when he sent the cypherpunks list a description of 'a partial hash collision based postage scheme' on 28 March 1997. The immediate application was anti-spam: make each outgoing email require a small but non-trivial computation — finding a nonce such that the hash of the message has k leading zero bits — and bulk mailing becomes economically costly without meaningfully burdening individual senders. The asymmetry was the point: expensive to produce, instantaneous to verify. The list produced a single technical reply. The proof-of-work primitive Back proposed would, eleven years later, become the Nakamoto consensus engine of Bitcoin, cited explicitly as reference [6] in the whitepaper. The connection is not merely that Satoshi borrowed a data structure; it is that Hashcash solved the problem of creating computational scarcity from scratch, without any trusted authority to certify the work. That's the precise property Bitcoin's mining mechanism requires. Back was the first person Satoshi Nakamoto contacted when drafting the Bitcoin paper in August 2008 — before Hal Finney, before Wei Dai, before anyone else on the pre-launch correspondence list. Back co-founded Blockstream in 2014 and remains its CEO. He is, by any reasonable accounting, the most consequential cypherpunk who is still actively shipping code — the figure who connects the anti-spam experiments of the mid-1990s to the block-subsidy mechanics of the longest-running proof-of-work chain. The Hashcash paper was formally published in 2002; the mailing-list post is the primary document. Facts • Email date: 28 March 1997, cypherpunks mailing list • Mechanism: SHA-1 partial pre-image; sender finds nonce n such that hash(message || n) has k leading zero bits • Formal paper: 'Hashcash — A Denial of Service Counter-Measure' (2002) • Cited as reference [6] in Satoshi Nakamoto's Bitcoin whitepaper • Back was the first correspondent Satoshi contacted when drafting the Bitcoin paper, August 2008 • Co-founded Blockstream (2014); remains CEO Primary Documents • Hashcash — A Denial of Service Counter-Measure (Adam Back, 2002) — http://www.hashcash.org/hashcash.pdf • Hashcash — Wikipedia — https://en.wikipedia.org/wiki/Hashcash