Hashcash is announced

Adam Back announces “hash cash postage” on the cypherpunks list: a sender proves work by finding a partial SHA-1 collision, which is costly to make and instant to check. The idea was aimed at spam and remailer abuse. (His 2002 paper dates the proposal to May 1997.)

“The idea of using partial hashes is that they can be made arbitrarily expensive to compute (by choosing the desired number of bits of collision), and yet can be verified instantly.”

People: Adam Back, John Gilmore.

Paper: “Hashcash – A Denial of Service Counter-Measure” (1 Aug 2002): Adam Back collects five years of Hashcash work in one paper, dated 1 August 2002. Six years later the whitepaper cites exactly this paper as reference [6].

The whitepaper cites Hashcash [6] (31 Oct 2008): Section 4 “Proof-of-Work” of the Bitcoin whitepaper builds on Hashcash. The paper also cites Wei Dai's b-money [1], Haber & Stornetta's time-stamping [3][4][5] and Merkle [7].