Hashcash Postage

VERIFIED INSTANTLY.

Adam Back’s 1997 Hashcash announcement proposed computational postage that was costly to produce and quick to verify.

An enormous sorting machine churns through a mountain of mail. Adam Back watches from the foreman's walkway as each letter spends time in the machinery before it can receive a stamp. On the far side, a much smaller gate checks the finished work in a moment. Letters with blue digital approval marks fill the outgoing belt. The factory is imaginary, but its unequal effort tells the story of Hashcash.

On 28 March 1997, Back announced a working hash cash postage implementation to the cypherpunks mailing list. His idea gave a message a kind of computational postage. A sender's machine would search for a suitable partial hash collision. Finding one took repeated attempts; checking the result was quick. In his announcement, Back described the result as something that could be verified instantly.

For someone sending a few messages, that work could be a modest inconvenience. For a sender attempting a vast mailing, the cost would multiply with each message. Hashcash was a proposal to make mass abuse of email and anonymous remailers harder to sustain. It was neither money spent at a post office nor a machine that could read an envelope and decide whether its words were spam.

Years later, Bitcoin's whitepaper named Back's Hashcash as a model for its proof-of-work system. Bitcoin applied proof of work to a different job: securing a shared record of transactions. The small block by the foreman's feet is a wink towards that future. One familiar envelope in the upper sorter looks further back, to card 001 and Chaum's privacy work. This imagined factory makes the connection between separate ideas visible without pretending that any of its machinery existed in 1997.