Hashcash

The surviving mailing-list record dates Back's implementation announcement to March 28, 1997. His later paper is dated August 1, 2002. The paper retrospectively places the original proposal in May 1997, which differs from the earlier announcement. This entry dates the public record by the contemporary message while preserving the paper's account.

Hashcash targeted spam and remailer abuse: senders compute an easily checked stamp. Bulk sending requires more work. Here, “cash” means computational postage.

The version described in the 2002 paper repeatedly tries candidates until a stamp's hash meets a leading-zero requirement. Generation usually takes many computations; verification is quick. Stamps are tied to a particular service. The server records spent stamps to reject reuse and can expire them to bound its records.

Back noted that service-specific stamps were not directly transferable. Exchanging them for bank-issued cash would require trusting the bank against overissuance. This illustrates why computation alone was insufficient.

Section 4 of the Bitcoin whitepaper explicitly uses Hashcash-like proof-of-work for a distributed timestamp chain. On August 20, 2008, Satoshi also asked Back to check the paper's citation and described a new electronic-cash use for hash-based proof-of-work. Both the paper and their correspondence document this connection.

An applied cryptographer and the author of Hashcash, Back earned a PhD in distributed systems and computer science at the University of Exeter. He later co-founded Blockstream and became its CEO in 2016.