British cryptographer and cypherpunk who invented Hashcash (1997), the proof-of-work system cited in the Bitcoin white paper. On August 20, 2008, he became the first known person Satoshi Nakamoto contacted about Bitcoin. He later co-founded Blockstream in 2014.
Adam Back (born 1970, United Kingdom) is a cryptographer, cypherpunk, and the inventor of Hashcash. He earned a PhD in computer science from the University of Exeter. He is the first person known to have been contacted by Satoshi Nakamoto about what would become Bitcoin.
Hashcash (1997): In March 1997, Back proposed Hashcash, a proof-of-work system originally designed to combat email spam and denial-of-service attacks. The system required senders to compute a partial hash collision — a computationally expensive operation — before sending an email, making mass spam economically impractical. Hashcash’s proof-of-work concept became a fundamental building block of Bitcoin’s consensus mechanism. The Bitcoin white paper cites Hashcash as one of its key references.
First Contact from Satoshi: On August 20, 2008, Satoshi Nakamoto emailed Back, making him the first known person Satoshi contacted about the ideas that would become Bitcoin. Satoshi referenced Back’s Hashcash and asked about related prior work. Back directed Satoshi to Wei Dai’s b-money proposal. Two days later, on August 22, Satoshi emailed Wei Dai. This chain of referrals — from Back to Dai — helped Satoshi connect with the key intellectual precursors to Bitcoin’s design.
Testimony and Email Publication: Back’s email correspondence with Satoshi was published by Bitcoin Magazine, providing important primary source documentation of Bitcoin’s earliest conceptual phase. In February 2024, Back testified in the COPA v Wright trial in London, providing first-hand testimony about his interactions with Satoshi and the timeline of Bitcoin’s creation.
Blockstream: In 2014, Back co-founded Blockstream, a blockchain technology company focused on Bitcoin infrastructure. He has served as the company’s CEO. Blockstream developed the Liquid Network sidechain and has been involved in satellite-based Bitcoin broadcasting and other Bitcoin infrastructure projects.
Significance: Back’s Hashcash is one of the most direct technical predecessors to Bitcoin’s mining mechanism. His position as the first person Satoshi contacted places him at the very beginning of Bitcoin’s documented creation history.
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Wei Dai's reply to Adam Back on the Cypherpunks mailing list, discussing b-money's practical limitations. Dai acknowledged that 'b-money will at most be a niche currency/contract enforcement mechanism' and revealed a shift in his political views, stating 'I now tend to think that the government monopoly of force is a net benefit.'
Satoshi Nakamoto's earliest known email. He contacts Adam Back to verify the correct citation for the Hashcash paper and shares a pre-release draft titled 'Electronic Cash Without a Trusted Third Party' (ecash.pdf).
Adam Back reflects on his August 2008 email exchange with Satoshi Nakamoto, his regret at not reading the Bitcoin whitepaper carefully, and his testimony during the COPA v. Wright trial where the complete email chain was made public.
During Day 13 of the COPA v. Craig Wright trial, Martti Malmi testified via video link and submitted 260 emails (140,000 words) exchanged with Satoshi Nakamoto between May 2009 and February 2011. The complete email archive was published on GitHub.
Blockstream CEO Adam Back stated that Bitcoin faces no quantum computing threat for roughly 20–40 years, pointing to NIST-standardized post-quantum signatures like SLH-DSA that Bitcoin can adopt well before any threat materializes.