Adam Back

Adam Back (born July 1970) is a British cryptographer and one of the original Cypherpunks who in 1997 devised Hashcash, the proof-of-work scheme cited in the 2008 Bitcoin white paper, and who co-founded and runs the blockchain company Blockstream. He holds a PhD in computer science from the University of Exeter. He was the first of the two people Satoshi Nakamoto is known to have emailed before the white paper, and he is one of the figures most often named as a Nakamoto candidate, which he denies.¹²³

Hashcash

Back proposed Hashcash in 1997 as a measure against email spam and denial-of-service attacks, building on the proof-of-work concept that Cynthia Dwork and Moni Naor had set out in 1992. Hashcash required a sender to compute a partial hash collision, a value whose hash begins with a set number of zero bits, before a message would be accepted; finding such a value takes a predictable amount of processor work while checking it is instant, so the cost falls on the sender and a mass spammer faces a prohibitive aggregate burden. Back circulated the scheme on the cypherpunks mailing list and published the design in a 1997 announcement and a 2002 paper, "Hashcash - A Denial of Service Counter-Measure."²⁴

The asymmetry at the center of Hashcash, work that is hard to produce and trivial to verify, became the mechanism Nakamoto used to govern coin issuance and to secure the Bitcoin ledger. The white paper cites Back's scheme directly, and Nakamoto's description of the system to the cryptography mailing list said new coins are made from "Hashcash style proof-of-work." Back's earlier cypherpunk work also included a 1998 implementation of an anonymous remailer and writing on the export-control politics of cryptography, including printing the RSA algorithm on T-shirts and other items to test the rule that classified strong cryptography as a munition.⁵⁴

Contact With Satoshi Nakamoto

The earliest known message from Satoshi Nakamoto to any named individual was an email to Back on 20 August 2008, asking how Hashcash should be cited and sending an early description of the project. Back has recounted that he pointed Nakamoto toward Wei Dai's b-money proposal, which Nakamoto then cited after emailing Dai on 22 August 2008. Back has said he did not engage deeply with the project at the time, having seen many digital-cash schemes fail, and did not return to Bitcoin until years later.¹⁶

Back co-founded Blockstream in 2014 and serves as its chief executive; the company develops Bitcoin infrastructure including the Liquid sidechain and the c-lightning implementation of the Lightning Network, and employs several core Bitcoin developers. Back is among the people Nakamoto cited by name in the white paper and one of the few still publicly active in Bitcoin.³⁷

Satoshi Speculation

Back has been named as a Nakamoto candidate repeatedly and has denied it each time. The case rests on his authorship of Hashcash, his place as the first person Nakamoto emailed, his British background and 1990s cypherpunk activity, and stylistic and biographical overlaps. In April 2026 a New York Times investigation by the reporter John Carreyrou named Back as the most likely candidate, citing his nationality, his cypherpunk archive, the Hashcash citation, more than a hundred matching words and phrases between his posts and Nakamoto's, his C++ and distributed-computing background, and a 2002 question he had raised about the 1933 United States gold seizure that surfaced in Bitcoin's design.⁸³

Back answered the New York Times investigation with a flat denial, telling reporters "I'm not Satoshi" and adding that his heavy posting volume on the cypherpunks lists naturally produced extensive evidence of his interests and made him an easy target for that kind of pattern-matching, which he characterized as confirmation bias. He has also said that the absence of a known creator is good for Bitcoin because it leaves the network looking like "a mathematically scarce digital commodity" rather than any individual's project. Back has consistently declined to provide cryptographic proof either way, as have the other living candidates.³⁹