Adam Back (London, 1970), a cryptographer and cypherpunk, proposes Hashcash in 1997 as an anti-spam mechanism for email: to send a message, the sender must prove they have performed verifiable computational work — finding a value (nonce) such that the SHA-1 hash of the message plus the nonce begins with a set number of zeros. The work is costly to perform but trivial to verify. This proof-of-work mechanism is explicitly cited by Nakamoto as the first reference in the Bitcoin whitepaper (2008) as the basis of the consensus system: Bitcoin miners perform exactly this calculation to add blocks to the chain, using SHA-256 instead of SHA-1. Back later founded Blockstream (2014), a Bitcoin infrastructure company. Nakamoto's explicit citation makes Back the inventor most directly acknowledged in the whitepaper, but his name rarely appears in popular narratives about Bitcoin's origin.