Satoshi Nakamoto — an unknown identity, possibly an individual or collective pseudonym — publishes on 31 October 2008 "Bitcoin: A Peer-to-Peer Electronic Cash System," a nine-page technical document synthesizing proof-of-work (Back, 1997), time-stamped blockchain (Haber and Stornetta, 1991–1992), Merkle trees (Merkle, 1979), and public-key cryptography (Diffie-Hellman, RSA) into a distributed consensus system with no central authority. The genesis block is mined on 3 January 2009. Bitcoin introduces the UTXO (Unspent Transaction Output) model for distributed accounting, adjustable proof-of-work difficulty to maintain a constant interval between blocks, and the mempool as a space for pending transactions. It is the first practical, verified solution to the double-spending problem in an open, permissionless network — the problem that had blocked digital money for decades. Nakamoto's identity remains unknown; he withdrew from public communication in 2010.