Blockchain first appeared in 2008 when Satoshi Nakamoto introduced the blockchain structure for bitcoin, the first cryptocurrency. Prior work on blockchain technology dates back to 1991, but it was Nakamoto who improved the structure by using a hash cash-like technique to record timestamps of added blocks without central authorization. Since 2008, blockchain has grown significantly, with the size of the bitcoin blockchain network reaching over 200 gigabytes by 2020 as adoption of the technology has increased. Blockchain allows transactions to be recorded in a decentralized, immutable public ledger.