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A blockchain is a distributed ledger in which transactions are incorporated into cryptographically secure blocks. This allows a trustless, peer-to-peer method for achieving consensus, and was a solution to a long standing unsolved problem in computer science known as the Byzantine Generals Problem. A valid chain is one which at least 51% of the network agrees is the longest. Computations supporting the network are performed by 'miners', programs which maintain a copy of the longest chain and use brute force computation to discover the next block, which will be some cryptographic hash function that satisfies the encrypted header of the previous block, for bitcoin this function is SHA-2, other cryptocurrencies use different functions, for example Litecoin uses Scrypt. Blockchain was first described by Satoshi Nakamoto in 2009 and applied to the bitcoin protocol. Distributed currency grew from the cypherpunk movement in the 1980s and 1990s and is a successor to the earlier hashcach system.