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In Hashcash, the client has to concatenate a random number with a string several times and hash this new string. It then has to do so over and over until a hash beginning with a certain number of zeros is found. Later on, in 2008, Satoshi Nakamoto adapted the concept first used in Hashcash to establish Bitcoin’s mining proof-of-work mechanism to secure the future BTC network. Satoshi mentioned both Back and Hashcash in the Bitcoin whitepaper released in 2008.
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