<P> This is usually implemented using an online central trusted third party that can verify whether a token has been spent . This normally represents a single point of failure from both availability and trust viewpoints . </P> <P> By 2007, a number of distributed systems for double - spending prevention had been proposed . </P> <P> The cryptocurrency bitcoin implemented a solution in early 2009 . It uses a cryptographic protocol called a proof - of - work system to avoid the need for a trusted third party to validate transactions . Instead, transactions are recorded in a public ledger called a blockchain . A transaction is considered valid when it is included in the blockchain that contains the most amount of computational work . This makes double - spending more difficult as the size of the overall network grows . Other cryptocurrencies also have similar features . </P> <P> Decentralized currencies that rely on blockchain are vulnerable to the 51% problem, in which a malicious actor can rewrite the ledger if they control enough of the computational work being done . For example, one could theoretically spend cryptocurrency then erase the transaction so it appears it never happened . In May 2018 this double - spending technique was used against Bitcoin Gold, the 26th largest cryptocurrency to defraud cryptocurrency exchanges of millions of dollars . In response, exchanges repeatedly raised the threshold needed to confirm a transaction, but the criminal had enough computing power to exceed those thresholds and continued double - spending for three days . </P>

How does proof of work prevent double spending
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