Hashcash is a proof-of-work system, based on SHA-1 algorithm and invented by Adam Back in 1997. Its bedrock is based on the concept that some mathematical results are easy to verify but difficult to discover values that went into calculating the results.

It is used to identify duplicate emails, limit email spam and reduce denial of service attacks. Hashcash based on Proof of Work, became foundation of Bitcoin to solve double counting problem that existed in decentralized cryptocurrencies.