This is a mechanism for protecting specific information by requiring the requester to perform a certain action. With regards to cryptocurrencies, certain information is requested, which is difficult to reproduce and satisfy all the criteria. The idea behind proof of work is to verify that the calculation was actually made during the creation of the last block (files containing information about transactions). Cryptocurrencies use various functions to carry out a proof of work. Bitcoin, for example, uses Hashcash; a function that requests proof of work in order to prevent spam in email newsletters.