Most cryptocurrency systems rely on a public distributed ledger known as a blockchain, which is basically an ever-growing list of records that are highly resistant to modification. As the name suggests, a blockchain is made of a linear chain of blocks and, in the context of cryptocurrencies, it is responsible for keeping a permanent record of all confirmed transactions (and related data) – all secured by cryptography. Generally speaking, every cryptocurrency works on top of a blockchain that works according to a predefined set of rules (i.e. an underlying protocol). The protocol is what defines how the blockchain and the cryptocurrency system should operate.