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Users broadcast cryptographically signed messages to the network using bitcoin cryptocurrency wallet software. These messages are proposed transactions, changes to be made in the ledger. Each node has a copy of the ledger's entire transaction history. If a transaction violates the rules of the bitcoin protocol, it is ignored, as transactions only occur when the entire network reaches a consensus that they should take place. This "full network consensus" is achieved when each node on the network verifies the results of a proof-of-work operation called mining. Mining packages groups of transactions into blocks, and produces a hash code that follows the rules of the bitcoin protocol. Creating this hash requires expensive energy, but a network node can verify the hash is valid using very little energy.