Blockchains are immutable digital ledger systems implemented in a distributed fashion (i.e., without a central repository) and usually without a central authority. At their most basic level, they enable a community of users to record transactions in a ledger that is public to that community, such that no transaction can be changed once published. In 2008, the blockchain idea was combined in an innovative way with several other technologies and computing concepts to enable the creation of modern cryptocurrencies: electronic money protected through cryptographic mechanisms instead of a central repository. The first such blockchain based approach was Bitcoin. These currency blockchain systems are novel in that they store value, not just information. The value is attached to a digital wallet—an electronic device (or software) that allows an individual to make electronic transactions. The wallets are used to sign transactions sent from one wallet to another, recording the transferred value publicly, allowing all participants of the network to independently verify the validity of the transactions. Each participant can keep a full record of all transactions, making the network resilient to attempts to alter that record (or forge transactions) later.