architectures, see discussions in [@eyal2014majority; @barber2012bitter; @carroll2015creating] and references therein. Other examples of decentralized crypto-currencies include Litecoin, which was originally based on the Bitcoin protocol, and has a faster verification time, Ripple which is a monetary system based on trust networks, Dogecoin, Monero and Nxt.

The distinct nature of crypto-currencies
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To distinguish between centralized and decentralized currencies, one can consider for instance the definition from the central bank of Canada [^5] ‘Decentralized e-money is stored and flows through a peer-to-peer computer network that directly links users, much like a chat room. No single user controls the network.’

The ECB report on virtual currencies[^6] classified these currencies based on their interaction with fiat money and the real economy. [@peters2014opening] proposed to extend this classification to include the existence of a central repository and a single administrator, where the absence of both means that the currency is operated via a decentralised network consensus-type administration. Decentralised virtual currencies are then termed crypto-currencies, as the operation of these currencies is usually based on cryptographic proof provided by a network, rather than the existence of a trusted third part to verify transactions.

Differentiating between the different forms of virtual currencies is non-trivial as they 