Blockchain Terminology. We observed that the term blockchain is used arbitrarily in the schemes. Indeed, Xxxxx et al. [34] and Xxxxx [45] also state that many authors use the term blockchain in different ways. Interestingly, in the original work by Xxxxxxxx [46] the term blockchain is not used, but the term distributed time-stamp server is used. Xxxx [47], Xxxxx et al. [34] and Xxx [43] state that blockchain is a dis- tributed ledger. Xxxx [47] also calls blockchain is a distributed database, while Birch et al. [34] use the term ‘shared ledger’. Wu¨st and Gervais distinguish per- missionless and permissioned blockchains, and provide examples for each type. Their Corda example, however, can be considered a decentralized database [22]. Although Corda is heavily inspired by blockchain systems [26], Corda does not use a chain of blocks. These examples show that, indeed, various terms are used interchangeably and are not always correctly. The terminology for the different solutions we use in this paper is illustrated in Fig. 1 and explained below. We distinguish two types of databases: central databases (DBs) and distributed databases. In a central database, data is cen- trally stored. Following this, a central ledger is a central database with the inclusion of transaction interaction. Transaction interaction [12] refers to the interdependency of transactions of different participants. For example, a Bitcoin account with a balance of 0 can only create a valid transaction after it receives a transaction that increases its current balance. Additionally, a shared central ledger can be used when multiple writers are present. A distributed database stores data across multiple locations, and provides read and write access to participants. Following this, a distributed ledger is a distributed database with the inclusion of transaction interaction. We consider blockchain (BC) to be particular form of distributed ledger technology (DLT), as here unknown participants can read from and write to the ledger, and reach consensus on the state of the ledger.
