Distributed Access Control Through Blockchain Technology

31 A blockchain is a distributed, always available, irreversible, and tamper­ resistant public database where the con­ trol over data and its evolution is distrib­ uted among a variable set of peers. Blockchain technology does not require the existence of trust relationships among the system’s users. Consequently, it employs a distributed consensus algorithm to allow the users to agree on immutable and auditable append­only operation without requiring interaction with a trusted third party. We are interested in the auditability of the data stored in the blockchain, since the blockchain can be used as a publicly verifiable proof that the data existed at the time it was saved in it.