Roughly described, a method of recording a change of authorization state of one or more authorization agents, the method comprising: establishing a copy of a blockchain ledger at each of a plurality of blockchain nodes, wherein each of the blockchain nodes is associated with a different controlling entity; providing a public key/private key pair for a first of the blockchain nodes; receiving, from the communication device, a first message comprising: first data indicative of a change of authorization state of a first authorization agent associated with the first controlling entity, the first data being encrypted; and a digital signature based on the blockchain ledger and the private key; authenticating the message using the public key; adding a block to the blockchain ledger based on the first message, thereby to generate a new blockchain ledger that records the change of authorization state of the first authorization agent.