A method of authorizing an operation on a remote device with a cryptographic signature verification component, the remote device being operable in a communications network having human-readable messages with message signatures, comprising receiving at an arbitrator an authorization request to perform an operation requiring authorization on the remote device; retrieving from the request an operation identifier and plaintext data; sending a human-readable request with the identifier and the plaintext data to an authorizer; receiving a reply from an authorizer, the reply message comprising at least the plaintext data and a verifiable cryptographic signature identifying the authorizer derived from the request; and on receiving the reply, sending a request to perform the operation to the remote device with an authorization derived from at least the cryptographic signature, the cryptographic signature being suitable for verification by the cryptographic signature verification component on the remote device.