ISO 15764:2004 describes an extension of data link protocols for enhancing the security of data transfers between electronic control units (ECUs) connected by a communication network used in road vehicles. It is based on cryptographic methods that include encryption, digital signatures and message authentication codes (MACs). It provides a description of services to establish ECUs as trusted parties in respect of one another and to protect against specific threats. It is applicable to all data links between pairs of ECUs capable of storing and processing secret data so that unauthorised third parties are denied access to it. Parameters are provided to enable the level of security in the data link to be selected.