A method comprises: a first data processing device requesting attestation of a second data processing device; the second data processing device generating a device-specific attestation message in dependence upon a device-specific key, a hardware configuration of the second data processing device and a software configuration of software running on the second data processing device; the second data processing device generating an application-specific attestation message in dependence upon an interaction protocol by which the first data processing device and the second data processing device interact; the second data processing device cryptographically binding the application-specific attestation message to the device-specific attestation message; the first data processing device verifying the application-specific attestation message, the verifying step comprising detecting a trusted status of the application-specific attestation message by verifying the device-specific attestation message cryptographically bound to the application-specific attestation message; and the first data processing device establishing an interaction with the second data processing device according to the interaction protocol, in dependence upon the verified application-specific attestation message.