To provide a secure service to an application virtual machine running in a first domain of a virtualized computing platform, a second domain is arranged to run a corresponding service driver exclusively for the application virtual machine. As part of the secure service, the service driver effects a key-based cryptographic operation; to do so, the service driver has to obtain the appropriate key from a key manager. The key manager is arranged to store the key and to release it to the service driver only upon receiving evidence of its identity and being satisfied of compliance with release policies associated with the key. These policies include receipt of valid integrity metrics, signed by trusted-device functionality of the virtualized computing platform, for the service driver and the code on which it depends.