Key establishment for constrained resource devices

It is disclosed a method and a constrained resource device (502, 70, 90) for establishing a secret first key between a client device (506) and the constrained resource device. The invention also relates to a method and an authorization server (504, 60, 80) for enabling establishing a secret first key between a client device (506) and the constrained resource device. Based on a secret second key shared (508) between the constrained RD and the AS, the secret first key shared between the constrained resource device and the client device can be established. Devices having constrained resources cannot use protocols with which additional messages are required to share a secure identity. Embodiments of the present invention have the advantage that a secret identity can be established within an authentication protocol and that no additional messages are required to establish the secret identity.