A device including a network interface, a memory and a processor. The network interface is configured to communicate with a verifier over a communication network. The memory is configured to store multiple layers of mutable code, the layers identifiable by respective measurements. The processor is configured to generate, for a given boot cycle, a nonce associated uniquely with the given boot cycle, to receive a challenge from the verifier for attestation of a given layer of the mutable code, to calculate an attestation key based on (i) a Unique Device Secret (UDS) stored securely in the device, (ii) a measurement of the given layer taken by another layer, and (iii) the nonce generated for the given boot cycle, to calculate a response for the challenge, by signing the challenge using the attestation key, and to send the response to the verifier for verification of the given layer.