This disclosure provides techniques for recovering a root key from measurement of a circuit function. In some embodiments, a checkpointing feature is used to periodically mark measurements of this function and thereby track drift in the value of the root key over the life of a digital device; the checkpointing feature permits rollback of any measurement of the function in a manner that negates incremental drift and permits recovery of the root key for the life of a device (e.g., an IC circuit or product in which the IC is embedded). This disclosure also provides novel PUF designs and applications.