A system including a host and a guest device, where the guest device can be implemented on a single packaged integrated circuit or a multichip circuit and have logic to use a physical unclonable function to produce a security key. The device can include logic on the guest to provide the PUF key to the host in a secure manner. The physical unclonable function can use entropy derived from non-volatile memory cells to produce the initial key. Logic is described to disable changes to PUF data, and thereby freeze the key after it is stored in the set.