PUF Physically Unclonable Function
A circuit deriving a unique key from the silicon's own manufacturing variation rather than storing it anywhere. The key therefore doesn't exist while the power is off.
The commonest variant reads which state uninitialised SRAM settles into at power-up. Every cell is theoretically symmetric but never actually is, so it falls whichever way its transistors happen to lean — a pattern unique to that particular chip and impossible to deliberately manufacture a copy of. The appeal is obvious: somebody who takes the device apart and reads out memories and fuses finds nothing, because the key only exists during the milliseconds it is regenerated. Since the response varies slightly with temperature, voltage and ageing, error-correcting helper data is needed to get a stable key. Caveats exist — several designs have been broken by machine-learned models of the circuit's behaviour.