Circuitry and methods prevent unauthorized programming, or reprogramming, of a programmable device, by requiring a signature in the configuration data to match a signature previously stored in the programmable device. A programmable integrated circuit device includes an input for configuration data, and programming control circuitry operable to derive a current signature from the configuration data, examine a first bit stored in the programmable integrated circuit device, and when the first bit is in a first state, compare the current signature to a first predetermined signature stored in the programmable integrated circuit device and configure the programmable integrated circuit device according to the configuration data only when the current signature matches the first predetermined signature, and when the first bit is in a second state, configure the programmable integrated circuit device according to the configuration data without comparing the current signature to the first predetermined signature.