Protection of digital data contained within an integrated circuit with a JTAG interface

The method involves making a digital quantity dependent on a stored value (HID-VAL) in a microcontroller in a non volatile manner, where the quantity is made inaccessible if signals are present in input of joint test action group (JTAG) interface (17`). The value is constituted of a parameter for generating a key used by a cryptographic function of the microcontroller. Another value (DEB VAL) is used to generate the key of the cryptographic function when the signals are present in the input of the interface. An independent claim is also included for a microcontroller comprising central processing unit, non volatile storage element and JTAG interface.