An integrated circuit (10) comprises a scan chain (14) with parallel inputs and outputs coupled to a functional circuit (12a-c). A scan chain modifying circuit (43, 47, 70a-c) is provided coupled to the scan chain (14). When testing is authorized the scan chain modifying circuit operates in a mode wherein a normal shift path is provided through the scan chain. When testing is not authorized the scan chain modifying circuit (43, 47, 70a-c) operates to effect spontaneous dynamic changes in the shift path, which dynamically vary the length of the shift path between external terminals of the integrated circuit while shifting takes place. In an embodiment the dynamical variations are controlled by a running key comparison. In other embodiments running key comparison is used to disable transfer through the scan chain and/or operation of functional circuits.