This invention concerns a method of shielding manipulations of secret data in an authentication chip from observation. In another aspect it concerns an authentication chip for performing the method. The secret data is manipulated in non-flashing CMOS structures, in which pMOS and nMOS transistors are driven such that they do not have intermediate resistance simultaneously during a change of state of the structure. And at the same time conventional CMOS inverters are operated adjacent the non-flashing CMOS structures.