The invention relates to a data carrier having a semiconductor chip (5) with at least one memory. The memory contains an operating program that is able to perform at least one operation (h). In order to prevent unauthorized access to the data (x) processed with the operation (h), both said data and the operation (h) itself are disguised. The disguising of the data (x) and the operation (h) is coordinated such that the disguised operation (h.sub.R1R, h.sub.R1R2) generates either the output data (y) of the undisguised operation (h) or disguised output data (y{circle around (x)}R.sub.2) from which the output data (y) can be determined.