A portable data carrier (1) includes a non-volatile memory (11), a processor (15) and a watermark application (131) executable by the processor (15) which is also stored on the data carrier (1). The watermark application (131) is configured to check (152) data (110, 140) stored on the data carrier (1) or data intended to be stored on the data carrier (1) as to the presence of a digital watermark, or to mark (151) such data (110, 140) with a digital watermark. The check (152) or marking (151) is carried out on the data carrier (1) and after the check (152) or marking (151) the checked or marked data (110, 140) are stored in the non-volatile memory (11) in order to remain there for an indefinite time.