Abstract: A data carrier with an electronic circuit for storing and processing data is provided with an information strip with optically readable markings, and preferably a test field and an authentication feature. An optical interface, for example, can be provided for communication with a write/read device. The markings represent a digital signature. Testing of the authenticity of the data carrier can be done on the basis of the physical properties of the markings, the internal structure of the signature and/or the proof that a test number stored in the electronic circuit is equal to a number calculated from the signature according to a pre-determined function. The signature can also serve as a cryptographic key for encoding and decoding the data to be exchanged between the electronic circuit and the write/read device. The test field can be irreversibly modified when the data carrier is used.