Abstract: Methods and apparatuses for forgery security, issuing and monitoring or reducing the value of data carriers such as value cards, entitlement cards, travel tickets, etc., are described, the data being written into a magnetic layer. According to the method, physical authenticating features of the data carrier are evaluated as inhomogeneities in the magnetic layers which carry the data. Suitable data carriers comprise an individual and characteristic inhomogeneity produced, for example, by fluctuations in thickness, discontinuities (13) etc. The second side of the data carrier (1) is provided with an absorbent coating (15) which serves to receive ink sprayed on by an ink jet printer. For reading and/or coding the data carriers which are provided with two magnetic strips (12, 12') independently of the position of the data carriers, there are in an apparatus two reading-writing heads (60, 60') and two pressure applying rollers (61, 61.degree.