Biometric applications are stepping into a new dimension with respect to size and complexity, due to the decision of many countries to introduce biometric travel documents. Thus, many problems become increasingly relevant, two of which we address: secure and privacy-preserving storage of biometric references and issues of sensor interoperability. For the first problem, a review of alternative storage methods (physical, optical, centralized database) is presented under consideration of security aspects. The second problem is addressed by a review of scientific activities in the domain and a detailed discussion for one specific exemplary biometric: online handwriting. Results of an experimental intersensor cross-validation are presented, which consist of different semantic classes, forgery strengths, and sensor characteristics. The main conclusions are that the degradation of recognition acuracy in online signature verification may reach up to a factor of 4.4 for nonskilled forgeries in cross-sensor verification scenarios, but is not as significant for skilled forgeries and is of the same order as for fingerprint systems. Further, the enrollment data representation of the chosen reference algorithm, BioHash, is compact enough for storage in identity (ID) documents. Thus, the application of online handwriting as supplementary modality appears feasible in future interoperable ID document scenarios.