Domain-Specific Pseudonymous Signatures for the German Identity Card

The restricted identification protocol for the new German identity card basically provides a method to use pseudonyms such that they can be linked by individual service providers, but not across different service providers (even not malicious ones). The protocol can be augmented to allow also for signatures under the pseudonyms. In this paper, we thus view —and define— this idea more abstractly as a new cryptographic signature primitive with some form of anonymity, and use the term domain-specific pseudonymous signatures. We then analyze the restricted identification solutions in terms of the formal security requirements.