Signing with multiple ID´s and a single key

We propose a solution for electronic identity cards that makes it possible to create multiple identities for a person creating digital signatures with just one private key stored on his personal electronic identity card. The public keys corresponding to this secret key belong to different sectors, devoted to different applications or activity areas (like private use and signing in behalf of a company or an authority). We provide strong privacy guarantees called unlinkability. It means that given two public keys from different sectors and some signatures corresponding to these keys, it is unfeasible to say if these keys (and signatures) come from the same person. The proof is performed in the random oracle model and reduces linkability to the Decisional Diffie-Hellman Problem. Our proposal extends the idea of Restricted Identification introduced in German personal identity cards from unlinkable sector identification to unlinkable sector signatures.