Restricted Identification Scheme and Diffie-Hellman Linking Problem
We concern schemes designed for user authentication in different systems (called sectors) with a single private key so that activities of the same person in different sectors are not linkable. In particular, we consider Restricted Identification scheme implemented on personal identity cards (neuer Personalausweis) issued by German authorities. The schemes we concern are devoted for practical application on personal identity cards where limitations of memory size is a critical issue.
Unlinkability for German Restricted Identification is silently based on random oracle model. We prove that the construction can be simplified by eliminating hiding certain values with hash functions: we show that unlinkability can be based on a problem that we call Linking Diffie-Hellman Problem (LDHP). We prove that LDHP is as hard as Decisional DHP. Thereby we justify unlinkability in the standard model.
We also introduce and analyze a variant of German Restricted Identification providing active authentication. This protocol is intended for application areas where the right to access a sector is not by default (as for German Restricted Identification) and can be both granted and blocked. It is intended to serve as anonymous identity for sectors such as access to medical data and law enforcement, where prevention of Sybil attacks is a fundamental requirement.
Keywordsanonymous identifier unlinkability ephemeral-static Diffie-Hellman authentication
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