Anonymous k -Show Credentials

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Privacy-preserving digital credentials are cryptographic tools that allow a user to prove a predicate about his/her identity or qualifications, without the verifying party learning additional information beyond the status of that predicate. The Identity Mixer (Idemix) [CL01] is a framework providing such credentials. In Idemix, we can distinguish two types of credentials: (1) one-time show credentials which can be shown only once before unveiling the identity of their holder, and (2) multi-show credentials which can be shown infinitely many times without the showings being linked to each other, or to the identity of their holder. In this paper, we bridge the gap between the two previous types of credentials, and extend Idemix to k-show credentials (for k > 1.) The k-show credentials we propose can be shown anonymously, but linkably, up to k times.