Anonymous tokens with private metadata bit

B Kreuter, T Lepoint, M Orrù, M Raykova - … , CA, USA, August 17–21, 2020 …, 2020 - Springer
Advances in Cryptology–CRYPTO 2020: 40th Annual International Cryptology …, 2020Springer
We present a cryptographic construction for anonymous tokens with private metadata bit,
called PMBTokens. This primitive enables an issuer to provide a user with a lightweight,
single-use anonymous trust token that can embed a single private bit, which is accessible
only to the party who holds the secret authority key and is private with respect to anyone
else. Our construction generalizes and extends the functionality of Privacy Pass (PETS'18)
with this private metadata bit capability. It provides unforgeability, unlinkability, and privacy …
Abstract
We present a cryptographic construction for anonymous tokens with private metadata bit, called PMBTokens. This primitive enables an issuer to provide a user with a lightweight, single-use anonymous trust token that can embed a single private bit, which is accessible only to the party who holds the secret authority key and is private with respect to anyone else. Our construction generalizes and extends the functionality of Privacy Pass (PETS’18) with this private metadata bit capability. It provides unforgeability, unlinkability, and privacy for the metadata bit properties based on the DDH and CTDH assumptions in the random oracle model. Both Privacy Pass and PMBTokens rely on non-interactive zero-knowledge proofs (NIZKs). We present new techniques to remove the need for NIZKs, while still achieving unlinkability. We implement our constructions and we report their efficiency costs.
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