Efficient Non-transferable Anonymous Multi-show Credential System with Optional Anonymity Revocation

A credential system is a system in which users can obtain credentials from organizations and can demonstrate possession of these credentials. Such a system is anonymous when transactions carried out by the same user cannot be related. An anonymous credential system is of significant practical relevance because it is the best means of providing privacy for users. In this paper, we propose a practical anonymous credential system considerably superior to existing ones:
(1) We give the first practical solution that allows a user to unlinkably demonstrate possession of a credential as many times as necessary without involving the issuing organization.
(2) We suggest more effective means of preventing users from sharing their identity, by introducing all-or-nothing sharing: a user who allows a friend to use her identity once, gives him the ability to use all of her credentials. This is
implemented by a new cryptographic primitive, called circular encryption, which is of independent interest.
(3) To prevent misuse of anonymity, our scheme is the first to offer optional anonymity revocation for particular transactions.
(4) Our scheme offers separability: all organizations can choose their cryptographic keys independently of each other.

Keywords. Privacy protection, credential system, e-cash, blind-signatures, circular encryption.