The SPARTA Pseudonym and Authorization System

This paper deals with privacy-preserving (pseudonymized) access to a service resource. In such a scenario, two opposite needs seem to emerge. On one side, the service provider may want to control in first place the user accessing its resources, i.e., without being forced to delegate the management of access permissions to third parties to meet privacy requirements. On the other side, it should be technically possible to trace back the real identity of an user upon dishonest behavior, and of course this must be necessary accomplished by an external authority distinct from the provider itself. The framework described in this paper aims at coping with these two opposite needs. This is accomplished through i) a distributed third-party-based instrastructure devised to assign and manage pseudonym certificates, decoupled from ii) a two-party procedure, devised to bind an authorization permission to a pseudonym certificate with no third-party involvement. The latter procedure is based on a novel blind signature approach which allows the provider to blindly verify, at registration time, that the user possesses the private key of the still undisclosed pseudonym certificate, thus avoiding transferability of the authorization permission.