Abstract
Messaging systems, where a user maintains a set of contacts and broadcasts messages to them, are very common. In a situation where a user only sends messages directly to a set of online contacts, a contact might miss a message if it is not available to receive it directly from the user. This work addresses the problem of a trusted contact's obtaining a message that it missed, from other trusted contacts of the user, while maintaining the anonymity of all participating contacts. A protocol is presented to facilitate this communication. An experimental framework is developed to evaluate various possible configurations of the entities involved.
The techniques developed to address the above problem are extended to address the problem of a user's authenticating with a service provider while ensuring that multiple sessions are unlinkable. The proposed approach achieves this by setting up an authenticated secure channel between the user and the service provider. Information exchanged for the setup of this secure channel is unique over multiple authentications. The proposed protocol is further enhanced to accommodate service provider policies that use credentials with relationship constraints among them. In such cases, the service provider will not be able to analyze and identify sets of users who authenticate with different credential subsets. The proposed credential revocation scheme allows an identity provider to revoke user credentials without compromising user privacy, even while relying on a public channel. Moreover, these protocols do not require the identity provider to remain online during authentication and revocation. Finally, details on how to adapt the proposed identity management system to privately manage healthcare records is presented as an application of the proposed system.
