Recently, researches on the shared authentication based on single sign-on have been actively performed so as to solve the problems of current service provider-centric and isolated Internet authentications, including low usability, high cost structure, and difficulty in privacy protection. In order for the shared authentication model, where the authentications of an authentication provider are shared by several Internet service providers, to be accepted in real Internet environment, trustworthiness among users, service providers, and authentication providers on the level of authentication assurance and the level of authentication information protection is necessarily required. This paper proposes a framework for trustworthy and privacy-protected shared authentication protocol based on the user-centric operation and open trust provider network. The proposed framework is differentiated from previous works in the points that it is able to provide interoperable shared authentication services on the basis of open trust infrastructure.