Identity authentication sharing technology which allows many service providers to share the result of identity authentication of an identity provider provides several important advantages including high usability achieved by avoiding repeated registration of identity information to service providers and single sign-on, cost effectiveness of service providers achieved by outsourcing identity authentication services from identity providers, and privacy protection achieved by exposing identity information only to a limited number of controlled identity providers. However, in order for the identity authentication sharing technologies to be widely deployed in global Internet scale, the trustworthiness issue among the participating identity providers, service providers, and users should be resolved in advance. This paper firstly analyzes existing trust frameworks for identity authentication sharing. And then, based on the result of analysis, this paper proposes a dynamic and open trust framework for identity authentication sharing.