The lack of effective trust establishment mechanisms for web services impedes the deployment of trust models for online services. One important issue is the lack of privacy protection in trust establishment. Current web service technology encourages a client to reveal all its attributes in a standard credential to the service provider for trust establishment. We propose a mechanism whereby the client formulates a single trust primitive by associating a subset of required attributes in a standard credential to negotiate a trust relationship. Client privacy is preserved because only those required attributes are revealed. After negotiation, a trust group element with dynamic validation is used to represent this trust relationship.