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Exchange of attribute credentials is a means to establish mutual trust between strangers that wish to share resources or conduct business transactions. Automated Trust Negotiation (ATN) is an approach to regulate the exchange of sensitive credentials by using access control policies. Existing ATN work makes unrealistic simplifying assumptions about credential-representation languages and credential storage. Moreover while existing work protects the transmission of credentials, it fails to hide the contents of credentials, thus providing uncontrolled access to potentially sensitive attributes. To protect information about sensitive attributes, we introduce the notion of attribute acknowledgment policies (Ack policies). We then introduce the trust target graph (TTG) protocol, which supports a more realistic credential language, Ack policies, and distributed storage of credentials.