We propose a novel entity authentication approach for pervasive computing environments. A person uses a single device, the Master Key, which aggregates all his digital forms of access tokens for entity authentication. The Master Key discovers and selects proper tokens for its owner. With an emphasis on usability, the Master Key secures authentication, protects privacy information from outsiders and insiders, and supports various claimant-verifier relations. We analyze privacy and security properties of our approach and protocols, and we investigate the overhead. Performance measurements show that our protocols are efficient.