A system and method provide efficient, secure, and highly reliable authentication for transaction processing and/or access control applications. A Personal Digital Key is a portable device carried by an individual that stores one or more profiles (e.g., a biometric profile) in a tamper-proof memory. When multiple PDKs are present at the point of the transaction, the system automatically determines which PDK to associate with the authentication and transaction processes. The differentiation decision is based on one or more differentiation metrics including distance information, location information, and detection duration information associated with each of the PDKs within range. Profile samples comprising subsets of the profile information are received to provide a quick correlation between a PDK an input sample (e.g., a subset of a biometric input). After determining which PDK should be associated with the transaction, a full authentication process is executed.