An apparatus and method are disclosed for determining authentication frequency (i.e., the length of time between authenticating and re-authenticating a user) and challenge type (e.g., username/password, fingerprint recognition, voice recognition, etc.) based on one or more environmental properties (e.g., ambient noise level, ambient luminosity, temperature, etc.), or one or more physiological properties of a user (e.g., heart rate, blood pressure, etc.), or both. Advantageously, the illustrative embodiment enables authentication frequency and challenge type to be adjusted based on the likelihood of malicious activity, as inferred from these properties. In addition, the illustrative embodiment enables the authentication challenge type to be tailored to particular environmental conditions (e.g., noisy environments, dark environments, etc.).