Abstract: A self-contained biometric device, method, and system for secure digital transactions is provided. The device comprises a housing that may be a subdermal implant, dermally-applied biometric tattoo, ergonomic external wearable, or smart card. The housing maintains consistent secure functionality during offline and online operation modes and includes a biometric verification module that captures a biometric input of a user and performs user authentication entirely within the device. A hardware-isolated storage module physically isolated from external networks stores cryptographic keys, credentials, and biometric templates. The device permits access to the cryptographic keys and credentials only upon successful user authentication. A transaction processing module resident inside the hardware-isolated storage module executes digital asset transactions entirely within the device in offline mode.