A Time-based One-Time Password (TOTP) validator is interposed between a principal and a network service. The validator interacts with a mobile application (app) on the mobile device associated with the principal to dynamically supply a validator secret. The secret and, perhaps, other information are processed by the app to generate a TOTP when the principal attempts to access a protected resource of the network service. The validator independently generates the TOTP and compares the app generated TOTP, and on a successful match, a principal's access device is redirected for access to the protected resource.