A flexible authentication system is described herein that fluidly switches between a federated authentication model and a local short-lived token model that does not require sophisticated authentication infrastructure at the relying party site. Upon detecting an event that causes the identity provider to be unavailable for authentication, the relying party switches to a temporary token model. The system generates a bearer token or challenge associated with the user's identity and (optionally) associated with time data that limits the period during which the token is valid. The relying party communicates the short-lived token to the user using contact information associated with the user and already stored by the relying party. Upon receiving the short-lived token, the user provides the short-lived token to the relying party, and the relying party processes the token to validate the user's identity and then allows the user to access the relying party's online services.