Reliable Password Hardening Service with Opt-Out

As the most dominant authentication mechanism, password-based authentication suffers catastrophic offline password guessing attacks once the authentication server is compromised and the password database is leaked. Password hardening (PH) service, an external/third-party crypto service, has been recently proposed to strengthen password storage and reduce the damage of authentication server compromise. However, all existing schemes are unreliable in that they overlook the important restorable property: PH service opt-out. In existing PH schemes, once the authentication server has subscribed to a PH service, it must adopt this service forever, even if it wants to stop the external/third-party PH service and restore its original password storage (or subscribe to another PH service). To fill the gap, we propose a new PH service called PW-Hero that equips its PH service with an option to terminate its use (i.e., opt-out). In PW-Hero, password authentication is strengthened against offline attacks by adding external secret spices to password records. With the opt-out property, authentication servers can proactively request to end the PH service after successful authentications. Then password records can be securely migrated to their traditional salted hash state, ready for subscription to other PH services. Besides, PW-Hero achieves all existing desirable properties, such as comprehensive verifiability, rate limits against online attacks, and user privacy. We define PW-Hero as a suite of protocols that meet desirable properties and build a simple, secure, and efficient instance. Moreover, we develop a prototype implementation and evaluate its performance, establishing the practicality of our PW-Hero service.