T/Key: Second-Factor Authentication From Secure Hash Chains
This addresses security vulnerabilities in widely used two-factor authentication systems, offering a practical alternative for users and organizations concerned with server-side attacks.
The paper tackles the problem of server-side secret exposure in time-based one-time password (TOTP) systems by introducing T/Key, a system that requires no secrets on the server, making it secure against server compromises, with the trade-off of longer passwords.
Time-based one-time password (TOTP) systems in use today require storing secrets on both the client and the server. As a result, an attack on the server can expose all second factors for all users in the system. We present T/Key, a time-based one-time password system that requires no secrets on the server. Our work modernizes the classic S/Key system and addresses the challenges in making such a system secure and practical. At the heart of our construction is a new lower bound analyzing the hardness of inverting hash chains composed of independent random functions, which formalizes the security of this widely used primitive. Additionally, we develop a near-optimal algorithm for quickly generating the required elements in a hash chain with little memory on the client. We report on our implementation of T/Key as an Android application. T/Key can be used as a replacement for current TOTP systems, and it remains secure in the event of a server-side compromise. The cost, as with S/Key, is that one-time passwords are longer than the standard six characters used in TOTP.