CHIP and CRISP: Protecting All Parties Against Compromise through Identity-Binding PAKEs
Recent advances in password-based authenticated key exchange (PAKE) protocols can offer stronger security guarantees for globally deployed security protocols. Notably, the OPAQUE protocol [Eurocrypt2018] realizes Strong Asymmetric PAKE (saPAKE), strengthening the protection offered by aPAKE to compromised servers: after compromising an saPAKE server, the adversary still has to perform a full brute-force search to recover any passwords or impersonate users. However, (s)aPAKEs do not protect client storage, and can only be applied in the so-called asymmetric setting, in which some parties, such as servers, do not communicate with each other using the protocol. Nonetheless, passwords are also widely used in symmetric settings, where a group of parties share a password and can all communicate (e.g., Wi-Fi with client devices, routers, and mesh nodes; or industrial IoT scenarios). In these settings, the (s)aPAKE techniques cannot be applied, and the state-of-the-art still involves handling plaintext passwords. In this work, we propose the notions of (strong) identity-binding PAKEs that improve this situation: they protect against compromise of any party, and can also be applied in the symmetric setting. We propose counterparts to state-of-the-art security notions from the asymmetric setting in the UC model, and construct protocols that provably realize them. Our constructions bind the local storage of all parties to abstract identities, building on ideas from identity-based key exchange, but without requiring a third party. Our first protocol, CHIP, generalizes the security of aPAKE protocols to all parties, forcing the adversary to perform a brute-force search to recover passwords or impersonate others. Our second protocol, CRISP, additionally renders any adversarial pre-computation useless, thereby offering saPAKE-like guarantees for all parties, instead of only the server. We evaluate prototype implementations of our protocols and show that even though they offer stronger security for real-world use cases, their performance is in line with, or even better than, state-of-the-art protocols.