Stateless 2PC Signatures for Internet-Scale Authentication and Authorization
Résumé
The industry is moving away from passwords for authentication and authorization, with hardware devices for storing long-term cryptographic keys emerging as the leading alternative. However, these devices often have limited displays and remain vulnerable to theft, malware, or tricking users into signing malicious payloads. Current systems provide little fallback security in such cases. Any solution must also meet strict requirements: compatibility with industry standards, scalability to handle high request volumes, and high availability.
Our approach virtualizes the authenticating/authorizing party via a two-party protocol with an informed helper. Keys are never assembled in one place, ensuring that the keys are protected even in the event of device compromise; all signatures are policy-checked; and the protocol is stateless, permitting helper clones to assume one another's work without coordination and thereby supporting horizontal scalability.
We formalize the required properties for such protocols and show how they are met by existing schemes (e.g., FROST for Schnorr, Boneh-Haitner-Lindell-Segev'25 for ECDSA). Motivated by the widespread use of ECDSA (FIDO2/Passkeys, blockchains), we introduce a new, optimized two-party ECDSA protocol that improves performance by about 4×-10× relative to the stateless BHLS'25 protocol at similar communication costs. At its core is a new variant of exponent-VRF, improving on earlier constructions and of independent interest. We validate our design with a proof-of-concept virtual authenticator for the FIDO2 Passkeys framework.
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