QES-Backed Virtual FIDO2 Authenticators: Architectural Options for Secure, Synchronizable WebAuthn Credentials

FIDO2 and the WebAuthn standard offer phishing-resistant, public-key based authentication but traditionally rely on device-bound cryptographic keys that are not naturally portable across user devices. Recent passkey deployments address this limitation by enabling multi-device credentials synchronized via platform-specific cloud ecosystems. However, these approaches require users and organizations to trust the corresponding cloud or phone providers with the protection and availability of their authentication material. In parallel, qualified electronic signature (QES) tokens and smart-card--based PKCS#11 modules provide high-assurance, hardware-rooted identity, yet they are not directly compatible with WebAuthn flows. This paper explores architectural options for bridging these technologies by securing a virtual FIDO2 authenticator with a QES-grade PKCS#11 key and enabling encrypted cloud synchronization of FIDO2 private keys. We first present and implement a baseline architecture in which the cloud stores only ciphertext and the decryption capability remains anchored exclusively in the user's hardware token. We then propose a hardened variant that introduces an Oblivious Pseudorandom Function (OPRF)-based mechanism bound to a local user-verification factor, thereby mitigating cross-protocol misuse and ensuring that synchronization keys cannot be repurposed outside the intended FIDO2 semantics; this enhanced design is analyzed but not implemented. Both architectures preserve a pure WebAuthn/FIDO2 interface to relying parties while offering different trust and deployment trade-offs. We provide the system model, threat analysis, implementation of the baseline architecture, and experimental evaluation, followed by a discussion of the hardened variant's security implications for high-assurance authentication deployments.