OpenSK
OpenSK is an open-source implementation for security keys
...The project emphasizes defense-in-depth: memory-safe Rust, hardened crypto, isolation via an embedded OS, and explicit user presence/verification flows. Developers can flash reference hardware, customize UX (LEDs, buttons, PIN), and experiment with extensions while staying interoperable with major browsers and platforms. A test and conformance suite validates protocol behavior against the WebAuthn/CTAP specs to ensure compatibility.