[ehzy]

Does anyone know if this works with Yubikey 4? I have a lightning to USB adapter but I can't seem to get it to work with these keys. It worked fine with A USB-C + NFC Yubikey Security key and a Google Titan key, but not Yubikey 4.

[chrizel]

According to this recent support article by Yubico, the YubiKey 4 series should be compatible: https://support.yubico.com/hc/en-us/articles/7449189070620

[lxgr]

The Yubikey 4 might not be FIDO certified? Apparently Apple accepts only certified authenticators, i.e. they presumably check the attestation certificate against an allowlist that the Yubikey 4 might not be on.

Maybe they also require CTAP2 (as opposed to U2F together with the browser‘s compatibility layer)?