I asked the same question earlier, then found out that Yubikeys can work with a lightning-USB adapter.
[1]https://support.yubico.com/support/solutions/articles/150000...
[1]https://support.yubico.com/support/solutions/articles/150000...
Again iOS 11 only supports the One-time password (OTP) featureset:
“When a YubiKey is connected to an iOS device, only the OTP mode functions listed below are available. Yubico OTP Static Password OATH-HOTP”
Does anyone know if iOS 12 will update Core NFC to overcome: “Core NFC [in iOS 11] does not allow for write operations that are required for authentication protocols like FIDO U2F.” [1] https://www.yubico.com/2017/10/iphone-support-yubikey-otp-vi...