Does this require Google Play Services on Android? If so that is a shame. There are other FIDO SDKs other than Google's which would be nice for us degoogled users. Edit: it requires play services. Sigh.
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So if I add a security key on my phone, I'll have to use it on the web too? Or would I need to make sure I have multiple keys so I can log in with either one?
Not sure I understand the question. A key can be used on Web as well as on the mobile apps now, instead of TOTP (which however cannot be yet disabled). It is generally advised to have a multiple key pair, incase one is lost / defect.
I thought I read that some phones can be the security key, hence my confusion as to how would that work