A new login technique is becoming available in 2023: the passkey. The passkey promises to solve phishing and prevent password reuse. But lots of smart and security-oriented folks are confused about what exactly a passkey is. There’s a good reason for that. A passkey is in some sense one of two (or three) different things, depending on how it’s stored.
I’m not sure I follow you - if someone can compromise the key material on my phone that is protected by a different factor, then it doesn’t matter whether the 2FA is server-side or not, it’s compromised either way.
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It seems like you are trying to protect against a compromise of the user’s device. But if their device is compromised then their session is compromised after auth anyway and you aren’t solving much with extra auth factors.
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If the user can perform all steps on the same device then it doesn’t make sense to assume only specific set of keys will be disclosed, you have to assume everything on the device can be compromised