[vader1]

The difference between WebAuthn and U2F isn't really clear to me, but I hope the older generations will continue to work with common browser's FIDO2 implementations for a long time to come. YubiKeys are great, but too expensive to replace on a whim every year.

[StavrosK]

U2F can only be used as a second factor. FIDO2 can be used as a replacement for a username/password, so you can go to a site, insert your FIDO2 key and log in without any other information.

Old Yubikeys only support U2F, and there's a Yubico FIDO2 key. Browser support isn't there yet, I've been trying to write a Django library for it but no browser will support the complete FIDO2 flow as far as I know.

[lvh]

> U2F can only be used as a second factor.

I mean, certificationally, sure. But what prevents a website from trusting you to input your identifier (user name or e-mail address) and then accepting a U2F signed blob as your only credential?

[StavrosK]

The fact that if someone steaks the key they can impersonate you anywhere. U2F doesn't support pin authentication, as far as I know.

[lvh]

It's not built into the protocol, sure, but the device can do whatever it wants before it decides to sign something. You need to unlock my phone and SEP before Krypton signs anything, for example. But you still raise a good point: I suppose it's a good thing that we can do that with cheaper devices and safe, browser-provided UX for PIN entry.

(I would suggest that a single physical device that takes a PIN before it does some signing is still 2FA even if there is only one _communicated_ credential, but I appreciate we need better terminology for this. Other versions of this to consider: if you username + password + Duo into your SSO portal and then sign into a service with SAML, is that not 2FA? If it isn't, does using a session cookie prevent something from being 2FA? For the latter, I'd say obviously not :-) I think the "who can impersonate you" is a good line in the sand, since in the former the SSO system holds full authority in most cases.)

[StavrosK]

I completely agree with everything you said, but a U2F token that takes a PIN would be hard in the sense that it would be hard to make it cross-platform (unless the PIN pad were on the actual device or something).

[Leace]

> FIDO2 can be used as a replacement for a username/password, so you can go to a site, insert your FIDO2 key and log in without any other information.

Technically U2F can be used to design passwordless scheme too (returning a big array of all key handles known to a service) but FIDO2 probably has some optimizations in this area.

[vader1]

Ah, that's perfect. A hardware token as the /only/ factor sounds like a bridge too far anyhow.

[StavrosK]

I don't know, it sounds perfect to me.

[emlun]

It's a hardware token that supports a local PIN as a second factor.

[cflee]

WebAuthn is backward compatible with U2F tokens, but naturally only for use as second factor. They defined CTAP1 as the U2F protocol for existing tokens, then defined the new CTAP2 for communication with the new tokens, and made both part of the spec.

From a Yubico blog post (https://www.yubico.com/2018/05/what-is-fido2/):

> WebAuthn and CTAP2 are both required to deliver the FIDO2 passwordless login experience, but WebAuthn still supports FIDO U2F authenticators, since CTAP1 is also part of the WebAuthn specification.