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Comment by lifeisstillgood | Hacker News Reader

I need to get past the press releases and read up, but is this using my phone to authenticate to an IDp (ie google) who then oks me with the relying party (example.com) or am I directly signing up with example.com (ie my phone has a key pair in its secure enclave for google only, for for hundreds of sites I visit

(from memory this will need much bigger storage in the secure enclaves - thousands of accounts is quite feasible)

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You sign up directly with the relying party and are authenticating to a hardware device which then OKs you to the relying party. Each relying party gets a different public key. The corresponding private keys can be stored inside the secure enclave (this setup is called resident keys), or they can be stored in a "key handle" that the relying party stores and provides every time you attempt a login. The key handle would contain the service-specific private key encrypted with a key held inside the secure enclave.

I have not heard of this key handle before - is there any docs on it?

Oh wait, example.com sends me a encrypted key, that I decrypt and then use? That sounds ... odd.

I mean, why not just keep the same encrypted data on my local phone ? The attack surface seems much smaller.

The U2F spec

https://fidoalliance.org/specs/fido-u2f-v1.2-ps-20170411/fid...

has a clear description in the "Allowing for Inexpensive U2F Devices" section.

For webauthn, they don't use key handle terminology anymore, but the same fuctionality is provided by the "Credential ID":

https://www.w3.org/TR/webauthn-2/#credential-id

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