Requirements for attestation for DPK

[keikoit]

Based on discussions with various RPs as well as system integrators for RPs, here are the requirements for attestation for DPK (device-bound public key).

 If DPK is supported, 
         1. an attestation statement shall be included, and
         2. the attestation shall be protected from replay attacks

Replay attack protection may be achieved by such a way like including RP-challenge in DPK signature. Using clientDataHash will enable the protection with a minimum extension from the current draft specification.
Without replay attack protection, DPK is equivalent to a bearer token. RPs who need DPK cannot trust such DPK and DPK will not be so useful for RPs.

Related issue: #1798

[kkoiwai]

My understanding is that at the TPAC in 2022-9-13, we confirmed that DPK is replay-protected as DPK signs over clientDataHash in the current PR1663, yet it is unclear.
I suggest to close this issue after the PR is modified to make it clear.

[ko-koiwai]

Two comments added to PR1663.
https://github.com/w3c/webauthn/pull/1663/files#r971374524
https://github.com/w3c/webauthn/pull/1663/files#r971375390

[agl]

I believe the comments on #1663 have been addressed with a clarifying note, thus closing this issue as suggested.