Hi, I „solved“ it with fido2 not doing signing but gatekeeping the signing process and placed share A into the Secure Enclave of the phone and share B on the Secure Enclave of the server, this way we have no share or signing ever exposed anywhere, not even in RAM, does this sound right to you?

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This sounds like it would work but I know one issue (for signing) is whatever process is doing the signing needs to be able to compute on the secp curve. I know iPhone for example does not have libsecp for their secure element

Does any fido2 standards support secp256k1?

Unfortunately you do not get secp256k1 with fido2, except some old YubiKeys... I could hack-it into the card, but then i lose the fido2 certification, so the idea now is to just use the passkey as a hardware gatekeeper for the MPC shares.

Like this:

https://www.mermaidchart.com/app/projects/bb9914a7-5c02-4735-a741-6177a3bf1230/diagrams/5218575b-0bfa-4f0d-9253-5dc5c4de8da2/version/v0.1/edit