EndUserCredentialToRegister
A credential the client will sign with from now on. Its shape follows the mechanism, so what
belongs to one cannot be stated for another: a key the client keeps encrypted has to arrive with
the encrypted half, and an authenticator has none to give.
{
"kind": "webAuthn",
"credentialId": "string",
"clientData": "string",
"attestationData": "string"
}