Explain that first factor is up to you as to how you want to implement it.
Sequence Diagram
pre_sign
dictionary ObtainU2fRequest {
DOMString type;
DOMString username; // TODO(mirko): need to get this another way.
};
type of type DOMString
The type of U2F Request to obtain is either "pre_sign" or "pre_enroll".
username of type DOMString
The username to obtain a Request for. This needs to be authenticated instead.
The response is a U2F Request dictionary. (see fido-u2f-javascript-api; 3.1.1 Dictionary Request Members)