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