Page Comparison

Explain that first factor and other forms of second factor authentication is up to you the developer as to how you they want to implement it.

Sequence Diagram 

Both the U2F Register and Authenticate have the same general sequence of events that occur.

First the application must obtain a U2F Request from the U2F Server.

That U2F Request is then passed to the U2F Client. which handles special processing of that Request and passing the 

The U2F Client takes the Request and properly formats it and sends it to the U2F Authenticator.

Obtain U2F Request

The first thing that the application needs to to for both Register and Authenticate is obtain a U2F Request from the U2F Server. This is done with an ObtainU2fRequest. There is a SurePassID U2F Client API class that encapsulates this functionality. It is the ObtainU2fRequestTask. 

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)The Sign In Activity must extend SurePassIdU2fSignActivity. It must implement the following abstract methods.

protected abstract void displayMessage(int messageId);

protected abstract void displayMessage(CharSequence message);

protected abstract void gotoTargetActivity();

protected abstract void gotoAlternateVerificationActivity();

protected abstract String getU2fServerUrl();

protected abstract void resetFormValues();

protected abstract void enableForm();

Upon successfully authenticating the user it should call

protected void u2fSign(String username)

That method will perform all the necessary U2F operations.