# File lib/u2f/fake_u2f.rb, line 29
  def register_response(challenge, error = false)
    if error
      JSON.dump(:errorCode => 4)
    else
      client_data_json = client_data(U2F::ClientData::REGISTRATION_TYP, challenge)
      JSON.dump(
        :registrationData => reg_registration_data(client_data_json),
        :clientData => U2F.urlsafe_encode64(client_data_json)
      )
    end
  end