Use a single challenge for U2F authentication.

1. According to the spec, either we have a single challenge with
   a number of `signRequests`, or a number of `signRequests`, each with
   it's own challenge.

2. Previously, we had both these - per-request challenges, as well as a
   single extra challenge.

3. This commit changes this so that the per-request challenges are
   removed, leaving only a single challenge, as per the v1.1 U2F API.

4. The existing implementation didn't work in Firefox, because the
   Firefox (extension) implementation is less flexible with regard to
   the inputs.

5. Fix teaspoon specs.

6. References: https://fidoalliance.org/specs/fido-u2f-v1.0-nfc-bt-amendment-20150514/fido-u2f-javascript-api.html#h2_background

app/assets/javascripts/u2f/authenticate.js.coffee

@@ -6,8 +6,17 @@
 class @U2FAuthenticate
   constructor: (@container, u2fParams) ->
     @appId = u2fParams.app_id
-    @challenges = u2fParams.challenges
-    @signRequests = u2fParams.sign_requests
+    @challenge = u2fParams.challenge
+
+    # The U2F Javascript API v1.1 requires a single challenge, with _no
+    # challenges per-request_.
+    #
+    # The U2F Javascript API v1.0 requires a challenge per-request, which
+    # is done by copying the single challenge into every request.
+    #
+    # In either case, we don't need the per-request challenges that the server
+    # has generated, so we can remove them.
+    @signRequests = u2fParams.sign_requests.map (request) -> _(request).omit('challenge')
   start: () =>
     if U2FUtil.isU2FSupported()
@@ -16,7 +25,7 @@ class @U2FAuthenticate
       @renderNotSupported()
   authenticate: () =>
-    u2f.sign(@appId, @challenges, @signRequests, (response) =>
+    u2f.sign(@appId, @challenge, @signRequests, (response) =>
       if response.errorCode
         error = new U2FError(response.errorCode)
         @renderError(error);

app/controllers/concerns/authenticates_with_two_factor.rb

@@ -57,7 +57,7 @@ module AuthenticatesWithTwoFactor
   # Authenticate using the response from a U2F (universal 2nd factor) device
   def authenticate_with_two_factor_via_u2f(user)
-    if U2fRegistration.authenticate(user, u2f_app_id, user_params[:device_response], session[:challenges])
+    if U2fRegistration.authenticate(user, u2f_app_id, user_params[:device_response], session[:challenge])
       # Remove any lingering user data from login
       session.delete(:otp_user_id)
       session.delete(:challenges)
@@ -77,9 +77,8 @@ module AuthenticatesWithTwoFactor
     if key_handles.present?
       sign_requests = u2f.authentication_requests(key_handles)
-      challenges = sign_requests.map(&:challenge)
-      session[:challenges] = challenges
-      gon.push(u2f: { challenges: challenges, app_id: u2f_app_id,
+      session[:challenge] ||= u2f.challenge
+      gon.push(u2f: { challenge: session[:challenge], app_id: u2f_app_id,
                       sign_requests: sign_requests })
     end
   end

spec/javascripts/u2f/authenticate_spec.coffee

@@ -5,13 +5,12 @@
 #= require ./mock_u2f_device
 describe 'U2FAuthenticate', ->
-  U2FUtil.enableTestMode()
   fixture.load('u2f/authenticate')
   beforeEach ->
     @u2fDevice = new MockU2FDevice
     @container = $("#js-authenticate-u2f")
-    @component = new U2FAuthenticate(@container, {}, "token")
+    @component = new U2FAuthenticate(@container, {sign_requests: []}, "token")
     @component.start()
   it 'allows authenticating via a U2F device', ->

spec/javascripts/u2f/register_spec.js.coffee

@@ -5,7 +5,6 @@
 #= require ./mock_u2f_device
 describe 'U2FRegister', ->
-  U2FUtil.enableTestMode()
   fixture.load('u2f/register')
   beforeEach ->

spec/support/fake_u2f_device.rb

@@ -18,8 +18,8 @@ class FakeU2fDevice
   def respond_to_u2f_authentication
     app_id = @page.evaluate_script('gon.u2f.app_id')
-    challenges = @page.evaluate_script('gon.u2f.challenges')
-    json_response = u2f_device(app_id).sign_response(challenges[0])
+    challenge = @page.evaluate_script('gon.u2f.challenge')
+    json_response = u2f_device(app_id).sign_response(challenge)
     @page.execute_script("
     u2f.sign = function(appId, challenges, signRequests, callback) {