Use a single challenge for U2F authentication.

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) {