use secure string comparisons for basic auth username / password

this will avoid timing attacks against applications that use basic auth.

Conflicts:
	activesupport/lib/active_support/security_utils.rb

CVE-2015-7576

actionpack/lib/action_controller/metal/http_authentication.rb

@@ -1,4 +1,5 @@
 require 'base64'
+require 'active_support/security_utils'
 
 module ActionController
   # Makes it dead easy to do HTTP Basic, Digest and Token authentication.
@@ -70,7 +71,11 @@ module ClassMethods
           def http_basic_authenticate_with(options = {})
             before_action(options.except(:name, :password, :realm)) do
               authenticate_or_request_with_http_basic(options[:realm] || "Application") do |name, password|
-                name == options[:name] && password == options[:password]
+                # This comparison uses & so that it doesn't short circuit and
+                # uses `variable_size_secure_compare` so that length information
+                # isn't leaked.
+                ActiveSupport::SecurityUtils.variable_size_secure_compare(name, options[:name]) &
+                  ActiveSupport::SecurityUtils.variable_size_secure_compare(password, options[:password])
               end
             end
           end

activesupport/lib/active_support/security_utils.rb

@@ -0,0 +1,27 @@
+require 'digest'
+
+module ActiveSupport
+  module SecurityUtils
+    # Constant time string comparison.
+    #
+    # The values compared should be of fixed length, such as strings
+    # that have already been processed by HMAC. This should not be used
+    # on variable length plaintext strings because it could leak length info
+    # via timing attacks.
+    def secure_compare(a, b)
+      return false unless a.bytesize == b.bytesize
+
+      l = a.unpack "C#{a.bytesize}"
+
+      res = 0
+      b.each_byte { |byte| res |= byte ^ l.shift }
+      res == 0
+    end
+    module_function :secure_compare
+
+    def variable_size_secure_compare(a, b) # :nodoc:
+      secure_compare(::Digest::SHA256.hexdigest(a), ::Digest::SHA256.hexdigest(b))
+    end
+    module_function :variable_size_secure_compare
+  end
+end