Fix timing attack vulnerability in ActiveSupport::MessageVerifier.

Use a constant-time comparison algorithm to compare the candidate HMAC with the calculated HMAC to prevent leaking information about the calculated HMAC.
Signed-off-by: NMichael Koziarski <michael@koziarski.com>

activesupport/lib/active_support/message_verifier.rb

@@ -25,10 +25,10 @@ def initialize(secret, digest = 'SHA1')
     
     def verify(signed_message)
       data, digest = signed_message.split("--")
-      if digest != generate_digest(data)
-        raise InvalidSignature
-      else
+      if secure_compare(digest, generate_digest(data))
         Marshal.load(ActiveSupport::Base64.decode64(data))
+      else
+        raise InvalidSignature
       end
     end
     
@@ -38,6 +38,19 @@ def generate(value)
     end
     
     private
+      # constant-time comparison algorithm to prevent timing attacks
+      def secure_compare(a, b)
+        if a.length == b.length
+          result = 0
+          for i in 0..(a.length - 1)
+            result |= a[i] ^ b[i]
+          end
+          result == 0
+        else
+          false
+        end
+      end
+
       def generate_digest(data)
         require 'openssl' unless defined?(OpenSSL)
         OpenSSL::HMAC.hexdigest(OpenSSL::Digest::Digest.new(@digest), @secret, data)