Use faster OpenSSL secure_compare if available

Comparing a 16 byte string:
openssl_secure_compare:  9397508.4 i/s
   rack_secure_compare:   515938.0 i/s - 18.21x  (± 0.00) slower

CHANGELOG.md

@@ -19,6 +19,7 @@ All notable changes to this project will be documented in this file. For info on
 - `Rack::Request#[]` and `#[]=` now warn even in non-verbose mode. ([#1277](https://github.com/rack/rack/issues/1277), [@jeremyevans](https://github.com/jeremyevans))
 - Decrease default allowed parameter recursion level from 100 to 32. ([#1640](https://github.com/rack/rack/issues/1640), [@jeremyevans](https://github.com/jeremyevans))
 - Attempting to parse a multipart response with an empty body now raises Rack::Multipart::EmptyContentError. ([#1603](https://github.com/rack/rack/issues/1603), [@jeremyevans](https://github.com/jeremyevans))
+- `Rack::Utils.secure_compare` uses OpenSSL's faster implementation if available. ([#1711](https://github.com/rack/rack/pull/1711), [@bdewater](https://github.com/bdewater))
 
 ### Fixed
 

lib/rack/utils.rb

@@ -374,14 +374,22 @@ def get_byte_ranges(http_range, size)
     # 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
+    if defined?(OpenSSL.fixed_length_secure_compare)
+      def secure_compare(a, b)
+        return false unless a.bytesize == b.bytesize
 
-      l = a.unpack("C*")
+        OpenSSL.fixed_length_secure_compare(a, b)
+      end
+    else
+      def secure_compare(a, b)
+        return false unless a.bytesize == b.bytesize
 
-      r, i = 0, -1
-      b.each_byte { |v| r |= v ^ l[i += 1] }
-      r == 0
+        l = a.unpack("C*")
+
+        r, i = 0, -1
+        b.each_byte { |v| r |= v ^ l[i += 1] }
+        r == 0
+      end
     end
 
     # Context allows the use of a compatible middleware at different points