bultin constant time checking

- if hmac.compare_digest is there, we should use it instead of our own
fallback.
- jwt handler has been updated to use utils.compare (reported in
#web2py-users)
- includes the same mods as https://github.com/web2py/web2py/pull/1146

gluon/utils.py

@@ -64,6 +64,10 @@ else:
         except (ImportError, ValueError):
             HAVE_PBKDF2 = False
 
+HAVE_COMPARE_DIGEST = False
+if hasattr(hmac, 'compare_digest'):
+    HAVE_COMPARE_DIGEST = True
+
 logger = logging.getLogger("web2py")
 
 
@@ -77,6 +81,8 @@ def AES_new(key, IV=None):
 
 def compare(a, b):
     """ Compares two strings and not vulnerable to timing attacks """
+    if HAVE_COMPARE_DIGEST:
+        return hmac.compare_digest(a, b)
     if len(a) != len(b):
         return False
     result = 0
@@ -143,6 +149,7 @@ DIGEST_ALG_BY_SIZE = {
     512 / 4: 'sha512',
 }
 
+
 def get_callable_argspec(fn):
     if inspect.isfunction(fn) or inspect.ismethod(fn):
         inspectable = fn
@@ -154,6 +161,7 @@ def get_callable_argspec(fn):
         inspectable = fn
     return inspect.getargspec(inspectable)
 
+
 def pad(s, n=32, padchar=' '):
     return s + (32 - len(s) % 32) * padchar
 
@@ -172,7 +180,7 @@ def secure_dumps(data, encryption_key, hash_key=None, compression_level=None):
 
 
 def secure_loads(data, encryption_key, hash_key=None, compression_level=None):
-    if not ':' in data:
+    if ':' not in data:
         return None
     if not hash_key:
         hash_key = sha1(encryption_key).hexdigest()