Use constant time string comparisons for auth.

Fix bug 942644.

Use constant time string comparisons when doing authentication to help
guard against timing attacks.

Change-Id: I88c4c5cd9edd9e5d60db07b6ae2638b74a2a2e17

swift/common/middleware/formpost.py

@@ -110,7 +110,7 @@ from time import gmtime, strftime, time
 from time import time
 from urllib import quote, unquote
 
-from swift.common.utils import get_logger
+from swift.common.utils import get_logger, strcmp_const_time
 
 
 #: The size of data to read from the form at any given time.
@@ -442,7 +442,8 @@ class FormPost(object):
                         attributes.get('expires') or '0'
                     )
         sig = hmac.new(key, hmac_body, sha1).hexdigest()
-        if sig != (attributes.get('signature') or 'invalid'):
+        if not strcmp_const_time(sig,(attributes.get('signature') or
+                                      'invalid')):
             return '401 Unauthorized', 'invalid signature'
         subenv['swift.authorize'] = lambda req: None
         subenv['swift.authorize_override'] = True

swift/common/utils.py

@@ -1117,3 +1117,23 @@ def listdir(path):
         if err.errno != errno.ENOENT:
             raise
     return []
+
+
+def strcmp_const_time(s1, s2):
+    """Constant-time string comparison.
+
+    :params s1: the first string
+    :params s2: the second string
+
+    :return: True if the strings are equal.
+
+    This function takes two strings and compares them.  It is intended to be
+    used when doing a comparison for authentication purposes to help guard
+    against timing attacks.
+    """
+    if len(s1) != len(s2):
+        return False
+    result = 0
+    for (a, b) in zip(s1, s2):
+        result |= ord(a) ^ ord(b)
+    return result == 0

test/unit/common/test_utils.py

@@ -850,6 +850,11 @@ log_name = %(yarr)s'''
         for v in utils.TRUE_VALUES:
             self.assertEquals(v, v.lower())
 
+    def test_strcmp_const_time(self):
+        self.assertTrue(utils.strcmp_const_time('abc123', 'abc123'))
+        self.assertFalse(utils.strcmp_const_time('a', 'aaaaa'))
+        self.assertFalse(utils.strcmp_const_time('ABC123', 'abc123'))
+
 
 if __name__ == '__main__':
     unittest.main()