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: I5fa5c8f07e57201e129903f71b3dea19071cac5e

AUTHORS

@@ -76,6 +76,7 @@ Ramana Juvvadi <rrjuvvadi@gmail.com>
 Robin Norwood <robin.norwood@gmail.com>
 root <root@bsirish.(none)>
 root <root@newapps.(none)>
+Russell Bryant <rbryant@redhat.com>
 saikrishna1511@gmail.com <psaikrishna@ubudesk1004.(none)>
 Sai Krishna <saikrishna1511@gmail.com>
 Salvatore Orlando <salvatore.orlando@eu.citrix.com>

keystone/common/utils.py

@@ -241,3 +241,23 @@ def unixtime(dt_obj):
 
     """
     return time.mktime(dt_obj.utctimetuple())
+
+
+def auth_str_equal(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

keystone/contrib/ec2/core.py

@@ -105,15 +105,15 @@ def __init__(self):
     def check_signature(self, creds_ref, credentials):
         signer = utils.Ec2Signer(creds_ref['secret'])
         signature = signer.generate(credentials)
-        if signature == credentials['signature']:
+        if utils.auth_str_equal(signature, credentials['signature']):
             return
         # NOTE(vish): Some libraries don't use the port when signing
         #             requests, so try again without port.
         elif ':' in credentials['signature']:
             hostname, _port = credentials['host'].split(':')
             credentials['host'] = hostname
             signature = signer.generate(credentials)
-            if signature != credentials.signature:
+            if not utils.auth_str_equal(signature, credentials.signature):
                 # TODO(termie): proper exception
                 msg = 'Invalid signature'
                 raise webob.exc.HTTPUnauthorized(explanation=msg)

keystone/contrib/s3/core.py

@@ -25,6 +25,7 @@
 from hashlib import sha1
 
 from keystone import config
+from keystone.common import utils
 from keystone.common import wsgi
 from keystone.contrib import ec2
 
@@ -47,5 +48,5 @@ def check_signature(self, creds_ref, credentials):
         key = str(creds_ref['secret'])
         signed = base64.encodestring(hmac.new(key, msg, sha1).digest()).strip()
 
-        if credentials['signature'] != signed:
+        if not utils.auth_str_equal(credentials['signature'], signed):
             raise Exception('Not Authorized')

tests/test_utils.py

@@ -61,3 +61,8 @@ def test_isotime(self):
         output = utils.isotime(dt)
         expected = '1987-10-13T01:02:03Z'
         self.assertEqual(output, expected)
+
+    def test_auth_str_equal(self):
+        self.assertTrue(utils.auth_str_equal('abc123', 'abc123'))
+        self.assertFalse(utils.auth_str_equal('a', 'aaaaa'))
+        self.assertFalse(utils.auth_str_equal('ABC123', 'abc123'))