From 09102e0767d3c24e0be7988dc22113993cbd3d3d Mon Sep 17 00:00:00 2001
From: Brett Smith <brettcsmith@brettcsmith.org>
Date: Sun, 24 Mar 2013 16:27:20 -0400
Subject: [PATCH] Harden It's Dangerous key management.

The previous code was theoretically subject to timing attacks, where
an attacker could read the key in between the time it was saved to the
file and when the chmod happened.  This version prevents that by using
umasks to ensure the files always have the right permissions.

This version also avoids using a key that cannot be saved due to some
system setup bug.
---
 mediagoblin/tools/crypto.py | 65 +++++++++++++++++++++++++------------
 1 file changed, 45 insertions(+), 20 deletions(-)

diff --git a/mediagoblin/tools/crypto.py b/mediagoblin/tools/crypto.py
index 0fb2ba2e..55811aea 100644
--- a/mediagoblin/tools/crypto.py
+++ b/mediagoblin/tools/crypto.py
@@ -14,10 +14,12 @@
 # You should have received a copy of the GNU Affero General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
-import os.path
+import errno
+import itsdangerous
 import logging
+import os.path
 import random
-import itsdangerous
+import tempfile
 from mediagoblin import mg_globals
 
 _log = logging.getLogger(__name__)
@@ -25,33 +27,56 @@ _log = logging.getLogger(__name__)
 
 # Use the system (hardware-based) random number generator if it exists.
 # -- this optimization is lifted from Django
-if hasattr(random, 'SystemRandom'):
+try:
     getrandbits = random.SystemRandom().getrandbits
-else:
+except AttributeError:
     getrandbits = random.getrandbits
 
 
 __itsda_secret = None
 
 
-def setup_crypto():
+def load_key(filename):
     global __itsda_secret
-    dir = mg_globals.app_config["crypto_path"]
-    if not os.path.isdir(dir):
-        os.makedirs(dir)
-        os.chmod(dir, 0700)
-        _log.info("Created %s", dir)
-    name = os.path.join(dir, "itsdangeroussecret.bin")
-    if os.path.exists(name):
-        __itsda_secret = file(name, "r").read()
-    else:
-        __itsda_secret = str(getrandbits(192))
-        f = file(name, "w")
-        f.write(__itsda_secret)
-        f.close()
-        os.chmod(name, 0600)
-        _log.info("Created %s", name)
+    key_file = open(filename)
+    try:
+        __itsda_secret = key_file.read()
+    finally:
+        key_file.close()
 
+def create_key(key_dir, key_filepath):
+    global __itsda_secret
+    old_umask = os.umask(077)
+    key_file = None
+    try:
+        if not os.path.isdir(key_dir):
+            os.makedirs(key_dir)
+            _log.info("Created %s", dirname)
+        key = str(getrandbits(192))
+        key_file = tempfile.NamedTemporaryFile(dir=key_dir, suffix='.bin',
+                                               delete=False)
+        key_file.write(key)
+        key_file.flush()
+        os.rename(key_file.name, key_filepath)
+        key_file.close()
+    finally:
+        os.umask(old_umask)
+        if (key_file is not None) and (not key_file.closed):
+            key_file.close()
+            os.unlink(key_file.name)
+    __itsda_secret = key
+    _log.info("Saved new key for It's Dangerous")
+
+def setup_crypto():
+    global __itsda_secret
+    key_dir = mg_globals.app_config["crypto_path"]
+    key_filepath = os.path.join(key_dir, 'itsdangeroussecret.bin')
+    try:
+        load_key(key_filepath)
+    except IOError, error:
+        if error.errno != errno.ENOENT:
+            raise
+        create_key(key_dir, key_filepath)
 
 def get_timed_signer_url(namespace):
     """
-- 
2.25.1