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

@@ -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 logging
-import random
+import errno
 import itsdangerous
+import logging
+import os.path
+import random
+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 load_key(filename):
+    global __itsda_secret
+    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
-    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_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):
     """