Question

I am using Google App Engine and PyCrypto to do some encryption. The error I am getting, which is below, occurs only on my local developement server, which is running Linux Mint Maya (13). I deployed the same code to the GAE cloud, and it runs without error.

ERROR    2012-06-29 16:04:20,717 webapp2.py:1553] [Errno 13] file not accessible: '/dev/urandom'
Traceback (most recent call last):
  File "/home/eric/google_appengine/lib/webapp2/webapp2.py", line 1536, in __call__
    rv = self.handle_exception(request, response, e)
  File "/home/eric/google_appengine/lib/webapp2/webapp2.py", line 1530, in __call__
    rv = self.router.dispatch(request, response)
  File "/home/eric/google_appengine/lib/webapp2/webapp2.py", line 1278, in default_dispatcher
    return route.handler_adapter(request, response)
  File "/home/eric/google_appengine/lib/webapp2/webapp2.py", line 1102, in __call__
    return handler.dispatch()
  File "/home/eric/google_appengine/lib/webapp2/webapp2.py", line 572, in dispatch
    return self.handle_exception(e, self.app.debug)
  File "/home/eric/google_appengine/lib/webapp2/webapp2.py", line 570, in dispatch
    return method(*args, **kwargs)
  File "/home/eric/workspace/commentbox/src/controller/api.py", line 55, in get
    self.response.out.write(encrypt(json.dumps(to_json)))
  File "/home/eric/workspace/commentbox/src/controller/api.py", line 27, in encrypt
    iv = Random.new().read(AES.block_size)
  File "/usr/lib/python2.7/dist-packages/Crypto/Random/__init__.py", line 33, in new
    return _UserFriendlyRNG.new(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/Crypto/Random/_UserFriendlyRNG.py", line 206, in new
    return RNGFile(_get_singleton())
  File "/usr/lib/python2.7/dist-packages/Crypto/Random/_UserFriendlyRNG.py", line 200, in _get_singleton
    _singleton = _LockingUserFriendlyRNG()
  File "/usr/lib/python2.7/dist-packages/Crypto/Random/_UserFriendlyRNG.py", line 144, in __init__
    _UserFriendlyRNG.__init__(self)
  File "/usr/lib/python2.7/dist-packages/Crypto/Random/_UserFriendlyRNG.py", line 86, in __init__
    self._ec = _EntropyCollector(self._fa)
  File "/usr/lib/python2.7/dist-packages/Crypto/Random/_UserFriendlyRNG.py", line 53, in __init__
    self._osrng = OSRNG.new()
  File "/usr/lib/python2.7/dist-packages/Crypto/Random/OSRNG/posix.py", line 60, in new
    return DevURandomRNG(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/Crypto/Random/OSRNG/posix.py", line 42, in __init__
    f = open(self.name, "rb", 0)
  File "/home/eric/google_appengine/google/appengine/tools/dev_appserver_import_hook.py", line 592, in __init__
    raise IOError(errno.EACCES, 'file not accessible', filename)
IOError: [Errno 13] file not accessible: '/dev/urandom'
ERROR    2012-06-29 16:04:20,721 webapp2.py:1549] Exception 
ERROR    2012-06-29 16:04:20,721 webapp2.py:1549] AttributeError
ERROR    2012-06-29 16:04:20,721 webapp2.py:1549] : 
ERROR    2012-06-29 16:04:20,721 webapp2.py:1549] "'DevURandomRNG' object has no attribute 'closed'"
ERROR    2012-06-29 16:04:20,721 webapp2.py:1549]  in 
ERROR    2012-06-29 16:04:20,721 webapp2.py:1549] <bound method DevURandomRNG.__del__ of <Crypto.Random.OSRNG.posix.DevURandomRNG object at 0x52707d0>>
ERROR    2012-06-29 16:04:20,721 webapp2.py:1549]  ignored

The python code that is throwing the error is the second line in this block:

from Crypto.Cipher import AES
from Crypto import Random

key = b'Sixteen byte key' 
iv = Random.new().read(AES.block_size)
cipher = AES.new(key, AES.MODE_CBC, iv)
return iv + cipher.encrypt(plaintext)

After seeing this error, I realized it might be a permissions error. So then I did a quick check of the permissions on /dev/urandom:

eric@eric-Latitude-E5400 ~ $ dpkg -L udev | xargs grep urandom
/lib/udev/rules.d/50-udev-default.rules:KERNEL=="null|zero|full|random|urandom", MODE="0666"
eric@eric-Latitude-E5400 ~ $ ls -lart /dev/*random
crw-rw-rw- 1 root root 1, 9 Jun 29 10:53 /dev/urandom
crw-rw-rw- 1 root root 1, 8 Jun 29 10:53 /dev/random

So it looks like my permissions are fine. I have also tried running the development server as root, but I get the same error. For some reason this only happens with the development server, and not when deployed to google’s cloud. Any ideas on what to try next?

Thanks!

Answer 1

The error you are getting is because GAE restricts file access, and /dev/urandom is blocked.

Note that the error is not when you import PyCrypto, it’s when you do AES.new(key, AES.MODE_CBC, iv)

You can fix it, either by editing Crypto/Random/OSRNG/__init__.py and moving the lines

if hasattr(os, 'urandom'):
    from Crypto.Random.OSRNG.fallback import new

to the top, or by modifying os.name to something different than posix or nt, at the beginning of your script. I suggest the first option.

ps: I assume you are using python 2.5 and pycrypto 2.2, because of your Traceback. Next time please include these details.