Improved get_random_string().

Improved the behavior of get_random_string to re-seed itself each time it is called
if the system does not have a secure random number generator. This will change the
properties of the random string produced, but will be unpredictable to an attacker.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@17581 bcc190cf-cafb-0310-a4f2-bffc1f526a37

django/utils/crypto.py

@@ -7,12 +7,18 @@ import struct
 import hashlib
 import binascii
 import operator
+import time
 
+# Use the system PRNG if possible
 import random
 try:
     random = random.SystemRandom()
+    using_sysrandom = True
 except NotImplementedError:
-    pass
+    import warnings
+    warnings.warn('A secure pseudo-random number generator is not available '
+                  'on your system. Falling back to Mersenne Twister.')
+    using_sysrandom = False
 
 from django.conf import settings
 
@@ -47,11 +53,25 @@ def get_random_string(length=12,
                       allowed_chars='abcdefghijklmnopqrstuvwxyz'
                                     'ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789'):
     """
-    Returns a random string of length characters from the set of a-z, A-Z, 0-9.
+    Returns a securely generated random string.
 
     The default length of 12 with the a-z, A-Z, 0-9 character set returns
     a 71-bit value. log_2((26+26+10)^12) =~ 71 bits
     """
+    if not using_sysrandom:
+        # This is ugly, and a hack, but it makes things better than
+        # the alternative of predictability. This re-seeds the PRNG
+        # using a value that is hard for an attacker to predict, every
+        # time a random string is required. This may change the
+        # properties of the chosen random sequence slightly, but this
+        # is better than absolute predictability.
+        random.seed(
+            hashlib.sha256(
+                "%s%s%s" % (
+                    random.getstate(),
+                    time.time(),
+                    settings.SECRET_KEY)
+                ).digest())
     return ''.join([random.choice(allowed_chars) for i in range(length)])