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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: bcc190cf-cafb-0310-a4f2-bffc1f526a37
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1 parent 239e41f commit 1525874238fd705ec17a066291935a9316bd3044 @PaulMcMillan PaulMcMillan committed
Showing with 22 additions and 2 deletions.
  1. +22 −2 django/utils/
24 django/utils/
@@ -7,12 +7,18 @@
import hashlib
import binascii
import operator
+import time
+# Use the system PRNG if possible
import random
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,
- 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)])

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