[ python-Bugs-1680034 ] Importing SystemRandom wastes entropy.

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Bugs item #1680034, was opened at 2007-03-13 12:17
Message generated for change (Comment added) made by rhettinger
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Category: Python Library
Group: Python 2.4
>Status: Closed
>Resolution: Wont Fix
Priority: 5
Private: No
Submitted By: Steve Tyler (stephent98)
Assigned to: Nobody/Anonymous (nobody)
Summary: Importing SystemRandom wastes entropy.

Initial Comment:
Importing SystemRandom wastes entropy.
The strace snippet shows a 16 byte read from /dev/urandom, which is presumably done to seed a random number generator. However SystemRandom does not need a seed, so the read is not needed.

test case:
#!/usr/bin/python
from random import SystemRandom

strace snippet:
open("/dev/urandom", O_RDONLY|O_LARGEFILE) = 4
read(4, "\\\333\277Q\243>K\350 \321\316\26_\271\364~", 16) = 16
close(4)                                = 0

Python version:
python-2.4.4-1.fc6 (Fedora Core 6)


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>Comment By: Raymond Hettinger (rhettinger)
Date: 2007-03-13 15:17

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Sorry, am closing this as won't fix.  The 16 bytes are used to seed the
MersenneTwister which is used by tempfile.py upon startup.  That is a
reasonable use of the resource.  

FWIW, it is possible for you to recover most of those 16 bytes of entropy
just by calling the twister itself.  Also, it is my understanding that
/dev/urandom is continuously refilling its hardware based entropy source
(so the supply is limitless, but not instant).



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Comment By: Steve Tyler (stephent98)
Date: 2007-03-13 14:08

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Here is a little more background on why wasting entropy is a problem.

"When accessed as /dev/urandom, as many bytes as are requested are
returned even when the entropy pool is exhausted."
http://www.linux.com/howtos/Secure-Programs-HOWTO/random-numbers.shtml

When the entropy pool is exhausted, the Linux RNG (accessed via
/dev/urandom) behaves like a pseudo-random number generator, which is not
acceptable for cryptographic applications such as password generators.

Analysis of the Linux Random Number Generator
http://www.pinkas.net/PAPERS/gpr06.pdf

Of course one can work around this issue by not using the "random" module
and accessing /dev/urandom or /dev/random directly.

For some perspective, simply importing the gnome.ui module consumes 4096
bytes of random data in a library I have not been able to completely
identify. (I don't think it is Python, though.)

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Comment By: Steve Tyler (stephent98)
Date: 2007-03-13 13:30

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Entropy is not an unlimited quantity,
therefore the existing behavior is undesirable.
My app is a random password generator which may need the entropy for
itself.
https://sourceforge.net/projects/gnome-password/


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Comment By: Georg Brandl (gbrandl)
Date: 2007-03-13 12:32

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This is not caused by SystemRandom, but by instantiating (and thereby
seeding) the normal (Mersenne Twister) random number generator, which is
done automatically when random is imported.

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