[Python-Dev] Should we use getentropy() for os.urandom()?

[Guido van Rossum]

I'm no expert, but from the bug report and the man page you quoted it does
sound like getentropy() should only be used to seed a PRNG. It also sounds
like reading /dev/[u]random should be considered a PRNG. For evidence, the
man page on OS X says: "The random device produces uniformly distributed
random byte values of potentially high quality." It goes on to describe the
difference between the two on Linux (where urandom produces lower quality
when it runs out of entropy while random apparently blocks). It does sound
like we should not call getentropy() except to get a good seed for our own
PRNG.

On Fri, Sep 4, 2015 at 3:45 PM, haypo s <victor.stinner at gmail.com> wrote:

> Hi,
>
> I followed discussions on the new systems getrandom() on Linux and
> getentropy() on OpenBSD. I wanted to use them in Python to avoid the
> need of a file descriptor to read /dev/urandom.
>
> Linux getrandom() is also more secure than /dev/urandom because it
> blocks until /dev/urandom is feeded with enough entropy.
>
> getentropy() and getrandom() are now used in Python 2.7.10, Python 3.4
> and newer.
>
> Today, an issue was reported on Solaris because os.urandom() is much
> slower with Python 2.7.10:
> https://bugs.python.org/issue25003
>
> It looks like Solaris has getrandom() and getentropy(), and
> getentropy() is slow.
>
> Now I'm not sure that I understood the purpose of getentropy() even on
> OpenBSD. Should it be used to feed a PRNG in user-space, or can it be
> used as a PRNG?
>
>
> http://www.openbsd.org/cgi-bin/man.cgi/OpenBSD-current/man2/getentropy.2?query=getentropy&sec=2
>
> If getentropy() must only be used to feed a PRNG (which only use a few
> bytes), we should remove the code using getentropy() (but getrandom()
> should be kept).
>
> Note: I didn't know that other operating systems supported getrandom()
> and getentropy()! The feature looks recent in Solaris:
> " Solaris 11.3 adds two new system calls, getrandom(2) and
> getentropy(2), for getting random bit streams or raw entropy."
> https://blogs.oracle.com/darren/entry/solaris_new_system_calls_getentropy
> (article published at July, 2015)
>
> Note2: There is an open discussion proposing to "Use arc4random under
> OpenBSD for os.urandom() if /dev/urandom is not present"
> https://bugs.python.org/issue22542
>
> getentropy() issue in Python (closed):
> https://bugs.python.org/issue22585
>
> getrandom() issue in Python (closed):
> https://bugs.python.org/issue22181
>
> Victor
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-- 
--Guido van Rossum (python.org/~guido)
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