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

haypo s victor.stinner at gmail.com
Sat Sep 5 00:45:52 CEST 2015

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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