[Python-Dev] BDFL ruling request: should we block forever waiting for high-quality random bits?

Nick Coghlan ncoghlan at gmail.com
Thu Jun 16 13:03:34 EDT 2016

On 16 June 2016 at 09:39, Paul Moore <p.f.moore at gmail.com> wrote:
> I'm willing to accept the view of the security experts that there's a
> problem here. But without a clear explanation of the problem, how can
> a non-specialist like myself have an opinion? (And I hope the security
> POV isn't "you don't need an opinion, just do as we say").

If you're not writing Linux (and presumably *BSD) scripts and
applications that run during system initialisation or on embedded ARM
hardware with no good sources of randomness, then there's zero chance
of any change made in relation to this affecting you (Windows and Mac
OS X are completely immune, since they don't allow Python scripts to
run early enough in the boot sequence for there to ever be a problem).

The only question at hand is what CPython should do in the case where
the operating system *does* let Python scripts run before the system
random number generator is ready, and the application calls a security
sensitive API that relies on that RNG:

- throw BlockingIOError (so the script developer knows they have a
potential problem to fix)
- block (so the script developer has a system hang to debug)
- return low quality random data (so the script developer doesn't even
know they have a potential problem)

The last option is the status quo, and has a remarkable number of
vocal defenders.

The second option is what we changed the behaviour to in 3.5 as a side
effect of switching to a syscall to save a file descriptor (and *also*
inadvertently made a gating requirement for CPython starting at all,
without which I'd be very surprised if anyone actually noticed the
potentially blocking behaviour in os.urandom itself)

The first option is the one I'm currently writing a PEP for, since it
makes the longstanding advice to use os.urandom() as the low level
random data API for security sensitive operations unequivocally
correct (as it will either do the right thing, or throw an exception
which the developer can handle as appropriate for their particular


Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia

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