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

Larry Hastings larry at hastings.org
Sat Jun 11 04:24:15 EDT 2016

On 06/11/2016 12:49 AM, Steven D'Aprano wrote:
> Will there be platforms where os.getrandom doesn't exist? If not, then
> secrets can just rely on it, otherwise what should it do?
> if hasattr(os, 'getrandom'):
>      return os.getrandom(n)
> else:
>      # Fail? Fall back on os.urandom?


  * Only Linux and Solaris have getrandom() right now.  IIUC Solaris
    duplicated Linux's API, but I don't know that for certain, and I
    don't know in particular what GRND_RANDOM does on Solaris.  (Of
    course, you don't need GRND_RANDOM for secrets.token_bytes().)
  * Only Linux and OS X have never-blocking /dev/urandom.  On Linux, you
    can choose to block by calling getrandom().  On OS X you have no
    choice, you can only use the never-blocking /dev/urandom.  (OS X
    also has a /dev/random but it behaves identically to /dev/urandom.) 
    OS X's man page reassuringly claims blocking is never necessary; the
    blogosphere disagrees.

If I were writing the function for the secrets module, I'd write it like 
you have above: call os.getrandom() if it's present, and os.urandom() if 
it isn't.  I believe that achieves current-best-practice everywhere: it 
does the right thing on Linux, it does the right thing on Solaris, it 
does the right thing on all the other OSes where reading from 
/dev/urandom can block, and it uses the only facility available to us on 


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