[Python-ideas] Python's Source of Randomness and the random.py module Redux

[Steven D'Aprano]

On Wed, Sep 09, 2015 at 08:01:16PM -0400, Donald Stufft wrote:
[...]
> Looking on google, the first result for "python random password" is
> StackOverflow which suggests:
> 
>     ''.join(random.choice(string.ascii_uppercase + string.digits) for _ in range(N))
> 
> However, it was later edited to, after that, include:
> 
>     ''.join(random.SystemRandom().choice(string.ascii_uppercase + string.digits) for _ in range(N))

You're worried about attacks on the random number generator that 
produces the characters in the password? I think I'm going to have to 
see an attack before I believe that this is meaningful.

Excluding PRNGs that are hopelessly biased ("nine, nine, nine, nine...") 
or predictable, how does knowing the PRNG help in an attack? Here's a 
password I just generated using your "corrected" version using 
SystemRandom:

    06XW0X0X

(Honest, that's exactly what I got on my first try.)

Here's one I generated using the "bad" code snippet:

    V6CFKCF2

How can you tell them apart, or attack one but not the other based on 
the PRNG?


> So it wasn't obvious to the person who answered that question that the random
> module's module scoped functions were not appropiate for this use. It appears
> that the original answer lasted for roughly 4 years before it was corrected,

Shouldn't it be using a single instance of SystemRandom rather than a 
new instance for each call?


[...]
> According to Theo, modern userland CSPRNGs can create random bytes faster than
> memcpy 

That is an astonishing claim, and I'd want to see evidence for it before 
accepting it.



-- 
Steve