Re: [PATCH 0/2] Generate temporary files using a CSPRNG

From: Jeff King <peff@peff.net>
Date: Nov 16, 2021, 15:44 UTC
Message-ID: <YZPR4Ya1ocWvKbAf@coredump.intra.peff.net>
In-Reply-To: <20211116033542.3247094-1-sandals@crustytoothpaste.net>

On Tue, Nov 16, 2021 at 03:35:40AM +0000, brian m. carlson wrote:

Show 11 quoted lines

> For those who are interested, I computed the probability of spurious
> failure for the self-test mode like so:
> 
>   256 * (255/256)^65536
> 
> This Ruby one-liner estimates the probability at approximately 10^-108:
> 
>   ruby -e 'a = 255 ** 65536; b = 256 ** 65536; puts b.to_s.length - a.to_s.length - 3'
> 
> If I have made an error in the calculation, please do feel free to point
> it out.

Yes, I think your math is correct there.

A more interesting question is whether generating 64k of PRNG bytes per test run is going to a problem for system entropy pools. For that matter, I guess the use of it for tempfiles will produce a similar burden, since we run so many commands. My understanding is that modern systems will just produce infinite output for /dev/urandom, etc, but I wonder if there are any systems left where that is not true (because they have a misguided notion that they need to stir in more "real" entropy bits).

-Peff