Re: /dev/urandom

[Mark H Weaver <mhw@netris.org>]

Hi Danny,

Danny Milosavljevic <dannym@scratchpost.org> writes:

> u-boot tools/sunxi-spl-image-builder.c reads from /dev/urandom .
>
> For reproducibility I'd like this to be a static file.  Do we already have one
> for this purpose?

I think it would be better to patch software as needed to avoid trying
to generate random numbers in specific places, mainly because it would
force us to always be aware of what the random numbers are being used
for, so that we can evaluate the potential security implications.

If we simply make /dev/urandom deterministic, then we will not be in the
position to know what the (non)random numbers are being used for, and we
run the risk of introducing serious security issues.

Note that it would not be sufficient to audit the software once to see
what the random numbers were being used for, because future versions of
the software might add additional uses for random numbers.  If that ever
happens, then the difference between the two approaches will become
important.

If we make /dev/urandom deterministic, then new uses for random numbers
introduced in future versions will be *automatically* made non-random,
and we might not notice.  On the other hand, if we patch out specific
uses of randomness, then new uses for randomness will be left intact
until we add patches for them.  This is as it should be, I think.

What do you think?

      Mark