Re: [PATCH 1/2] wrapper: add a helper to generate numbers from a CSPRNG

From: Jeff King <peff@peff.net>
Date: Nov 17, 2021, 20:01 UTC
Message-ID: <YZVfrhos+lZas7hk@coredump.intra.peff.net>
In-Reply-To: <CAPUEsphh-enSYS66mi7_XaS0n1bmUvGXRcgVp6iqhg94xSHVog@mail.gmail.com>

On Tue, Nov 16, 2021 at 07:36:51PM -0800, Carlo Arenas wrote:

Show 14 quoted lines

> > > for the little amount of random data we need, it might be wiser to
> > > fallback to something POSIX like lrand48 which is most likely to be
> > > available, but of course your tests that consume lots of random data
> > > will need to change.
> >
> > Unfortunately that won't help. You have to seed lrand48 with something,
> > which usually means pid and/or timestamp. Which are predictable to an
> > attacker, which was the start of the whole conversation. You really need
> > _some_ source of entropy, and only the OS can provide that.
> 
> again, showing my ignorance here; but that "something" doesn't need to
> be guessable externally; ex: git add could use as seed contents from
> the file that is adding, or even better mix it up with the other
> sources as a poor man's /dev/urandom

Those contents are still predictable. So you've made the attacker's job a little harder (now they have to block tempfiles for, say, each tag you're going to verify), but haven't changed the fundamental problem.

It definitely would help in _some_ threat models, but I think we should strive for a solution that can be explained clearly as "nobody can DoS your tempfiles" without complicated qualifications.

-Peff