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Date:	Thu, 15 Jul 2010 21:41:22 -0500
From:	Matt Mackall <mpm@...enic.com>
To:	Henrique de Moraes Holschuh <hmh@...ian.org>
Cc:	Herbert Xu <herbert@...dor.apana.org.au>,
	Theodore Ts'o <tytso@....edu>,
	Michael Biebl <biebl@...ian.org>, 587665@...s.debian.org,
	linux-kernel@...r.kernel.org, Petter Reinholdtsen <pere@...gry.com>
Subject: Re: [Pkg-sysvinit-devel] Bug#587665: Safety of early boot init of
 /dev/random seed

On Thu, 2010-07-15 at 20:33 -0300, Henrique de Moraes Holschuh wrote:
> On Mon, 05 Jul 2010, Matt Mackall wrote:
> > > > Here are our questions:
> > > > 
> > > > 1. How much data of unknown quality can we feed the random pool at boot,
> > > >    before it causes damage (i.e. what is the threshold where we violate the
> > > >    "you are not goint to be any worse than you were before" rule) ?
> > 
> > There is no limit. The mixing operations are computationally reversible,
> > which guarantees that no unknown degrees of freedom are clobbered when
> > mixing known data.
> 
> Good.  So, whatever we do, we are never worse off than we were before we did
> it, at least by design.
> 
> > > > 2. How dangerous it is to feed the pool with stale seed data in the next
> > > >    boot (i.e. in a failure mode where we do not regenerate the seed file) ?
> > 
> > Not at all.
> >  
> > > > 3. What is the optimal size of the seed data based on the pool size ?
> > 
> > 1:1.
> 
> We shall try to keep it at 1:1, then.
> 
> > > > 4. How dangerous it is to have functions that need randomness (like
> > > >    encripted network and partitions, possibly encripted swap with an
> > > >    ephemeral key), BEFORE initializing the random seed ?
> > 
> > Depends on the platform. For instance, if you've got an unattended boot
> > off a Live CD on a machine with a predictable clock, you may get
> > duplicate outputs.
> 
> I.e. it is somewhat dangerous, and we should try to avoid it by design, so
> we should try to init it as early as possible.  Very well.
> 
> > > > 5. Is there an optimal size for the pool?  Does the quality of the randomness
> > > >    one extracts from the pool increase or decrease with pool size?
> > 
> > Don't bother fiddling with the pool size.
> 
> We don't, but local admins often do, probably in an attempt to better handle
> bursts of entropy drainage.  So, we do want to properly support non-standard
> pool sizes in Debian if we can.

Unless they're manually patching their kernel, they probably aren't
succeeding. The pool resize ioctl was disabled ages ago. But there's
really nothing to support here: even the largest polynomial in the
source is only 2048 bits, or 256 bytes.

> > > > Basically, we need these answers to find our way regarding the following
> > > > decisions:
> > > > 
> > > > a) Is it better to seed the pool as early as possible and risk a larger time
> > > >    window for problem (2) above, instead of the current behaviour where we
> > > >    have a large time window where (4) above happens?
> > 
> > Earlier is better.
> > 
> > > > b) Is it worth the effort to base the seed file on the size of the pool,
> > > >    instead of just using a constant size?  If a constant size is better,
> > > >    which size would that be? 512 bytes? 4096 bytes? 16384 bytes?
> > 
> > 512 bytes is plenty.
> >
> > > > c) What is the maximum seed file size we can allow (maybe based on size of
> > > >    the pool) to try to avoid problem (1) above ?
> > 
> > Anything larger than a sector is simply wasting CPU time, but is
> > otherwise harmless.
> 
> Well, a filesystem block is usually 1024 bytes, and a sector is 4096 bytes
> nowadays... :-)
> 


-- 
Mathematics is the supreme nostalgia of our time.


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