Re: [PATCH][RESEND 3] hwrng: add randomness to system from rng
sources
[Posted March 12, 2014 by jake]
| From: |
| Matt Mackall <mpm-AT-selenic.com> |
| To: |
| Jason Cooper <jason-AT-lakedaemon.net> |
| Subject: |
| Re: [PATCH][RESEND 3] hwrng: add randomness to system from rng sources |
| Date: |
| Wed, 05 Mar 2014 18:52:27 -0600 |
| Message-ID: |
| <1394067147.17842.45.camel@calx> |
| Cc: |
| Kees Cook <keescook-AT-chromium.org>, Theodore Ts'o <tytso-AT-mit.edu>, LKML <linux-kernel-AT-vger.kernel.org>, Herbert Xu <herbert-AT-gondor.apana.org.au>, Rusty Russell <rusty-AT-rustcorp.com.au>, Satoru Takeuchi <satoru.takeuchi-AT-gmail.com>, linux-crypto <linux-crypto-AT-vger.kernel.org>, Andrew Morton <akpm-AT-linux-foundation.org> |
| Archive‑link: | |
Article |
On Wed, 2014-03-05 at 16:11 -0500, Jason Cooper wrote:
> > In other words, if there are 4096 bits of "unknownness" in X to start
> > with, and I can get those same 4096 bits of "unknownness" back by
> > unmixing X' and Y, then there must still be 4096 bits of "unknownness"
> > in X'. If X' is 4096 bits long, then we've just proven that
> > reversibility means the attacker can know nothing about the contents of
> > X' by his choice of Y.
>
> Well, this reinforces my comfortability with loadable modules. The pool
> is already initialized by the point at which the driver is loaded.
>
> Unfortunately, any of the drivers in hw_random can be built in. When
> built in, hwrng_register is going to be called during the kernel
> initialization process. In that case, the unknownness in X is not 4096
> bits, but far less. Also, the items that may have seeded X (MAC addr,
> time, etc) are discoverable by a potential attacker. This is also well
> before random-seed has been fed in.
To which I would respond.. so?
If the pool is in an attacker-knowable state at early boot, adding
attacker-controlled data does not make the situation any worse. In fact,
if the attacker has less-than-perfect control of the inputs, mixing more
things in will make things exponentially harder for the attacker.
Put another way: mixing can't ever removes unknownness from the pool, it
can only add more. So the only reason you should ever choose not to mix
something into the pool is performance.
--
Mathematics is the supreme nostalgia of our time.
