On Sat, Dec 08, 2007 at 07:35:54PM -0500, Theodore Tso wrote:
> On Sat, Dec 08, 2007 at 05:20:17PM -0600, Matt Mackall wrote:
> > random: do extraction before mixing
> >
> > If an attacker manages to capture the current pool state, she can
> > determine the last 10 bytes extracted from the pool.
>
> That's not true; we aren't just extracting data in the
> __add_entropy_words() call. In fact, above that, the bulk of the
> extraction comes form when we hash the entire pool, feeding back a
> portion of the hash into the pool here:
>
> for (i = 0; i < r->poolinfo->poolwords; i += 16) {
> /* hash blocks of 16 words = 512 bits */
> sha_transform(buf, (__u8 *)(r->pool + i), buf + 5);
> /* feed back portion of the resulting hash */
> add_entropy_words(r, &buf[i % 5], 1);
> }
Ok, yes, I'd forgotten we were chaining in the final sha_transform. I
plead too many bufs and buf+5s, which I fix up in 6/6. Funny thing is
that I'd convinced myself that this attack didn't work (correct) last
year when I read the paper I mentioned earlier. But yesterday and
today I couldn't spot the problem with it. So again, I'll add an
explicit comment for future hackers and researchers.
--
Mathematics is the supreme nostalgia of our time.
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