Re: storing an explicit nonce

Bruce Momjian <bruce(at)momjian(dot)us> wrote:

> On Tue, May 25, 2021 at 04:48:21PM -0700, Andres Freund wrote:
> > Hi,
> >
> > On 2021-05-25 17:29:03 -0400, Bruce Momjian wrote:
> > > So, let me ask --- I thought CTR basically took an encrypted stream of
> > > bits and XOR'ed them with the data. If that is true, then why are
> > > changing hint bits a problem? We already can see some of the bit stream
> > > by knowing some bytes of the page.
> >
> > A *single* reuse of the nonce in CTR reveals nearly all of the
> > plaintext. As you say, the data is XORed with the key stream. Reusing
> > the nonce means that you reuse the key stream. Which in turn allows you
> > to do:
> > (data ^ stream) ^ (data' ^ stream)
> > which can be simplified to
> > (data ^ data')
> > thereby leaking all of data except the difference between data and
> > data'. That's why it's so crucial to ensure that stream *always* differs
> > between two rounds of encrypting "related" data.
> >
> > We can't just "hope" that data doesn't change and use CTR.
>
> My point was about whether we need to change the nonce, and hence
> WAL-log full page images if we change hint bits. If we don't and
> reencrypt the page with the same nonce, don't we only expose the hint
> bits? I was not suggesting we avoid changing the nonce in non-hint-bit
> cases.
>
> I don't understand your computation above. You decrypt the page into
> shared buffers, you change a hint bit, and rewrite the page. You are
> re-XOR'ing the buffer copy with the same key and nonce. Doesn't that
> only change the hint bits in the new write?

The way I view things is that the CTR mode encrypts each individual bit,
independent from any other bit on the page. For non-hint bits data=data', so
(data ^ data') is always zero, regardless the actual values of the data. So I
agree with you that by reusing the nonce we only expose the hint bits.

--
Antonin Houska
Web: https://www.cybertec-postgresql.com