Re: storing an explicit nonce

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?

--
Bruce Momjian <bruce(at)momjian(dot)us> https://momjian.us
EDB https://enterprisedb.com

If only the physical world exists, free will is an illusion.