Re: Moving forward with TDE [PATCH v3] - Mailing list pgsql-hackers

Greetings,

Thanks for your feedback on this.

* Andres Freund (andres@anarazel.de) wrote:
> I still am quite quite unconvinced that using the LSN as a nonce is a good
> design decision.

This is a really important part of the overall path to moving this
forward, so I wanted to jump to it and have a specific discussion
around this.  I agree that there are downsides to using the LSN, some of
which we could possibly address (eg: include the timeline ID in the IV),
but others that would be harder to deal with.

The question then is- what's the alternative?

One approach would be to change the page format to include space for an
explicit nonce.  I don't see the community accepting such a new page
format as the only format we support though as that would mean no
pg_upgrade support along with wasted space if TDE isn't being used.
Ideally, we'd instead be able to support multiple page formats where
users could decide when they create their cluster what features they
want- and luckily we even have such an effort underway with patches
posted for review [1].  Certainly, with the base page-special-feature
patch, we could have an option for users to choose that they want a
better nonce than the LSN, or we could bundle that assumption in with,
say, the authenticated-encryption feature (if you want authenticated
encryption, then you want more from the encryption system than the
basics, and therefore we presume you also want a better nonce than the
LSN).

Another approach would be a separate fork, but that then has a number of
downsides too- every write has to touch that too, and a single page of
nonces would cover a pretty large number of pages also.

Ultimately, I agree with you that the LSN isn't perfect and we really
shouldn't be calling it 'ideal' as it isn't, and we can work to fix that
language in the patch, but the lack of any alternative being proposed
that might be acceptable makes this feel a bit like rock management [2].

My apologies if there's something good that's already been specifically
pushed and I just missed it; if so, a link to that suggestion and
discussion would be greatly appreciated.

Thanks again!

Stephen

[1]: https://commitfest.postgresql.org/45/3986/
[2]: https://en.wikipedia.org/wiki/Wikipedia:Bring_me_a_rock ; though
that isn't great for a quick summary (which I tried to find on an
isolated page somewhere and didn't).

The gist is, without a suggestion of things to try, we're left
to our own devices to try and figure out things which might be
successful, only to have those turned down too when we come back with
them, see [1] for what feels like an example of this.  Given your
feedback overall, which I'm very thankful for, I'm hopeful that you see
that this is, indeed, a useful feature that people are asking for and
therefore are willing to spend some time on it, but if the feedback is
that nothing on the page is acceptable to use for the nonce, we can't
put the nonce somewhere else, and we can't change the page format, then
everything else is just moving deck chairs around on the titanic that
has been this effort.