Re: storing an explicit nonce

On Tue, May 25, 2021 at 05:04:50PM -0400, Stephen Frost wrote:
> > Now, if we want to consult some security experts and have them tell us
> > the hint bit visibility is not a problem, we could get by without using a
> > new nonce for hint bit changes, and in that case it doesn't matter if we
> > have a separate LSN or custom nonce --- it doesn't get changed for hint
> > bit changes.
> 
> I do think it's reasonable to consider having hint bits not included in
> the encrypted part of the page and therefore remove the need to produce
> a new nonce for each hint bit change.  Naturally, there's always an
> increased risk when any data in the system isn't encrypted but given
> the other parts of the system which aren't being encrypted as part of
> this effort it hardly seems like a significant increase of overall risk.
> I don't believe that any of the auditors and security teams I've
> discussed TDE with would have issue with hint bits not being encrypted-
> the principle concern has always been the primary data.

OK, this is good to know.  I know the never-reuse rule, so it is good to
know it can be relaxed for certain data without causing problems in
other places.  Should I modify my patch to do this?

FYI, technically, the hint bit is still encrypted, but could _flip_ in
the encrypted file if changed, so that's why we say it is visible.  If
we used a block cipher instead of a streaming one (CTR), this might not
work because the earlier blocks can be based in the output of later
blocks.

> Naturally, the more we are able to encrypt and the more we can do to
> provide data integrity validation, may open up the possibility for PG to
> be used in even more places, which argues for having some way of making
> these choices be options which a user could decide at initdb time, or at
> least contemplating a road map to where we could offer users the option
> to have other parts of the system be encrypted and ideally have data
> integrity checks, but I don't think we necessarily have to solve
> everything right now in that regard- just having TDE in some form will
> open up quite a few new possibilities for v15, even if it doesn't
> include data integrity validation beyond our existing checksums and
> doesn't encrypt hint bits.

I am thinking full-file system encryption should still be used by people
needing that.  I am concerned that if we add too many
restrictions/additions on this feature, it will not be very useful.

-- 
  Bruce Momjian  <bruce@momjian.us>        https://momjian.us
  EDB                                      https://enterprisedb.com

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