Re: storing an explicit nonce

On Thu, Oct  7, 2021 at 02:52:07PM -0400, Stephen Frost wrote:
> > > Is there a particular reason why you would prefer not to use LSN? I suggested
> > > it because in my view having a variable tweak is still better than not having
> > > it even if we deem the risks of XTS tweak reuse not important for our use case.
> > > The comment was made under the assumption that requiring wal_log_hints for
> > > encryption is acceptable.
> > 
> > Well, using the LSN means we have to store the LSN unencrypted, and that
> > means we have to carve out a 16-byte block on the page that is not
> > encrypted.
> 
> With XTS this isn't actually the case though, is it..?  Part of the
> point of XTS is that the last block doesn't have to be a full 16 bytes.

> What you're saying is true for XEX, but that's also why XEX isn't used
> for FDE in a lot of cases, because disk sectors aren't typically
> divisible by 16.

Oh, I was not aware of that XTS feature.  Nice.

> https://en.wikipedia.org/wiki/Disk_encryption_theory
> 
> Assuming that's correct, and I don't see any reason to doubt it, then
> perhaps it would make sense to have the LSN be unencrypted and include
> it in the tweak as that would limit the risk from re-use of the same
> tweak over time.

Yes, seems like a plan.

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

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