Greetings,
* Bruce Momjian (bruce@momjian.us) wrote:
> On Tue, Oct 5, 2021 at 04:29:25PM -0400, Bruce Momjian wrote:
> > On Tue, Sep 28, 2021 at 12:30:02PM +0300, Ants Aasma wrote:
> > > On Mon, 27 Sept 2021 at 23:34, Bruce Momjian <bruce@momjian.us> wrote:
> > > We are still working on our TDE patch. Right now the focus is on refactoring
> > > temporary file access to make the TDE patch itself smaller. Reconsidering
> > > encryption mode choices given concerns expressed is next. Currently a viable
> > > option seems to be AES-XTS with LSN added into the IV. XTS doesn't have an
> > > issue with predictable IV and isn't totally broken in case of IV reuse.
> >
> > Uh, yes, AES-XTS has benefits, but since it is a block cipher, previous
> > 16-byte blocks affect later blocks, meaning that hint bit changes would
> > also affect later blocks. I think this means we would need to write WAL
> > full page images for hint bit changes to avoid torn pages. Right now
> > hint bit (single bit) changes can be lost without causing torn pages.
> > This was another of the advantages of using a stream cipher like CTR.
>
> Another problem caused by block mode ciphers is that to use the LSN as
> part of the nonce, the LSN must not be encrypted, but you then have to
> find a 16-byte block in the page that you don't need to encrypt.
With AES-XTS, we don't need to use the LSN as part of the nonce though,
so I don't think this argument is actually valid..? As discussed
previously regarding AES-XTS, the general idea was to use the path to
the file and the filename itself plus the block number as the IV, and
that works fine for XTS because it's ok to reuse it (unlike with CTR).
Thanks,
Stephen
