Tomas Vondra <tomas.vondra@2ndquadrant.com> wrote:
> On Mon, Jul 15, 2019 at 06:11:41PM -0400, Bruce Momjian wrote:
> >On Mon, Jul 15, 2019 at 11:05:30PM +0200, Tomas Vondra wrote:
> >> On Mon, Jul 15, 2019 at 03:42:39PM -0400, Bruce Momjian wrote:
> >> > On Sat, Jul 13, 2019 at 11:58:02PM +0200, Tomas Vondra wrote:
> >> > > One extra thing we should consider is authenticated encryption. We can't
> >> > > just encrypt the pages (no matter which AES mode is used - XTS/CBC/...),
> >> > > as that does not provide integrity protection (i.e. can't detect when
> >> > > the ciphertext was corrupted due to disk failure or intentionally). And
> >> > > we can't quite rely on checksums, because that checksums the plaintext
> >> > > and is stored encrypted.
> >> >
> >> > Uh, if someone modifies a few bytes of the page, we will decrypt it, but
> >> > the checksum (per-page or WAL) will not match our decrypted output. How
> >> > would they make it match the checksum without already knowing the key.
> >> > I read [1] but could not see that explained.
> >> >
> >>
> >> Our checksum is only 16 bits, so perhaps one way would be to just
> >> generate 64k of randomly modified pages and hope one of them happens to
> >> hit the right checksum value. Not sure how practical such attack is, but
> >> it does require just filesystem access.
> >
> >Yes, that would work, and opens the question of whether our checksum is
> >big enough for this, and if it is not, we need to find space for it,
> >probably with a custom encrypted page format. :-( And that makes
> >adding encryption offline almost impossible because you potentially have
> >to move tuples around. Yuck!
> >
>
> Right. We've been working on allowing to disable checksum online, and it
> would be useful to allow something like that for encryption too I guess.
> And without some sort of page-level flag that won't be possible, which
> would be rather annoying.
>
> Not sure it needs to be in the page itself, though - that's pretty much
> why I proposed to store metadata (IV, key ID, ...) for encryption in a
> new fork. That would be a bit more flexible than storing it in the page
> itself (e.g. different encryption schemes might easily store different
> amounts of metadata).
>
> Maybe a new fork is way too complex solution, not sure.
One problem of this new fork would be that contents of its buffer (the MAC
values) is not determined until the corresponding buffers of the MAIN fork get
encrypted. However encryption is performed by the storage layer (md.c), which
is not expected to lock other buffers (such as those of the "MAC fork"), read
their pages from disk or insert their WAL records.
This is different from the FSM or VM forks whose buffers are only updated
above the storage layer.
--
Antonin Houska
Web: https://www.cybertec-postgresql.com
