Re: [Proposal] Table-level Transparent Data Encryption (TDE) and Key Management Service (KMS)

On Tue, Jul 30, 2019 at 4:48 PM Bruce Momjian <bruce(at)momjian(dot)us> wrote:

> I had more time to think about the complexity of adding relfilenode to
> the IV. Since relfilenode is only unique within a database/tablespace,
> we would need to have pg_upgrade preserve database/tablespace oids
> (which I assume are the same as the directory and tablespace symlinks).
> Then, to decode a page, you would need to look up those values. This is
> in addition to the new complexity of CREATE DATABASE and moving files
> between tablespaces. I am also concerned that crash recovery operations
> and cluster forensics and repair would need to also deal with this.
>
> I am not even clear if pg_upgrade preserving relfilenode is possible ---
> when we wrap the relfilenode counter, does it start at 1 or at the
> first-user-relation-oid? If the former, it could conflict with oids
> assigned to new system tables in later major releases. Tying the
> preservation of relations to two restrictions seems risky.
>

Agreed. Unless you know for sure the input is going to immutable across
copies or upgrades, including anything in either the IV or key derivation
gets risky and could tie you down for the future. That's partly why I like
the idea separate salt (basically you directly pay for the complexity by
tracking that).

Even if we do not include a separate per-relation salt or things like
relfilenode when generating a derived key, we can still include other types
of immutable attributes. For example the fork type could be included to
eventually allow multiple forks for the same relation to be encrypted with
the same IV = LSN + Page Number as the derived key per-fork would be
distinct.

> Using just the page LSN and page number allows a page to be be
> decrypted/encrypted independently of its file name, tablespace, and
> database, and I think that is a win for simplicity. Of course, if it is
> insecure we will not do it.
>

As LSN + Page Number combo is unique for all relations (not just one
relation) I think we're good for pages.

I am thinking for the heap/index IV, it would be:
>
> uint64 lsn;
> unint32 page number;
> /* only uses 11 bits for a zero-based CTR counter for 32k pages */
> uint32 counter;
>

Looks good.

> and for WAL it would be:
>
> uint64 segment_number;
> uint32 counter;
> /* guarantees this IV doesn't match any relation IV */
> uint32 2^32-1 /* all 1's */
>

I need to read up more on the structure of the WAL records but here's some
high level thoughts:

WAL encryption should not use the same key as page encryption so there's no
need to design the IV to try to avoid matching the page IVs. Even a basic
derivation with a single fixed WDEK = HKDF(MDEK, "WAL") and TDEK =
HKDF(MDEK, "PAGE") would ensure separate keys. That's the the literal
string "WAL" or "PAGE" being added as a salt to generate the respective
keys, all that matters is they're different.

Ideally WAL encryption would generating new derived keys as part of the WAL
stream. The WAL stream is not fixed so you have the luxury of being able to
add a "Use new random salt XZY going forward" records. Forcing generation
of a new salt/key upon promotion of a replica would ensure that at least
the WAL is unique going forward. Could also generate a new upon server
startup, after every N bytes, or a new one for each new WAL file. There's
much more flexibility compared to page encryption.

As WAL is a single continuous stream, we can start the IV for each derived
WAL key from zero. There's no need to complicate it further as Key + IV
will never be reused.

If WAL is always written as full pages we need to ensure that the empty
parts of the page are actual zeros and not "encrypted zeroes". Otherwise an
XOR of the empty section of the first write of a page against a subsequent
one would give you the plain text.

The non-fixed size of the WAL allows for the addition of a MAC though I'm
not sure yet the best way to incorporate it. It could be part of each
encrypted record or its own summary record (providing a MAC for a series of
WAL records). After I've gone through this a bit more I'm looking to put
together a write up with this and some other thoughts in one place.

Regards,
-- Sehrope Sarkuni
Founder & CEO | JackDB, Inc. | https://www.jackdb.com/