Re: Transparent Data Encryption (TDE) and encrypted files

On Fri, Oct 4, 2019 at 11:31:00PM +0200, Tomas Vondra wrote:
> On Fri, Oct 04, 2019 at 03:57:32PM -0400, Bruce Momjian wrote:
> > The current approach is to encrypt anything that contains user data,
> > which includes heap, index, and WAL files. I think replication slots
> > and logical replication might also fall into that category, which is why
> > I started this thread.
>
> Yes, I think those bits have to be encrypted too.
>
> BTW I'm not sure why you list replication slots and logical replication
> independently, those are mostly the same thing I think. For physical
> slots we probably don't need to encrypt anything, but for logical slots
> we may spill decoded data to files (so those will contain user data).

In this thread, I am really looking for experts who can explain exactly
where sensitive data is stored in PGDATA. Oh, pgsql_tmp must be
encrypted too. I would say we know which things must be encrypted, but
we now need to go through the rest of PGDATA to determine which parts
are safe to leave unencrypted, and which must be encrypted.

> > I can see some saying that all cluster files should be encrypted, and I
> > can respect that argument. However, as outlined in the diagram linked
> > to from the blog entry:
> >
> > https://momjian.us/main/blogs/pgblog/2019.html#September_27_2019
> >
> > I feel that TDE, since it has limited value, and can't really avoid all
> > information leakage, should strive to find the intersection of ease of
> > implementation, security, and compliance. If people don't think that
> > limited file encryption is secure, I get it. However, encrypting most
> > or all files I think would lead us into such a "difficult to implement"
> > scope that I would not longer be able to work on this feature. I think
> > the code complexity, fragility, potential unreliability, and even
> > overhead of trying to encrypt most/all files would lead TDE to be
> > greatly delayed or never implemented. I just couldn't recommend it.
> > Now, I might be totally wrong, and encryption of everything might be
> > just fine, but I have to pick my projects, and such an undertaking seems
> > far too risky for me.
> >
>
> I agree some trade-offs will be needed, to make the implementation at
> all possible (irrespectedly of the exact design). But I think those
> trade-offs need to be conscious, based on some technical arguments why
> it's OK to consider a particular information leak acceptable, etc. For
> example it may be fine when assuming the attacker only gets a single
> static copy of the data directory, but not when having the ability to
> observe changes made by a running instance.

Yes, we need to be explicit in what we don't encrypt --- that it is
reasonably safe.

> In a way, my concern is somehat the opposite of yours - that we'll end
> up with a feature (which necessarily adds complexity) that however does
> not provide sufficient security for various use cases.

Yep, if we can't do it safely, there is no point in doing it.

> And I don't know where exactly the middle ground is, TBH.

We spend a lot of time figuring out exactly how to safely encrypt WAL,
heap, index, and pgsql_tmp files. The idea of doing this for another
20 types of files --- to find a safe nonce, to be sure a file rewrite
doesn't reuse the nonce, figuring the API, crash recovery, forensics,
tool interface --- is something I would like to avoid. I want to avoid
it not because I don't like work, but because I am afraid the code
impact and fragility will doom the feature.

> > Just for some detail, we have solved the block-level encryption problem
> > by using CTR mode in most cases, but there is still a requirement for a
> > nonce for every encryption operation. You can use derived keys too, but
> > you need to set up those keys for every write to encrypt files. Maybe
> > it is possible to set up a write API that handles this transparently in
> > the code, but I don't know how to do that cleanly, and I doubt if the
> > value of encrypting everything is worth it.
> >
> > As far as encrypting the log file, I can see us adding documentation to
> > warn about that, and even issue a server log message if encryption is
> > enabled and syslog is not being used. (I don't know how to test if
> > syslog is being shipped to a remote server.)
> >
>
> Not sure. I wonder if it's possible to setup syslog so that it encrypts
> the data on storage, and if that would be a suitable solution e.g. for
> PCI DSS purposes. (It seems at least rsyslogd supports that.)

Well, users don't want the data visible in a mounted file system, which
is why we were thinking a remote secure server would help.

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

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