Re: Internal key management system

On Fri, 12 Jun 2020 at 16:17, Fabien COELHO <coelho(at)cri(dot)ensmp(dot)fr> wrote:
>
>
> Hello Masahiko-san,
>
> I'm not sure I understood your concern. I try to answer below.
>
> > If I understand your idea correctly we put both DEK and KEK
> > "elsewhere", and a postgres process gets only DEK from it.
>
> Yes, that is one of the option.
>
> > It seems to me this idea assumes that the place storing encryption keys
> > employees 2-tire key hierarchy or similar thing.
>
> ISTM that there is no such assumption. There is the assumption that there
> is an interface to retrieve DEK. What is done being the interface to
> retrieve this DEK should be irrelevant to pg. Having them secure by a
> KEK looks like an reasonable design, though. Maybe keys are actually
> stored. Maybe thay are computed based on something, eg key identifier and
> some secret. Maybe there is indeed a 2-tier something. Maybe whatever.
>
> > What if the user wants to or has to manage a single encryption key?
>
> Then it has one key identifier and it retrieve one key from the DMS.
> Having a "management system" for a singleton looks like overkill though,
> but it should work.
>
> > For example, storing an encryption key for PostgreSQL TDE into a file in
> > a safe server instead of KMS using DEK and KEK because of budgets or
> > requirements whatever.
>
> Good. If you have an interface to retrieve a key, then it can probably
> contact said server to get it when needed?
>
> > In this case, if the user does key rotation, that encryption key would
> > need to be rotated, resulting in the user would need to re-encrypt all
> > database data encrypted with old key.
>
> Sure, by definition actually changing the key requires a
> decryption/encryption cycle on all data.
>
> > It should work but what do you think about how postgres does key
> > rotation and re-encryption?
>
> If pg actually has the DEK, then it means that while the re-encryption is
> performed it has to manage two keys simultenaously, this is a question for
> what is done on pg server with the keys, not really about the DMS ?

Yes. Your explanation made my concern clear. Thanks!

>
> If the "elsewhere" service does the encryption, maybe the protocol could
> include it, eg something like:
>
> REC key1-id key2-id data-encrypted-with-key1
> -> data-encrypted-with-key2
>
> But it could also achieve the same thing with two commands, eg:
>
> DEC key1-id data-encrypted-with-key1
> -> clear-text-data
>
> ENC key2-id clear-text-data
> -> data-encrypted-with-key2
>
> The question is what should be put in the protocol, and I would tend to
> think that some careful design time should be put in it.
>

Summarizing the discussed points so far, I think that the major
advantage points of your idea comparing to the current patch's
architecture are:

* More secure. Because it never loads KEK in postgres processes we can
lower the likelihood of KEK leakage.
* More extensible. We will be able to implement more protocols to
outsource other operations to the external place.

On the other hand, here are some downsides and issues:

* The external place needs to manage more encryption keys than the
current patch does. Some cloud key management services are charged by
the number of active keys and key operations. So the number of keys
postgres requires affects the charges. It'd be worse if we were to
have keys per table.

* If this approach supports only GET protocol, the user needs to
create encryption keys with appropriate ids in advance so that
postgres can get keys by id. If postgres TDE creates keys as needed,
CREATE protocol would also be required.

* If we need only GET protocol, the current approach (i.g.
cluser_passphase_command) would be more simple. I imagine the
interface between postgres and the extension is C function. This
approach is more extensible but it also means extensions need to
support multiple protocols, leading to increase complexity and
development cost.

* This approach necessarily doesn’t eliminate the data leakage threat
completely caused by process compromisation. Since DEK is placed in
postgres process memory, it’s still possible that if a postgres
process is compromised the attacker can steal database data. The
benefit of lowering the possibility of KEK leakage is to deal with the
threat that the attacker sees database data encrypted by past or
future DEK protected by the stolen KEK.

* An open question is, as you previously mentioned, how to verify the
key obtained from the external place is the right key.

Anything else we need to note?

Finally, please understand I’m not controverting your idea but just
trying to understand which approach is better from multiple
perspectives.

Regards,

--
Masahiko Sawada http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services