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

On 7/13/19 9:38 AM, Joe Conway wrote:
> On 7/11/19 9:05 PM, Bruce Momjian wrote:
>> On Thu, Jul 11, 2019 at 08:41:52PM -0400, Joe Conway wrote:
>>> On 7/11/19 6:37 PM, Bruce Momjian wrote:
>>> > Our first implementation will encrypt the entire cluster. We can later
>>> > consider encryption per table or tablespace. It is unclear if
>>> > encrypting different parts of the system with different keys is useful
>>> > or feasible. (This is separate from key rotation.)
>>>
>>> I still object strongly to using a single key for the entire database. I
>>> think we can use a single key for WAL, but we need some way to split the
>>> heap so that multiple keys are used. If not by tablespace, then some
>>> other method.
>>
>> What do you base this on?

Ok, so here we go. See links below. I skimmed through the entire thread
and FWIW it was exhausting.

To some extent this degenerated into a general search for relevant
information:

---
[1] and [2] show that at least some file system encryption uses a
different key per file.
---
[2] also shows that file system encryption uses a KDF (key derivation
function) which we may want to use ourselves. The analogy would be
per-table derived key instead of per file derived key. Note that KDF is
a safe way to derive a key and it is not the same as a "related key"
which was mentioned on another email as an attack vector.
---
[2] also says provides additional support for AES 256. It also mentions
CBC versus XTS -- I came across this elsewhere and it bears discussion:

"Currently, the following pairs of encryption modes are supported:

AES-256-XTS for contents and AES-256-CTS-CBC for filenames
AES-128-CBC for contents and AES-128-CTS-CBC for filenames
Adiantum for both contents and filenames

If unsure, you should use the (AES-256-XTS, AES-256-CTS-CBC) pair.

AES-128-CBC was added only for low-powered embedded devices with crypto
accelerators such as CAAM or CESA that do not support XTS."
---
[2] also states this, which again makes me think in terms of table being
the moral equivalent to a file:

"Unlike dm-crypt, fscrypt operates at the filesystem level rather than
at the block device level. This allows it to encrypt different files
with different keys and to have unencrypted files on the same
filesystem. This is useful for multi-user systems where each user’s
data-at-rest needs to be cryptographically isolated from the others.
However, except for filenames, fscrypt does not encrypt filesystem
metadata."
---
[3] suggests 68 GB per key and unique IV in GCM mode.
---
[4] specifies 68 GB per key and unique IV in CTR mode -- this applies
directly to our proposal to use CTR for WAL.
---
[5] has this to say which seems independent of mode:

"When encrypting data with a symmetric block cipher, which uses blocks
of n bits, some security concerns begin to appear when the amount of
data encrypted with a single key comes close to 2n/2 blocks, i.e. n*2n/2
bits. With AES, n = 128 (AES-128, AES-192 and AES-256 all use 128-bit
blocks). This means a limit of more than 250 millions of terabytes,
which is sufficiently large not to be a problem. That's precisely why
AES was defined with 128-bit blocks, instead of the more common (at that
time) 64-bit blocks: so that data size is practically unlimited."

But goes on to say:
"I wouldn't use n*2^(n/2) bits in any sort of recommendation. Once you
reach that number of bits the probability of a collision will grow
quickly and you will be way over 50% probability of a collision by the
time you reach 2*n*2^(n/2) bits. In order to keep the probability of a
collision negligible I recommend encrypting no more than n*2^(n/4) bits
with the same key. In the case of AES that works out to 64GB"

It is hard to say if that recommendation is per key or per key+IV.
---
[6] shows that Azure SQL Database uses AES 256 for TDE. It also seems to
imply a single key is used although at one point it says "transparent
data encryption master key, also known as the transparent data
encryption protector". The term "master key" indicates that they likely
use derived keys under the covers.
---
[7] is generally useful read about how many of the things we have been
discussing are done in SQL Server
---
[8] was referenced by Sehrope. In addition to support for AES 256 for
long term use, table 5.1 is interesting. It lists CBC mode as "legacy"
but not "future".
---
[9] IETF RFC for KDF
---
[10] IETF RFC for Key wrapping -- this is probably how we should wrap
the master key with the Key Encryption Key (KEK) -- i.e. the outer key
provided by the user or command on postmaster start
---

Based on all of that I cannot find a requirement that we use more than
one key per database.

But I did find that files in an encrypted file system are encrypted with
derived keys from a master key, and I view this as analogous to what we
are doing.

As an aside to the specific question, I also found more evidence that
AES 256 is appropriate.

Joe

============================
[1]
https://www.postgresql.org/message-id/832657a1-b27a-a33a-5bc2-ce420f95f4be%40joeconway.com

[2]
https://www.postgresql.org/message-id/20190708194733.cztnwhqge4acepzw%40development

[3]
https://www.postgresql.org/message-id/20190708211811.sio5o36zxhps7snx%40momjian.us

[4] https://www.rfc-editor.org/pdfrfc/rfc3686.txt.pdf

[5]
https://security.stackexchange.com/questions/33434/rsa-maximum-bytes-to-encrypt-comparison-to-aes-in-terms-of-security

[6]
https://docs.microsoft.com/en-us/azure/sql-database/transparent-data-encryption-azure-sql?view=sql-server-2017

[7]
https://docs.microsoft.com/en-us/sql/relational-databases/security/encryption/transparent-data-encryption?view=sql-server-2017

[8] https://www.ecrypt.eu.org/csa/documents/D5.4-FinalAlgKeySizeProt.pdf

[9] https://www.ietf.org/rfc/rfc5869.txt

[10] https://tools.ietf.org/html/rfc3394

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