On Mon, Jul 15, 2019 at 06:08:28PM -0400, Sehrope Sarkuni wrote:
> Hi,
>
> Some more thoughts on CBC vs CTR modes. There are a number of
> advantages to using CTR mode for page encryption.
>
> CTR encryption modes can be fully parallelized, whereas CBC can only
> parallelized for decryption. While both can use AES specific hardware
> such as AES-NI, CTR modes can go a step further and use vectorized
> instructions.
>
> On an i7-8559U (with AES-NI) I get a 4x speed improvement for
> CTR-based modes vs CBC when run on 8K of data:
>
> # openssl speed -evp ${cipher}
> type 16 bytes 64 bytes 256 bytes 1024 bytes
> 8192 bytes 16384 bytes
> aes-128-cbc 1024361.51k 1521249.60k 1562033.41k 1571663.87k
> 1574537.90k 1575512.75k
> aes-128-ctr 696866.85k 2214441.86k 4364903.85k 5896221.35k
> 6559735.81k 6619594.75k
> aes-128-gcm 642758.92k 1638619.09k 3212068.27k 5085193.22k
> 6366035.97k 6474006.53k
> aes-256-cbc 940906.25k 1114628.44k 1131255.13k 1138385.92k
> 1140258.13k 1143592.28k
> aes-256-ctr 582161.82k 1896409.32k 3216926.12k 4249708.20k
> 4680299.86k 4706375.00k
> aes-256-gcm 553513.89k 1532556.16k 2705510.57k 3931744.94k
> 4615812.44k 4673093.63k
Wow, I am seeing CTR as 2x faster here too.
> For relation data where the encryption is going to be per page,
> there's flexibility in how the CTR nonce (IV + counter) is generated.
> With an 8K page, the counter need only go up to 512 for each page
> (8192-bytes per page / 16-bytes per AES-block). That would require
> 9-bits for the counter. Rounding that up to 16-bits allows for wider
> pages and it still uses only two bytes of the counter while ensuring
> that it'd be unique per AES-block. The remaining 14-bytes would be
> populated with some other data that is guaranteed unique per
> page-write to allow encryption via the same per-relation-file derived
> key. From what I gather, the LSN is a candidate though it'd have to be
> stored in plaintext for decryption.
Oh, for CTR, we need to increment the counter for each 16-byte block ---
got it.
> What's important is that writing the two pages (either different
> locations or the same page back again) never reuses the same nonce
> with the same key. Using the same nonce with a different key is fine.
Uh, I think we can use LSN and page-number to be unique.
> With any of these schemes the same inputs will generate the same
> outputs. With CTR mode for WAL this would be an issue if the same key
> and deterministic nonce (ex: LSN + offset) is reused in multiple
> places. That does not have to be the same cluster either. For example
Very good point, since CTR does not use the user data as part of the
later encryption.
> if two replicas are promoted from the same backup with the same master
> key, they would generate the same WAL CTR stream, reusing the
> key/nonce pair. Ditto for starting off with a master key and deriving
> per-relation keys in a cloned installation off some deterministic
> attribute such as oid.
Uh, when we promote a standby, don't we increment the timeline? Does
that help? I don't know what we could use to distingish two standbys
that are both promoted and using the same key --- there is nothing
unique about them.
> This can be avoided by deriving new keys per file (not just per
> relation) from a random salt. It'd be stored out of band and combined
> with the master key to derive the specific key used for that CTR
> stream. If there's a desire for supporting multiple ciphers or key
> sizes, that could be stored alongside the salt. Perhaps use the same
> location or lack of it to indicate "not encrypted" as well.
You mean the cluster would have its own random key? Unfortunately all
clusters in a replica set have the same Database system identifier as
the primary.
> Per-file salts and derived keys would facilitate re-keying a table
> piecemeal, file by file, by generating a new salt/derived-key,
> encrypting a copy of the decrypted file, and doing an atomic rename.
> The files contents would change but its length and any references to
> pages or byte offsets would stay valid. (I think this would work for
> CBC modes too as there's nothing CTR specific about it.)
Storing that might be a problem, particularly to access during crash
recovery.
> I'm not sure of is how to handle randomizing the relation file IV in a
> cloned database. Until the key for a relation file or segment is
> rotated it'd have the same deterministic IV generated as its source as
> the LSN would continue from the same point. One idea is with 128-bits
> for the IV, one could have 64-bits for LSN, 16-bits for AES-block
> counter, and the remaining 48-bits be randomized; though you'd need to
> store those 48-bits somewhere per-page (basically it's a salt per
> page). That'd give some protection from the clone's new data be
> encrypted with the same stream as the parent's. Another option would
> be to track ranges of LSNs and have a centralized list of 48-bit
> randomized salts. That would remove the need for additional salt per
> page though you'd have to do a lookup on that shared list to figure
> out which to use.
>
> CTR mode is definitely more complicated than a pure random-IV + CBC
> but with any deterministic generation of IVs for CBC mode you're going
> to have some of these same problems as well.
This is starting to sound unworkable for our usecase.
--
Bruce Momjian <br...@momjian.us> http://momjian.us
EnterpriseDB http://enterprisedb.com
+ As you are, so once was I. As I am, so you will be. +
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