On Wed, Jul 31, 2019 at 03:29:59PM +0900, Masahiko Sawada wrote:
> Just to confirm, we have 21 bits left for nonce in CTR? We have LSN (8
> bytes), page-number (4 bytes) and counter (11 bits) in 16 bytes nonce
> space. Even though we have 21 bits left we cannot store relfilenode to
> the IV.
No. The nonce is the LSN and page number, the CTR counter (11 bits),
and 21 extra bits. CTR needs a nonce for every 16-byte block, if you
want to think of it that way.
Even though there isn't space for the relfilenode in the nonce, We could
use the relfilenode/tablespace/database id by mixing into a derived key,
based on the master key, but as I stated in another email, we don't want
do that unles we have to.
> BTW I've received a review about the current design by some
> cryptologists in our company. They recommended to use CTR rather than
> CBC. The main reason is that in block cipher it's important to ensure
> the uniqueness even for every input block to the block cipher. CBC is
> hard to ensure that because the previous output is the next block's
> input. Whereas in CTR, it encrypts each blocks separately with
> key+nonce. Therefore if we can ensure the uniqueness of IV we can meet
> that. Also it's not necessary to encrypt IV as it's okey to be
> predictable. So I vote for CTR for at least for tables/indexes
> encryption, there already might be consensus though.
Yes, you are more likely to get duplicate nonce in CBC mode rather than
the CTR mode we are proposing.
--
Bruce Momjian <bruce@momjian.us> http://momjian.us
EnterpriseDB http://enterprisedb.com
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