Hi,
On 2021-05-27 12:28:39 -0400, Robert Haas wrote:
> All that having been said, I am pretty sure I don't fully understand
> what any of these modes involve. I gather that XTS requires two keys,
> but it seems like it doesn't require a nonce.
It needs a second secret, but that second secret can - as far as I
understand it - be generated using a strong prng and encrypted with the
"main" key, and stored in a central location.
> It seems to use a "tweak" that is generated from the block number and
> the position within the block (since an e.g. 8kB database block is
> being encrypted as a bunch of 16-byte AES blocks) but apparently
> there's no problem with the tweak being the same every time the block
> is encrypted?
Right. That comes with a price however: It leaks the information that a
block "version" is identical to an earlier version of the block. That's
obviously better than leaking information that allows decryption like
with the nonce reuse issue.
Nor does it provide integrity - which does seem like a significant issue
going forward. Which does require storing additional per-page data...
Greetings,
Andres Freund