On 7/10/19 2:45 AM, Masahiko Sawada wrote:
> On Wed, Jul 10, 2019 at 11:06 AM Stephen Frost <sfrost@snowman.net> wrote:
>>
>> Greetings,
>>
>> * Ryan Lambert (ryan@rustprooflabs.com) wrote:
>> > > What I think Tomas is getting at here is that we don't write a page only
>> > > once.
>> >
>> > > A nonce of tableoid+pagenum will only be unique the first time we write
>> > > out that page. Seems unlikely that we're only going to be writing these
>> > > pages once though- what we need is a nonce that's unique for *every
>> > > write* of the 8k page, isn't it? As every write of the page is going to
>> > > be encrypting something new.
>> >
>> > > With sufficient randomness, we can at least be more likely to have a
>> > > unique nonce for each 8K write. Including the LSN seems like it'd be a
>> > > possible alternative.
>> >
>> > Agreed. I know little of the inner details about the LSN but what I read
>> > in [1] sounds encouraging in addition to tableoid + pagenum.
>> >
>> > [1] https://www.postgresql.org/docs/current/datatype-pg-lsn.html
>>
>> Yes, but it's still something that we'd have to store somewhere- the
>> actual LSN of the page is going to be in the 8K block.
>
> Can we use CBC-ESSIV[1] or XTS[2] instead? IIUC with these modes we
> can use table oid and page number for IV or tweak and we don't need to
> change them each time to encrypt pages.
>
> [1] https://en.wikipedia.org/wiki/Disk_encryption_theory#Encrypted_salt-sector_initialization_vector_.28ESSIV.29
> [2]
https://en.wikipedia.org/wiki/Disk_encryption_theory#XEX-based_tweaked-codebook_mode_with_ciphertext_stealing_(XTS)
From what I can tell [1] is morally equivalent to the NIST method and
does nothing to change the fact that the input nonce needs to be unique
for each encryption operation. I have not had time to review [2] yet...
While it would be very tempting to convince ourselves that a unique
input nonce is not a requirement, I think we are better off being
conservative unless we find some extremely clear guidance that allows us
to draw that conclusion.
Joe
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