On 7/9/19 10:06 PM, Stephen Frost 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.
>
> Unless we decide that we can pull the LSN *out* of the 8K block and
> store it unencrypted, and then store the *rest* of the block
> encrypted... That might also allow things like backup software to work
> on these encrypted data files for page-level backups without needing
> access to the key and that'd be pretty neat.
>
> Of course, as with anything, the more data you expose, the higher the
> overall risk that someone can figure out some meaning from it. Still,
> if the idea was that we'd use the LSN in this way, then it'd need to be
> stored unencrypted regardless...
I don't think we are going to be able to eliminate every possible
side-channel anyway -- this seems like a good compromise to me.
Joe
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