Robert Haas <robertmhaas@gmail.com> writes:
> If the WAL *is* encrypted, the state at this point is that the block
> is unreadable, because the first 4kB of the block is the first half of
> the bits that resulted from encrypting 8kB of data that includes the
> new record, and the second 4kB of the block is the second half of the
> bits that resulted from encrypting 8kB of data that did not include
> the new record, and since the new record perturbed every bit on the
> page with probability ~50%, what that means is you now have garbage.
> That means that not only did we lose the new record, but we also lost
> the 3.5kB of good data which the page previously contained. That's
> NOT ok. Some of the changes associated with those WAL records may
> have been flushed to disk already, or there may be commits in there
> that were acknowledged to the client, and we can't just lose them.
ISTM that this is only a problem if you choose the wrong encryption
method. One not-wrong encryption method is to use a stream cipher
--- maybe that's not the exact right technical term, but anyway
I'm talking about a method which notionally XOR's the cleartext
data with a random bit stream generated from the encryption key
(probably along with other knowable inputs such as the block number).
In such a method, corruption of individual on-disk bytes doesn't
prevent you from getting the correct decryption of on-disk bytes
that aren't corrupted.
regards, tom lane
