Robert Haas <robertmh...@gmail.com> wrote: > On Tue, Aug 6, 2019 at 10:36 AM Bruce Momjian <br...@momjian.us> wrote:
> Seems reasonable (not that I am an encryption expert). > > > For WAL, we effectively create a 16MB bitstream, though we can create it > > in parts as needed. (Creating it in parts is easier in CTR mode.) The > > nonce is the segment number, but each 16-byte chunk uses a different > > counter. Therefore, even if you are encrypting the same 8k page several > > times in the WAL, the 8k page would be different because of the LSN (and > > other changes), and the bitstream you encrypt/XOR it with would be > > different because the counter would be different for that offset in the > > WAL. > > But, if you encrypt the same WAL page several times, the LSN won't > change, because a WAL page doesn't have an LSN on it, and if it did, > it wouldn't be changing, because an LSN is just a position within the > WAL stream, so any given byte on any given WAL page always has the > same LSN, whatever it is. > > And if the counter value changed on re-encryption, I don't see how > we'd know what counter value to use when decrypting. There's no way > for the code that is decrypting to know how many times the page got > rewritten as it was being filled. > > Please correct me if I'm being stupid here. In my implementation (I haven't checked whether Masahiko Sawada changed this in his patch) I avoided repeated encryption of different data using the same key+IV by omitting the unused part of the WAL page from encryption. Already written records can be encrypted repeatedly because they do not change. -- Antonin Houska Web: https://www.cybertec-postgresql.com