On Sat, 2 Nov 2019 at 21:33, Antonin Houska <a...@cybertec.at> wrote:
> 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. > > Yeah my patch doesn't change this part. IV for WAL encryption consists of the segment file number, page offset within segment file and the counter for CTR cipher mode. Regards, -- Masahiko Sawada http://www.2ndQuadrant.com/ <http://www.2ndquadrant.com/> PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
