Re: storing an explicit nonce

Hi,

On 2021-05-27 12:01:16 -0400, Bruce Momjian wrote:
> On Thu, May 27, 2021 at 08:34:51AM -0700, Andres Freund wrote:
> > On Thu, May 27, 2021, at 08:10, Bruce Momjian wrote:
> > > On Wed, May 26, 2021 at 05:11:24PM -0700, Andres Freund wrote:
> > > > On 2021-05-25 17:12:05 -0400, Bruce Momjian wrote:
> > > > > If we used a block cipher instead of a streaming one (CTR), this might
> > > > > not work because the earlier blocks can be based in the output of
> > > > > later blocks.
> > > >
> > > > What made us choose CTR for WAL & data file encryption? I checked the
> > > > README in the patchset and the wiki page, and neither seem to discuss
> > > > that.
> > > >
> > > > The dangers around nonce reuse, the space overhead of storing the nonce,
> > > > the fact that single bit changes in the encrypted data don't propagate
> > > > seem not great? Why aren't we using something like XTS? It has obvious
> > > > issues as wel, but CTR's weaknesses seem at least as great. And if we
> > > > want a MAC, then we don't want CTR either.
> > >
> > > We chose CTR because it was fast, and we could use the same method for
> > > WAL, which needs a streaming, not block, cipher.
> >
> > The WAL is block oriented too.
>
> Well, AES block mode only does 16-byte blocks, as far as I know, and I
> assume WAL is more granular than that.

WAL is 8kB blocks by default. We only ever write it out with at least
that granularity.

> Also, you need to know the bytes _before_ the WAL do write a new
> 16-byte block, so it seems overly complex for our usage too.

See the XTS reference. Yes, it needs the previous 16bytes, but only
within the 8kB page.

Greetings,

Andres Freund