Subject: Re: libpq compression (part 3)

From: Jacob Champion <[email-masked]>
Date: Mon, 20 May 2024 12:39:49 -0700

On Mon, May 20, 2024 at 11:05 AM Andrey M. Borodin <[email-masked]> wrote:
> > So, the data would be compressed first, with framing around that, and
> > then transport encryption would happen afterwards. I don't see how
> > that would leak your password, but I have a feeling that might be a
> > sign that I'm about to learn some unpleasant truths.
>
> Compression defeats encryption. That's why it's not in TLS anymore.
> The thing is compression codecs use data self correlation. And if you mix secret data with user's data, user might guess how correlated they are.

I'm slow on the draw, but I hacked up a sample client to generate
traffic against the compression-enabled server, to try to illustrate.

If my client sends an LDAP password of "hello", followed by the query
`SELECT 'world'`, as part of the same gzip stream, I get two encrypted
packets on the wire: lengths 42 and 49 bytes. If the client instead
sends the query `SELECT 'hello'`, I get lengths 42 and 46. We lost
three bytes, and there's only been one packet on the stream before the
query; if the observer controlled the query, it's pretty obvious that
the self-similarity has to have come from the PasswordMessage. Rinse
and repeat.

That doesn't cover the case where the password itself is low-entropy,
either. "hellohellohellohello" at least has length, but once you
compress it that collapses. So an attacker can passively monitor for
shorter password packets and know which user to target first.

--Jacob