Re: sniffing plaintext protocols

[Joachim Schipper <j.schipper () math uu nl>]

On Fri, Aug 11, 2006 at 06:19:38PM +0200, itsec.info wrote:
> On Friday 11 August 2006 07:34, Tonnerre Lombard wrote:
> > While this discussion doesn't quite seem to match the list subject, I
> > have something to say here.
> It is welcome when the topic evolves!
>
> > On Thu, 2006-08-10 at 10:27 -0700, Gary E. Miller wrote:
> > > Most modern pop3 and imap clients and servers support TLS.  That
> > > protentially gives you a certificate protected channel between the client
> > > and server.
> > >
> > > A nice setup is dovecot server and thunderbird client using all TLS.
> >
> > This doesn't protect your mail at all if one of the mail servers
> > underway demands to receive the mail unencrypted, which a lot of mail
> > servers still do these days. 
> Hopefully these emails can not be de-crypted on the fly.

The connection is encrypted, the mail isn't. You're looking for GnuPG,
which does decrypt the mail; however, this requires additional software
at the endpoint (i.e. something more or less PGP-compatible).

> > Even worse, this [i.e., other mail servers not accepting STARTTLS,
> > in my understanding - Joachim] gives anyone with the desire to crack
> > your local TLS certificate for pop3s/imaps a huge opportunity for a
> > known plaintext attack.
> I do not get this point.

An attacker with the ability to sniff traffic would know what mail is
being received; therefore, he could predict a very large part of the
conversation between MUA and IMAP server; in fact, knowing both
implementations and with a little experimentation, he would be capable
of knowing every single byte sent.

However, if the above is a correct representation of what Tonerre wrote,
this is still a bogus argument. Such an attacker could also presumably
send mail to an otherwise inactive account, and still know 100% of the
exchange; and while knowing the plaintext is helpful in cracking the
certificate, modern crypto algorithms can't be defeated by such a simple
attack - or, at least, you'd be rich and famous if you could show it was
possible.

> > The only real way to secure the contents (not the sender and
> > receipient!) of your mail is to use PGP encryption on it. If you want to
> > hide the metadata (who sent mail to who and about what) as well, you'll
> > have to go for mixes, but they're pretty uneasy to get right...
> Well pgp does address another area compared to TLS or pop3s. On the
> other hand I agree to combine both!

Only if you configure your MUA not to downgrade to plain SMTP when TLS
is not available, and so on - there isn't too much point otherwise.

                Joachim

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