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Re: sniffing plaintext protocols
From: Joachim Schipper <j.schipper () math uu nl>
Date: Sat, 12 Aug 2006 00:57:06 +0200

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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