From: Scott Brickner <sjb@universe.digex.net>
Date: Wed, 11 Oct 95 15:22:40 PDT
To: anon-remailer@utopia.hacktic.nl (Anonymous)
Subject: Re: MITM evasion MITM evasion
In-Reply-To: <199510110855.JAA17966@utopia.hacktic.nl>
Message-ID: <199510112222.SAA21067@universe.digex.net>
MIME-Version: 1.0
Content-Type: text/plain


Anonymous, claiming to be Tim May, writes:
>Two years ago, I pointed out that getting a single message past
>the man in the middle isn't good enough; you have to convince your
>readers that the key they received on one channel is more accurate
>than the key they're receiving on all the other channels.
>But if they'll believe that, they may also believe the man in the middle's
>announcement that the key in your name on all the keyservers is
>wrong, and the correct key is the one he's putting out.
>Can't win either way, but it's still important to get the key out.

I see two general categories of MITM attacks.  In one case, Mitch wants
to eavesdrop on Alice and Bob, but doesn't really care about other
communication they do.  In the other, Mitch wants to know about all of
Alice's communications, regardless of with whom they are.

Public key cryptography turns the first case into two instances of the
second.  If Mitch doesn't control all of both Alice and Bob's
communications with everyone, the will eventually discover that the key
they're using for the other isn't the same one everyone else uses.

In the second MITM model, Mitch has an unbelievable task.  Any public
key that goes from Alice to anyone else, or vice versa, must be
substituted with one Mitch holds.  Any messages *about* public keys
must be transformed into messages about the corresponding MITM keys.

This includes telephone conversations where Alice and Bob exchange
keyids, the business card Eve has printed with her keyid and gives
to Alice at Interop, the Betsi key Alice can read in the newspaper,
WWW pages, files FTP'd, and face-to-face meetings.

Anything short of total control gives Alice an opportunity to learn
about Mitch's presence.  If Alice can exploit the hole enough to get
one good key, Mitch must change his tactics to denial of service
with respect to that key, or Alice can ask the key owner for other
good keys.

If Mitch can successfully surround Alice in such a cloud, I submit
at least one of the following statements is true:

1. Alice is such a non-entity that no one really wants to communicate
with her.

2. Bob can safely assume that the new key he just got isn't really from
Alice, because an Alice-with-a-life surrounded by a nearly successful
Mitch-cloud wouldn't be sending out keys --- she'd be sending out
messages saying "HELP ME!!  I'M LOCKED IN MITCH'S SECRET BOMB
SHELTER!!!"