From: nelson@crynwr.com
Date: Fri, 7 Jun 1996 18:29:57 +0800
To: cypherpunks@toad.com
Subject: Re: Multiple Remailers at a site?
In-Reply-To: <19960604232157.2053.qmail@ns.crynwr.com>
Message-ID: <19960607042143.11138.qmail@ns.crynwr.com>
MIME-Version: 1.0
Content-Type: text/plain


nelson@crynwr.com writes:
 > Scott Brickner writes:

 >  > The "to" and "from" values that the traffic analyst will be using
 >  > are the IP addresses in the packets.  It doesn't matter whether
 >  > mixmaster, cypherpunks, or penet remailers are used, they still use
 >  > IP addresses.
 > 
 > Sure *does* matter.  There's no computationally feasible way to
 > associate one mixmaster message with another.  The only way you can
 > get a clue is by analyzing who sends mail into and out of the
 > mixmaster system.  If both of your endpoints are within the mixmaster
 > system, you have no entering or exiting mail to analyze.  It doesn't
 > matter if the mixmaster remailers are on the same or different systems.

Scott indicates, in private mail, that he needs another, clearer,
explanation.  Okay, here goes:

A is sender, B is recipient, M is mixmaster remailer network, W is
watcher.

A Mixmaster system (there can be more than one, although there is
currently only one published Mixmaster system of remailers) acts as a
single node for the purposes of traffic analysis.  Imagine, if you
will, a remailer that everyone trusts implicitly.  Why would you need
any other remailers??  All W can see is incoming mixmaster messages
(lets you identify A), and outgoing ASCII messages (lets you identify
B).  If W can correlate traffic between A and B, he does it by
watching what happens between A and B, not being privy to the
internals of M.

Now obviously there is no single trustable M.  So, you create many Ms,
who move traffic between themselves.  Let's assume that only one of
them is trustable (M') and happens to be used by A to send a message
to B.  W STILL doesn't know what happens inside M' and has no more
information about the correlation between the message sent by A and
the message received by B than in the first case.

Do you see now, Scott?  Adding mixmasters doesn't need to make traffic
analysis harder (it does, but it doesn't need to).  It makes finding
an M that you trust easier.  And to that end, it doesn't matter if
those mixmasters are all running on the same host or not.

Now, my point about increased security by sender and/or receiver
running a Mixmaster remailer is that W has an easy time identifying A
and B because he can see that A sends a mixmaster message, and B
receives an ASCII message from a mixmaster remailer.  If either A or B
is running a mixmaster, W is denied knowledge that A or B even exists.
He MUST assume that anyone running a remailer is receiving or sending
some or all of the messages.  Message counting (looking for a delta
implying an internally received or transmitted message) is no help.
Since mixmaster happily ignores bogus messages, I could receive a
message, fill its packets with junk, send them one or more hops, and
let someone *else* be under suspicion of having received a message.

As an aside, the TLAs *are* looking for A's and B's.  They spend
millions of dollars a year on telephone traffic analysis.  We MUST
assume that they would spend tens of thousands of dollars a year on
email traffic analysis.

-russ <nelson@crynwr.com>    http://www.crynwr.com/~nelson
Crynwr Software   | Crynwr Software sells packet driver support | PGP ok
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