From: loki@infonex.com (Lance Cottrell)
Date: Wed, 5 Jun 1996 19:05:59 +0800
To: Scott Brickner <stewarts@ix.netcom.com>
Subject: Re: Multiple Remailers at a site?
Message-ID: <addac43501021004bb2c@[206.170.115.3]>
MIME-Version: 1.0
Content-Type: text/plain


At 1:40 PM 6/4/96, Scott Brickner wrote:
>Bill Stewart writes:
>>>I don't think multiple remailers at the same site help anything.
>>
>>Assume Alice, Bob, and Carol are on abc.com and Xenu, Yak, and Zut
>>are on xyz.com.  Remailing between Alice, Bob, and Carol doesn't
>>make appear to make much difference, but it does reduce the damage
>>if one of the remailer's keys is compromised.  On the other hand,
>>mail from Alice -> Xenu -> Bob -> Yak -> Carol -> Zut adds traffic
>>to the system, and makes traffic analysis more difficult,
>>even if the Bad Guys are watching site abc.com and have stolen
>>Alice, Bob, and Carol's keys.
>
>Wait a minute.  More traffic should make analysis easier, since traffic
>analysis is mostly statistical work on the source and destination (not
>necessarily "from" and "to").  A bigger sample makes more reliable
>results.
>
>For traffic analysis, I don't know *who* sent the message (it was,
>after all, anonymized), but I do know a site which transmitted it and
>one which received it, the time it was transmitted, and maybe its
>size.  Multiply this times a whole bunch of messages, and I can infer
>information about "common interests" between those sources and
>destinations.
>
>The delays and mixing done by remailers make it harder by
>disassociating the true sender from the true receiver.  If a remailer
>were to ignore this step, the analyst can deduce from the two data
>points
>
>    "message a, source A, destination RemailerX, time t, size s"
>    "message b, source RemailerX, destination B, time t+0.001s, size s"
>
>that there's some connection between A and B.  The more such evidence,
>the stronger the connection.  If the remailer does a good job with
>the delays and shuffling, then it becomes difficult for the analyst
>to match message a with message b, leaving him with what he already
>knew (that A and RemailerX have a common interest, as to B and RemailerX,
>but the interests may be wholly unrelated).
>
>Multiple remailers on the same machine increases the resolution of
>the address information, at best, improving the analysts ability to
>make connections.  The same traffic load going to a single remailer
>at the site makes the analyst's job harder.
>
>>The other threat it helps with is that if XYZ.COM gets complaints
>>about that evil user Zut, she can kick her off (Bad Zut!)
>>and still leave Xenu and Yak alone; if the remailer service
>>were provided by the machine owner herself she might be directly liable.
>
>Hmm.  Nothing really stops the machine owner from creating a personal
>anonymous account to run the remailer.  When someone complains, shut it
>down and create a new one.  There isn't yet a law which requires that
>the owner be able to identify the user.  This affords the same
>protection that multiple users does.


The time correlation attack can be defeated by sending mail into the
remailer network with a period roughly equal to the propagation time of a
message through a chain. That way your messages correlate with absolutely
all receipts of all messages. That contains no information.

        -Lance


----------------------------------------------------------
Lance Cottrell   loki@obscura.com
PGP 2.6 key available by finger or server.
Mixmaster, the next generation remailer, is now available!
http://www.obscura.com/~loki/Welcome.html or FTP to obscura.com

"Love is a snowmobile racing across the tundra.  Suddenly
it flips over, pinning you underneath.  At night the ice
weasels come."
                        --Nietzsche
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