From: Hal <hfinney@shell.portal.com>
Date: Mon, 8 Aug 94 07:58:11 PDT
To: cypherpunks@toad.com
Subject: Re: Anonymous Transport Agents (Was: Latency vs. Reordering)
In-Reply-To: <940808023733B6Mjgostin@eternal.pha.pa.us>
Message-ID: <199408081457.HAA07967@jobe.shell.portal.com>
MIME-Version: 1.0
Content-Type: text/plain


Jeff Gostin <jgostin@eternal.pha.pa.us> writes:

>     Suppose an encryption-savvy mail transport agent, say ESMTP, was
>developed. Further suppose that part of handshaking protocol for this
>transport protocol included an ENCRYPTED reverse lookup on IP identities
>to check that the message is actually coming from where it claims it's
>coming from.  Suppose again that the results of this lookup were only
>checked for correctness (boolean), and then discarded WITHOUT LOGGING, or
>at least with minimal logging. If the reverse lookup was TRUE (IE: the
>sending machine was who it said it was), the message was accepted. If it
>failed, the message would be accepted, and then sent to the bit bucket.

I can see two problems.  First, at least the first machine on the trans-
port path will see both your origin address and your destination address.
So it is in a perfect position to do traffic analysis.  Many users may
not have the ability to control which machine this is since routing is
usually automatic these days.

Second, if each machine simply saves a message and sends it on, then even
if the messages are encrypted there will probably be timing relationships
between the incoming and outgoing messages which will allow them to be
linked.  So someone monitoring the intersite communication channels may be
able to track a message through the network just by noticing when it comes
into and goes out of each node.  This is why Chaum introduces message
batching and mixing at each node.

Hal