From: "Robert A. Rosenberg" <hal9001@panix.com>
Date: Tue, 11 Jun 1996 17:23:03 +0800
To: Hal <hfinney@shell.portal.com>
Subject: Re: Anonymous return addresses
In-Reply-To: <199606082329.QAA14538@jobe.shell.portal.com>
Message-ID: <v03007102ade2846cf37a@[166.84.254.3]>
MIME-Version: 1.0
Content-Type: text/plain


At 16:29 -0700 6/8/96, Hal wrote:


>There is no large amount of traffic needed, as each server only sends an
>amount of data equal to one message.  The individual servers do not get
>any information about which message the requestor wants (other than that
>it is one of the 50).  Only by colluding and XOR'ing their bit strings
>can they figure that out.  The same kind of collusion is needed to trace
>a sent message using two remailers, so the security is similar to what we
>get sending messages.

If the message is split into more than one part (to meet the message size
requirement) there is some potential leakage to each server of what message
is being requested. If User a requested 3 messages, then they MAY be
requesting all three parts of a 3 part message (or 2+1). If a record is
kept of the number of requests over time, then there can be some regression
checking based on the ID (ie: If the number of new messages for ANx in the
DB matches the number that User Y requests in the current session). I may
be in error with this thought but it looks like a possible problem.

>Messages would have a finite lifetime and would expire and be removed
>from the database after a while.  The authors propose breaking the
>database up into batches with a fixed number of messages, but I don't
>fully follow the reasoning behind this.  I guess it reduces the load on
>the server when it does its XOR's.

This can also affect the "attack" I speculated on above since it can "leak"
more info. Multi-part messages (or multiple messages to the same recipient)
which are retrieved in one session can be correlated between the groups
(ie: User Y asked for 5 messages [Selected from Groups 1&5] and ANx is the
one AN? that has the requested number of messages in each of the Groups
[ie: 3 from G1 and 2 from G5]).