Re: anonymous mailing lists

[David Mazieres <dm@amsterdam.lcs.mit.edu>]

iang@cs.berkeley.edu (Ian Goldberg) wrote:

> Yesterday, Dave and I discussed at length a design for a new
> remailer network...

If you are thinking of revamping the mixmaster protocol, I have a
couple of suggestions/requests.  One basic philosophy motivating all
of these ideas is that I would like to avoid requiring any
"centralized control" or consensus about exactly what remailers should
exist.  This can be achieved by pushing a lot of configuration
parameters into the anonymous messages, where the sender has control
over them

First, D-H (or RSA with short-lived keys) is an extremely good idea.
Long-lived encryption keys (like the current mixmaster secret keys)
should not be used for secrecy.  However, it would also be good if you
could avoid any man-in-the middle weaknesses.  Specifically, with
simple D-H, an active attack could be used to record all anonymous
messages from A to B, and weeks later if B is compromised the messages
could then be decrypted.

Thus, when sending from remailer A to remailer B, B's identity must be
proven with B's public key (either through RSA encrypting A's half of
the D-H secret key and a challenge with B's key, or by having B sign
his half of the D-H secret and a nonce).  Moreover, since not every
remailer will be known to every other, and since people may want to
set up and test new remailers for a while before announcing them to
the world, a strong cryptographic hash or MAC of B's public key should
be embedded in the remailed-message itself.  Thus, A can query B for
its public key and verify the public key, then use this public key to
know it is talking to the real "next hop".

It would also be nice to avoid having every message go through every
remailer unless the sender actually want's it to.  In particular, a
larger remailer network should not have to translate into more traffic
for all the remailers, as it would be nice to have as large a network
as possible.  Thus, if, for instance, remailer A sends messages out
every half hour, and A wants to send messages to B, C, and D--why not
send the three useful messages to B, C, and D all in the same round,
and just send garbage to all the other remailers.  Of course, messages
should be allowed to have as many next-hops as necessary, so that if
you don't want A to know that a message's next hop is B, you can ask
it to send the same message to C, F, and G as well as to B.  That way,
A won't know the real next hop.

Now the next question is, when sending garbage to all the other
remailers, should "all the other remailers" be defined by A or by the
anonymous message itself.  Here, A should definitely have some list of
remailers it knows about.  However, maybe at each hop a message should
be able to supply 6-byte (IP address/port number) addresses of other
remailers to which garbage should be send.  If there appears to be a
remailer at the address supplied, and that remailer is not already
known to A, perhaps the new remailer should automatically be added to
the list of garbage recipients (and then automatically deleted if it
stops responding for 24 hours).

In the event that A has a real backlog of messages for a particular
destination B, it might make sense for A to hand some of those
messages off to other remailers instead of just feeding them garbage.
That way, even when one remailer is receiving a lot of mail it won't
be immediately clear to it's operator which the preceeding hop is.

Given all these features, of course, it would be necessary to have
variable-length next-hop-descriptors instead of the fixed size and
number currently in mixmaster.  Is there some reason this can't be
done?  The total actual length of the 3-DES encrypted portion of the
mixmaster message shouldn't be available to any but the last hop.
Thus, is there something wrong with padding the message (or even just
the 10K header portion of the message if you want to keep the message
in two parts) with garbage to be 3-DES decrypted into more garbage at
the next hop?  Of course the padding should be done in such a way that
the final hop does not know how much space the remailing headers
originally took up, but this shouldn't be too hard (for instance the
padding could go between the headers and the message data).

Finally, another very useful feature would be some support for
improved response blocks.  Right now aliases like alpha.c2.org don't
offer very much security because they have to go through Type-1
remailers.  However, one could imagine mixmaster extensions to allow
it to work for replies as well as anonymous messages.  Imagine a nym
server with just a 10K mixmaster header as a response block.  The
server would pad a received message to 10K, prepend the 10K mixmaster
header, and send off the message.  At each hop of the way, the message
would get "decrypted" with some 3-DES key (and possibly a weird IV).
However, couldn't the recipient then just "encrypt" the message to
recover the plaintext?  Of course, this might undesireably weaken the
replay prevention, but there's got to be a good solution for response
blocks somewhere near what we currently have for mixmaster.

David