dc-nets implementation (protocol spec)

Yanek Martinson yanek at novavax.nova.edu
Sun Dec 13 11:35:50 PST 1992

I am writing a dc-net implementation.   What follows is some design 
discussion, a protocol specification that defines the interaction,
message format, and file formats, followed by notes, and plans for
future enhancements.  A basic familiarity with the concept of dc-nets is
assumed, for people new to this, see Chaums's article on the topic,
posted here recently.

I am assuming interaction through e-mail because this is, right now,
the most accessible medium.  A more efficient approach would be to
use a true broadcast media, such as coax cable, or satellite.  But
coax only works for local areas, and not everyone has satellite uplink

In the future, I envision all or most local area networks running their
own small dc-nets that use ethernet broadcast, these being linked
hierarchically to medium-level dc-nets that use any internet type of
communication medium (udp datagrams, tcp connections, smtp, usenet news,
shared ftp space, etc) or other regional communication such as radio
(amateur packet radio, spread-spectrum, or even CB) , and these finally
being linked into global dc-nets that use satellite broadcast.  

This type of system would achieve the truly anonymous global communication
system, similar to what Vinge describes in True Names:

      He dumped the last twenty-four hours of the world bulletin
      board into his home memory space and began checking through
      it.  The bulletin board was ideal for untraceable reception
      of messages: anyone on Earth could leave a message [...] If 
      a user copied the entire board, and /then/ searched it, there
      was no outside record of exactly what information he was
      interested in.  There were also simple ways to make nearly 
      untraceable entries on the board.

DC-nets will remove the "nearly" from the last sentence.

The system I am writing is for research purposes only, I would like to 
have about 6 to 8 people running it, and then observe it in action. 
I am not aware of any existing, operational dc-net.  If you know of
one, please let me know.  I would not like to develop a new, incompatible
protocol if it is not fundamentally better than something existing.

The dc-net system is independent of how the keys are distributed/created.
This system assumes that there is already a key stream existing between
any two users, that it can use.

There are several methods by which keys can be distributed.  The most
secure would probably be to use a hardware RNG to generate a one-time
pad, which is then trasfered by some secure means (exchanged on floppies
in person, or put on a 4GB DAT tape (4mm, $20 or less) and sent by
some courier service, etc).  Another possibility is to use
Diffie-Hellman protocol to agree on a set of keys.  The simplest method,
and one I chose to use in this project, is to generate a random key
stream, encrypt it with the other person's public key, sign it with
yours, and send it through e-mail.  See the next message for details
on message formats used for this.  

Again, the core dc-net system described in this message is in no way
dependent on this form of key exchange.  Any other key distribution
method can be "dropped in" without changing anything.

The design document follows:

ALGORITHM (pseudocode):

This algorithm is executed independently by each participant.  The list
of participants, RBSIZE, and MSGSIZE must be agreed on beforehand.  There
must be an existing keystream, or a way to receive one.

Begin Round (increment round number)

Stage 1 (reservation):
    Take RBSIZE bits off everyone's pad, xor all together.
    For every message you need to send, reverse a random bit.
    Sign with your public key, and transmit to all.
    When received everyone's block, xor all together.
    If result is all zeros, round ends.  Go on to the next round.

Stage 2 (transmission):
    For every '1' bit in reservation block:

    Begin Message (increment message number)

        Take MSGSIZE bits off everyone's pad, xor all together.
        If the bit was reserved by you, xor with your message.
        Sign with your public key, transmit to all.
        When received everyone's block, xor all together, getting message.
        If message was yours, verify it, mark it as sent.
        If not, it is a message from someone else. Process as an incoming

    End Message (go to next set bit of reservation block if any)

End Round (go to next round, optionally delay to save bandwidth)


(messages are sent as signed, armored, non-encrypted PGP messages)

 Subject: dc-net transmission

 DCNET <network>; ROUND <round>; PARTICIPANT <userid>; STAGE <stage> [MSG <n>]

 <binary data stream follows>



    your own participant name for that network


    list of participants and addresses in the following format:



    one-time pad for the user, straight 8-bit binary data
        optionally pgp-encrypted with your public key, signed to prevent
        disclosure and tampering; these options not implemented yet


        spool where you store responses received, until you receive
        all of them.  Your own responses are stored there too


        messages you need to send, one message per file.  
        should be stored encrypted too. 


        program that will process any messages received from this 
        network.  In the simplest case this would be a shell script
        that puts the message to the user's mailbox.  But it could
        also drop it into another net's outgoing directory if it
        matches some criteria, or it could be posted to a newsgroup,
        or anything else you want to do.


        this file contains log of transmissions received times and sizes.
        it should only contain public information, so it can be posted
        for analysis of net efficiency, bandwidth used, etc.


I must thank the ILF (Information Liberation Front) for their timely post
of Chaum's article.  I had started with the two-person example provided
in the cypherpunks glossary, and independently extended it to apply to
any number of participants, and had progressed up to the idea of
hierarchial dc-nets, and was struggling with the question of collision
detection, when the article was posted.  Chaum's reservation blocks are
an elegant and efficient solution.  His formal proof of security is
also reassuring.

RBSIZE is the number of bits in the reservation block.  This should be
much larger than number of participants, to minimize the chance of
two participants randomly reserving the same bit (in which case the
transmission has to be retried in the next round).  A convenient number
to use is 1024 bites (128bytes), if the number of users is small.

When operating in idle mode (no messages to be sent), RBSIZE bits are
sent to each participant each round. So the total bandwidth used is,
assuming 1kbit RBSIZE, 25 participants, and two rounds an hour, about
150 kilobytes per day sent and received by each participant, or about 14
bits per second.  This is quite insignificant, considering today's 
communication equipment.  

MSGSIZE is the size of a message.  This should be about the size of an
average message, not the largest one.  Making this too large will waste
bandwidth.  I will use message size of 5kilobytes.  This is quite
small, but enough for research purposes.  This will make the messages
several screenfuls long.  Longer messages can be split into parts, and
reassembled.  For example, this message would be split into two parts.

For each message that is sent, the number of bytes that every every
participant must send and receive is MSGSIZE times the number of 
participants (using the above assumptions, 125 kilobytes).

Using a true broadcast media would dramatically reduce these bandwidth
requirements, making large-scale systems with many participants

This implementation uses a <netname> to distinguish between separate
dc-nets a user may be participating in.  A netname can be any string, 
subject to filename limitations.  Keeping them alphanumeric and under 14
chars should work on most machines.. They might also be case insensitive.

Netnames do not have to be globally unique, one person just can not be on
two nets of the same name.

Persons are referenced to by <participant>, which would normally be the
username but can be an arbitrary string subject to the same limitations.
Two perople with the same participand name can not be on one dc-net.

Are any of you amateur radio (HAM) operators?  I would be interested if
this system could be run over packet radio.  Are there any regulations
against encrypted traffic?  


In addition to mail-based interaction, usenet newsgroups, ethernet broadcasts,
shared ftp access, and tcp connections should be supported.  

A protocol for dynamically adding participants to the net, removing
oneself from the net, or changing the address should exist.

A way to deal with malfunctions, such as not receiving a message from
a participant.  Currently the net will wait forever, for that one message
to arrive.  A time-out should probably established, after which everyone
will re-send their contribution without including the missing person.
This is potentially dangerous, since it is equivalent to disclosing that
participant's key streams.  Since one-time pad is used, this will not
compromise earlier communication, but the person must be notified if they
ever come back on-line to not use that portion of key again.

Support for encrypted and signed storage of keystreams and outgoing

A protocol for routing messages between hierarchical dc-nets.

Automatic splitting of messages longer than MSGSIZE.

A way to indicate the message size in the allocation block, so
communications would not be wasted on small messages, and large messages
would not have to be split into pieces.

PLEASE send any comments, criticism, or ideas to:

Yanek Martinson    mthvax.cs.miami.edu!safe0!yanek     uunet!medexam!yanek
this address preferred -->> yanek at novavax.nova.edu <<-- this address preferred
Phone (305) 765-6300 daytime   FAX: (305) 765-6708  1321 N 65 Way/Hollywood
      (305) 963-1931 evenings       (305) 981-9812  Florida, 33024-5819

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