trunk/Docs/draft-moeller-v2-01.txt

Internet-Draft                                              Ulf Moeller
Expires: December 31, 2003
                                                         Lance Cottrell
                                                        Anonymizer Inc.
                                                        Peter Palfrader
                                                  The Mixmaster Project
                                                           Len Sassaman
                                                 Nomen Abditum Services
                                                              July 2003


                      Mixmaster Protocol Version 2
                       <draft-moeller-v2-01.txt>

Status of This Memo

     This document is an Internet-Draft and is subject to all provisions
     of Section 10 of RFC2026.

     Internet-Drafts are working documents of the Internet Engineering
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     This Internet-Draft will expire on December 31, 2003.

Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.


Abstract

Most e-mail security protocols only protect the message body, leaving
useful information such as the the identities of the conversing
parties, sizes of messages and frequency of message exchange open to
adversaries. This document describes Mixmaster (version 2), a mail
transfer protocol designed to protect electronic mail against traffic
analysis.

Mixmaster is based on D. Chaum's mix-net protocol.
A mix (remailer) is a service that forwards messages, using public key
cryptography to hide the correlation between its inputs and outputs.
Sending messages through sequences of remailers achieves anonymity and
unobserveability of communications against a powerful adversary.


Table of Contents

   1. Introduction
   2. The Mix-Net Protocol
   2.1 Message Creation
   2.2 Remailing
   2.3 Message Reassembly
   3. Pool Behavior
   3.1 Pool Mix Algorithm
   3.2 Dummy Traffic
   4. Message Format
   4.1 Payload Format
   4.2 Cryptographic Algorithms
   4.3 Packet Format
   4.3.1 Header Section Format
   4.3.2 Body Format
   4.4 Mail Transport Encoding
   5. Key Format
   5.1 Key Rotation
   6. Administrative Commands
   7. Delivery of Anonymous Messages
   8. Security Considerations
   9. Acknowledgements
   10. References
   11. Authors' Addresses


1. Introduction

This document describes a mail transfer protocol designed to protect
electronic mail against traffic analysis. Most e-mail security
protocols only protect the message body, leaving useful information
such as the the identities of the conversing parties, sizes of
messages and frequency of message exchange open to adversaries.

Message transmission can be protected against traffic analysis by the
mix-net protocol. A mix (remailer) is a service that forwards
messages, using public key cryptography to hide the correlation
between its inputs and outputs. If a message is sent through a
sequence of mixes, one trusted mix is sufficient to provide anonymity
and unobserveability of communications against a powerful
adversary. Mixmaster is a mix-net implementation for electronic mail.

This memo describes version 2 of the Mixmaster message format, as used
on the Internet since 1995. An improved protocol is described in a
separate document.


2. The Mix-Net Protocol

The mix-net protocol [Chaum 1981] allows one to send messages while hiding
the relation of sender and recipient from observers
(unobserveability). It also provides the sender of a message with the
ability to remain anonymous to the recipient (sender anonymity). If
anonymity is not desired, authenticity and unobserveability can be
achieved at the same time by transmitting digitally signed messages.

This section gives an overview over the protocol used in Mixmaster. The
mixing algorithm is specified in section 3, and the message format is
specified in section 4.


2.1 Message Creation

To send a message, the user agent splits it into parts of fixed size,
which form the bodies of Mixmaster packets. If sender anonymity is
desired, care should be taken not to include identifying information
in the message. The message may be compressed.

The sender chooses a sequence of up to 20 remailers for each
packet. The final remailer must be identical for all packets.

The packet header consists of 20 sections. For a sequence of n
remailers, header sections n+1, ... , 20 are filled with random
data. For all sections i := n down to 1, the sender generates a
symmetric encryption key, which is used to encrypt the body and
all following header sections. This key, together with other control
information for the remailer, is included in the i-th header section,
which is then encrypted with the remailer's public key. The resulting
message is sent to the first remailer in an appropriate transport
encoding.

To increase reliability, redundant copies of the message may be sent
through different paths. The final remailer must be identical for all
paths, so that duplicates can be detected and the message is delivered
only once.


2.2 Remailing

When a remailer receives a message, it decrypts the first header
section with its private key. By keeping track of a packet ID, the
remailer verifies that the packet has not been processed before. The
integrity of the message is verified by checking the packet length and
verifying message digests included in the packet. Then the first
header section is removed, the others are shifted up by one, and the
last section is filled with random padding. All header sections and
the packet body are decrypted with the symmetric key found in the
header. This reveals a public key-encrypted header section for the
next remailer at the top, and removes the old top header
section. Transport encoding is applied to the resulting message.

The remailer collects several encrypted messages before sending the
resulting messages in random order. Thus the relation between the
incoming and outgoing messages is obscured to outside adversaries even
if the adversary can observe all messages sent. The message is
effectively anonymized by sending it through a chain of independently
operated remailers.


2.3 Message Reassembly

When a packet is sent to the final remailer, it contains an indication
that the chain ends at that remailer, and whether the packet contains
a complete message or part of a multi-part message. If the packet
contains the entire message, the packet body is decrypted and after
reordering messages the plain text is delivered to the recipient. For
partial messages, a message ID is used to identify the other parts as
they arrive. When all parts have arrived, the message is reassembled,
decompressed if necessary, and delivered. If the parts do not arrive
within a time limit, the message is discarded.

Only the last remailer in the chain can determine whether packets are
part of a certain message. To all the others, they are completely
independent.


3. Pool Behavior

3.1 Cottrell Pool Mix Algorithm

In order to obfuscate the link between incoming and outgoing messages,
Mixmaster uses a pooling scheme.  Messages that are to be forwarded
anonymously are stored in a pool.  In regular intervals the remailer fires
and sends some random messages from the pool to either the next hop or
their final recipients.

The pooling scheme used in Mixmaster is the "Cottrell Pool Mix", which has
the following three parameters:
  t       mixing interval
  min     minimum number of messages in the pool
  rate    percentage of messages to be send in one round

Every t seconds:
  - Let n be the number of messages currently in the pool
  - count is the smaller of (n - min) and (n * rate), or 0 if
    (n - min) is negative.
  - Select count random messages from the pool and send them.

3.2. Dummy Traffic

"Dummy messages" are multi-hop messages of type NULL with four randomly
selected nodes as their chain.

Older versions of Mixmaster (2.0.4 - 2.9.0) allowed for the creation of
dummy message cover traffic, but provided no automated means for introducing
this dummy traffic into the system. Beginning in version 3.0, Mixmaster employs
an internal dummy policy.

Every time a message is placed in the pool, the remailer chooses a random
number from a geometric distribution and creates that many dummy messages
which are also placed in the pool.

Similarly, prior to each execution of the mixing algorithm described in
section 3.1, the remailer selects a random number from a different geometric
distribution and adds that many dummy messages to the pool as well.

The distributions' parameters are chosen so that on average the remailer
creates one dummy for every 32 messages coming in and one every 9 mixing
rounds.


4. Message Format

4.1 Payload Format

The Mixmaster message payload can be an e-mail message, a Usenet
message or a dummy message.

The messages use the formats specified in [RFC 822] and [RFC 1036]
respectively, prepended with data specifying the payload type. An
additional, more restricted method of specifying message header lines
is defined for reasons of backward compability.

The payload format is as follows:

  Number of destination fields   [        1 byte]
  Destination fields             [ 80 bytes each]
  Number of header line fields   [        1 byte]
  Header lines fields            [ 80 bytes each]
  User data section              [ up to ~2.5 MB]

Each destination field consist of a string of up to 80 ASCII characters,
padded with null-bytes to a total size of 80 bytes. The following
strings are defined:

  null:                 dummy message
  post:                Usenet message
  post: [newsgroup]    Usenet message
  [address]            e-mail message

If no destination field is given, the payload is an e-mail message.

If the destination field is "post: [newsgroup]", a "Newsgroups:
[newsgroup]" field is added to the header of the resulting message. If
the destination field is of the fourth type, a "To: [address]" field
is added to the header of the resulting message. [address] and
[newsgroup] are strings of ASCII characters.  If the message
is a dummy message the node should discard it.

Message headers can be specified in header line fields. Each header
line field consists of a string of up to 80 ASCII characters, padded
with null-bytes to a total size of 80 bytes.

There are three types of user data sections:

A compressed user data section begins with the GZIP identification
header (31, 139). It contains another user data section. The data are
compressed using GZIP [RFC 1952]. The GZIP operating system field must
be set to Unix, and file names must not be given. Compression may be
used if the capabilities attribute of the final remailer contains the
flag "C".

An RFC 822 user data section begins with the identification "##<CR>"
(35, 35, 13). It contains an e-mail message or a Usenet message as
specified in [RFC 822] and [RFC 1036]. This type cannot be used if
the final remailer uses a Mixmaster software version prior to 2.0.4.

A user data section not beginning with one of the above identification
strings contains only the body of the message. When this type of user
data section is used, the message header fields must be included in
destination and header line fields.

The payload is limited to a maximal size of 2610180 bytes. Individual
remailers may use a smaller limit.

Remailer operators can choose to remove header fields supplied by the
sender and insert additional header fields, according to local policy
(see section 5).


4.2 Cryptographic Algorithms

The asymmetric encryption operation in Mixmaster version 2 uses RSA
with 1024 bit RSA keys and the PKCS #1 v1.5 (RSAES-PKCS1-v1_5) padding
format [RFC 2437]. The symmetric encryption uses EDE 3DES with cipher
block chaining (24 byte key, 8 byte initialization vector) [Schneier
1996]. MD5 [RFC 1321] is used as the message digest algorithm.


4.3 Packet Format

A Mixmaster packet consists of a header containing information for the
remailers, and a body containing payload data. To ensure that
packets are indistinguishable, the size of these encrypted data fields
is fixed.

The packet header consists of 20 header sections (specified in
section 3.3.1) of 512 bytes each, resulting in a total header size
of 10240 bytes. The header sections -- except for the first one -- and
the packet body are encrypted with symmetric session keys specified in
the first header section.


4.3.1 Header Section Format

     Public key ID                [  16 bytes]
     Length of RSA-encrypted data [   1 byte ]
     RSA-encrypted session key    [ 128 bytes]
     Initialization vector        [   8 bytes]
     Encrypted header part        [ 328 bytes]
     Padding                      [  31 bytes]

Total size: 512 bytes

To generate the RSA-encrypted session key, a random 24 byte Triple-DES
key is encrypted with RSAES-PKCS1-v1_5, resulting in 128 bytes (1024
bits) of encrypted data. This Triple-DES key and the initialization
vector provided in clear are used to decrypt the encrypted header part.
They are not used at other stages of message processing.

The 328 bytes of data encrypted to form the encrypted header part are
as follows:

     Packet ID                            [ 16 bytes]
     Triple-DES key                       [ 24 bytes]
     Packet type identifier               [  1 byte ]
     Packet information      [depends on packet type]
     Timestamp                            [  7 bytes]
     Message digest                       [ 16 bytes]
     Random padding               [fill to 328 bytes]

The possible packet type identifiers are:

 Intermediate hop            0
 Final hop                   1
 Final hop, partial message  2

The packet information depends on the packet type identifier, as follows:

     Packet type 0 (intermediate hop):
        19 Initialization vectors      [152 bytes]
        Remailer address               [ 80 bytes]

     Packet type 1 (final hop):
        Message ID                     [ 16 bytes]
        Initialization vector          [  8 bytes]

     Packet type 2 (final hop, partial message):
        Chunk number                   [  1 byte ]
        Number of chunks               [  1 byte ]
        Message ID                     [ 16 bytes]
        Initialization vector          [  8 bytes]

Packet ID: randomly generated packet identifier.

Triple-DES key: used to encrypt the following header sections and the
packet body.

Initialization vectors: For packet type 1 and 2, the IV is used to
symmetrically encrypt the packet body. For packet type 0, there is one IV
for each of the 19 following header sections. The IV for the last header
section is also used for the packet body.

Remailer address: e-mail address of next hop.

Message ID: randomly generated identifier unique to (all chunks of)
this message.

Chunk number: Sequence number used in multi-part messages, starting
with 1.

Number of chunks: Total number of chunks.

Timestamp: A timestamp is introduced with the byte sequence (48, 48, 48,
48, 0). The following two bytes specify the number of days since Jan 1,
1970, given in little-endian byte order. A random number of up to 3 may be
subtracted from the number of days in order to obscure the origin of the
message. 

Early implementations of Mixmaster did not generate a timestamp packet.
Implementors should be aware of the partitioning attack implications if
they chose to permit processing of packets without timestamps.

Message digest: MD5 digest computed over the preceding elements of the
encrypted header part.

In the case of packet type 0, header sections 2 .. 20 and the packet body
each are decrypted separately using the respective initialization vectors.
In the case of packet types 1 and 2, header sections 2 .. 20 are ignored,
and the packet body is decrypted using the given initialization vector.


4.3.2 Body Format

The message payload (section 3.1) is split into chunks of 10236
bytes. To each chunk, its length is prepended as a 4 byte
little-endian number to form the body of a Mixmaster packet.

A message may consist of up to 255 packets.


4.4 Mail Transport Encoding

Mixmaster packets are sent as text messages [RFC 822]. The RFC 822
message body has the following format:

  ::
  Remailer-Type: Mixmaster [version number]

  -----BEGIN REMAILER MESSAGE-----
  [packet length ]
  [message digest]
  [encoded packet]
  -----END REMAILER MESSAGE-----

The length field always contains the decimal number "20480", since the
size of Mixmaster packets is constant. An MD5 message digest [RFC
1321] of the (un-encoded) packet is encoded in base64.

The packet itself is encoded in base 64 encoding [RFC 1421], broken
into lines of 40 characters (except that the last line is shorter).


5. Key Format

Remailer public key files consist of a list of attributes and a
public RSA key:

  [attributes list]

  -----Begin Mix Key-----
  [key ID]
  [length]
  [encoded key]
  -----End Mix Key-----

The attributes are listed in one line separated by spaces. Individual
attributes must not contain whitespace:

 identifier:       a human readable string identifying the remailer
 address:          the remailer's Internet mail address
 key ID:           public key ID
 version:          the Mixmaster software version number
 capabilities:     flags indicating additional remailer capabilities
 validity date:    date from which the key is valid
 expiration date:  date of the key's expiration

The identifier consists of lowercase alphanumeric characters, beginning
with an alphabetic character. The identifier should consist of no more
than eight characters in length.

The encoded key packet consists of two bytes specifying the key length
(1024 bits) in little-endian byte order, and of the RSA modulus and the
public exponent in big-endian form using 128 bytes each, with preceding
null bytes for the exponent if necessary. The packet is encoded in base 64
[RFC 1421], and broken into lines of 40 characters each (except that the
last line is shorter). Its length (258 bytes) is given as a decimal
number.

The key ID is the MD5 message digest of the representation of the RSA
public key (not including the length bytes). It is encoded as a
hexadecimal string.

The version field consists of the protocol version number followed by a
colon and the software version information, limited to the ASCII
alphanumeric characters, plus dot (.) and dash (-). All implimentations of
this protocol should prepend the software version with "2:" to indicate
Mixmaster protocol version 2. Existing implimentations lacking a protocol
version number imply protocol version 2.

The capabilities field is optional. It is a list of flags represented
by a string of ASCII characters. Clients should ignore unknown
flags. The following flags are used in version 2.0.4 through 3.0:

 C     accepts compressed messages.
 M     will forward messages to another mix when used as final hop.
 Nm    supports posting to Usenet through a mail-to-news gateway.
 Np    supports direct posting to Usenet.

The date fields introduced in version 3.0 are optional. They are ASCII
date stamps in the format YYYY-MM-DD. The first date indicates the date
from which the key is first valid; the second date indicates its
expiration. If only one date is present, it is treated as the key creation
date. (The date stamp implies 00:00 UTC).

Digital signatures [RFC 2440] should be used to ensure the
authenticity of the key files.


5.1 Key Rotation

Beginning with version 3.0, Mixmaster offers automatic key rotation. Care
must be taken to minimize the possibility for partitioning attacks during
the key rotation window.

Keys are generated with a validity date and an expiration date. Mixmaster
clients only display keys which are currently valid and not expired.

Keys are valid for a 13 month period. Mixmaster remailers generate new
keys when the existing key's expiration date is one month or less in the
future. Remailers report the most recently generated key as the remailer
key when queried, effectively giving each key a 12 month service period.

Remailers continue to decrypt and process mail encrypted to expired keys
for one week past the expiration date on the key. One week after
expiration, an expired remailer key should be securely destroyed.


6. Administrative Commands

The remailer software understands a number of specific administrative
commands. These commands are sent via the Subject: line of an email to the
email address of the remailer:

 remailer-help:     returns information on using the remailer
 remailer-key:      returns the remailer's public key
 remailer-stats:    returns information on the number of messages processed
 remailer-conf:     returns local configuration information
 remailer-adminkey: returns the public key of the remailer operator

Upon receiving an administrative request, the remailer returns its reply
to the email address specified in the Reply-To: or From: header of the
request.

The remailer-help request should return basic information on using the
remailer. Remailers may also accept remailer-help requests with an ISO 639
two-letter language code appended following a hyphen. For example,
remailer-help-ar will return a help file in Arabic, if available.
Supported languages should be listed at the beginning of the remailer-help
response.

The remailer-key request returns the remailer key as described above. It
may also return keys and capability information for other remailer
protocols supported by the remailer.

The remailer-stats request returns statistics on the number of messages
processed per day by the remailer.

The remailer-conf request returns local configuration information, such as
the version of the remailer software, the supported remailer protocols,
filtered headers, blocked newsgroups and domains, and the attribute
strings for other remailers the remailer knows about.

The remailer-adminkey request returns a public OpenPGP key for use in
communication with the remailer operator.


7. Delivery of Anonymous Messages

When anonymous messages are forwarded to third parties, remailer
operators should be aware that senders might try to supply header
fields that indicate a false identity or to send Usenet control
messages [RFC 1036] unauthorized, which is a problem because many news
servers accept control messages automatically without any
authentication.

For these reasons, remailer software should allow the operator
to disable certain types of message headers, and to insert headers
automatically.

Remailers usually add a "From:" field containing an address controlled
by the remailer operator to anonymous messages. Using the word
"Anonymous" in the name field allows recipients to apply scoring
mechanisms and filters to anonymous messages. Appropriate additional
information about the origin of the message can be inserted in the
"Comments:" header field of the anonymous messages.

If the recipient does not wish to receive anonymous messages,
unobserveability of communications and authenticity can be achieved at
the same time by the remailer verifying that the message is
cryptographically signed [RFC 2440] by a known sender.

Anonymous remailers are sometimes used to send harassing e-mail. To
prevent this abuse, remailer software should allow operators to block
destination addresses on request. Real-life abuse and attacks on
anonymous remailers are discussed in [Mazieres 1998].


8. Security Considerations

The security of the mix-net relies on the assumption that the
underlying cryptographic primitives are secure. In addition, specific
attacks on the mix-net need to be considered ([Möller 1998] contains a
more detailed analysis of these attacks).

Passive adversaries can observe some or all of the messages sent to
mixes. The users' anonymity comes from the fact that a large number
of messages are collected and sent in random order. For that reason
remailers should collect as many messages as possible while keeping
the delay acceptable.

Statistical traffic analysis is possible even if single messages are
anonymized in a perfectly secure way: An eavesdropper may correlate
the times of Mixmaster packets being sent and anonymized messages
being received. This is a powerful attack if several anonymous
messages can be linked together (by their contents or because they are
sent under a pseudonym). To protect themselves, senders must mail
Mixmaster packets stochastically independent of the actual messages
they want to send. This can be done by sending packets in regular
intervals, using a dummy message whenever appropriate. To avoid
leaking information, the intervals should not be smaller than the
randomness in the delay caused by trusted remailers.

There is no anonymity if all remailers in a given chain collude with
the adversary, or if they are compromised during the lifetime of their
keys. Using a longer chain increases the assurance that the user's
privacy will be preserved, but in the same time causes lower
reliability and higher latency. Sending redundant copies of a message
increases reliability but may also facilitate attacks. An optimum must
be found according to the individual security needs and trust in the
remailers.

Active adversaries can also create, suppress or modify
messages. Remailers must check the packet IDs to prevent replay
attacks. Message integrity must be verified to prevent the adversary
from performing chosen ciphertext attacks or replay attacks with
modified packet IDs, and from encoding information in an intercepted
message in a way not affected by decryption (e.g. by modifying the
message length or inducing errors). This version of the protocol does
not provide integrity for the packet body. Because the padding for
header section is random, in this version of the protocol it is
impossible for a remailer to check the integrity of the encrypted
header sections that will be decrypted by the following remailers.
Chosen ciphertext attacks and replay attacks are detected by verifying
the message digest included in the header section.

The adversary can trace a message if he knows the decryption of all
other messages that pass through the remailer at the same time. To
make it less practical for an attacker to flood a mix with known
messages, remailers can store received messages in a reordering pool
that grows in size while more than average messages are received, and
periodically choose at random a fixed fraction of the messages in the
pool for processing. There is no complete protection against flooding
attacks in an open system, but if the number of messages required is
high, an attack is less likely to go unnoticed.

If the adversary suppresses all Mixmaster messages from one particular
sender and observes that anonymous messages of a certain kind are
discontinued at the same time, that sender's anonymity is compromised
with high probability. There is no practical cryptographic protection
against this attack in large-scale networks. The effect of a more
powerful attack that combines suppressing messages and re-injecting
them at a later time is reduced by using timestamps.

The lack of accountability that comes with anonymity may have
implications for the security of a network. For example, many news
servers accept control messages automatically without any
cryptographic authentication. Possible countermeasures are discussed
in section 7.


9. Acknowledgements

Several people contributed ideas and source code to the Mixmaster v2
software. "Antonomasia" <ant@notatla.demon.co.uk>, Adam Back
<adam@cypherspace.org> and Bodo Möller <bmoeller@acm.org> suggested
improvements to this document.


10. References

[Chaum 1981] Chaum, D., "Untraceable Electronic Mail, Return Addresses, and
Digital Pseudonyms", Communications of the ACM 24 (1981) 2.

[Mazieres 1998] Mazières, D., and Kaashoek, F., "The Design, Implementation and
Operation of an Email Pseudonym Server",
5th ACM Conference on Computer and Communications Security, 1998.
<URL: ftp://cag.lcs.mit.edu/pub/dm/papers/mazieres:pnym.ps.gz>.

[Möller 1998] Möller, U., "Anonymisierung von Internet-Diensten",
Studienarbeit, University of Hamburg, January 1998.
<URL: http://agn-www.informatik.uni-hamburg.de/people/3umoelle/st.ps>.

[RFC 822] Crocker, D., "Standard for the Format of ARPA Internet Text
Messages", STD 11, RFC 822, August 1982.

[RFC 1036] Horton, M., and Adams, R., "Standard for Interchange of
USENET Messages", RFC 1036, December 1987.

[RFC 1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992.

[RFC 1421] Linn, J., "Privacy Enhancement for Internet Electronic
Mail: Part I -- Message Encryption and Authentication Procedures", RFC
1421, February 1993.

[RFC 1952] Deutsch, P., "GZIP file format specification version 4.3",
RFC 1952, May 1996.

[RFC 2311] Dusse, S., Hoffman, P, Ramsdell, B, Lundblade, L., and
Repka, L., "S/MIME Version 2 Message Specification", RFC 2311, March
1998.

[RFC 2437] Kaliski, B., and Staddon, J., "PKCS #1: RSA Cryptography
Specifications, Version 2.0", RFC 2437, October 1998.

[RFC 2440] Callas, J., Donnerhacke, L., Finney, H., and Thayer, R.:
"OpenPGP Message Format", RFC 2440, November 1998.

[Schneier 1996] Schneier, B., "Applied Cryptography", 2nd Edition, Wiley,
1996.


11. Authors' Addresses


 Ulf Moeller

 EMail: ulf@fitug.de


 Lance Cottrell
 Anonymizer, Inc.
 5694 Mission Center Road #426
 San Diego, CA 92108-4380

 EMail: loki@infonex.com


 Peter Palfrader

 EMail: peter@palfrader.org


 Len Sassaman
 Nomen Abditum Services
 P.O. Box 99282
 Emeryville, CA 94662-9282

 EMail: rabbi@abditum.com