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Network Working Group                                         U. Moeller
Internet-Draft                                             Secardeo GmbH
Expires: June 29, 2005                                       L. Cottrell
                                                        Anonymizer, Inc.
                                                            P. Palfrader
                                                   The Mixmaster Project
                                                             L. Sassaman
                                                  Nomen Abditum Services
                                                       December 29, 2004


                      Mixmaster Protocol Version 2
                    draft-sassaman-mixmaster-03.txt

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of section 3 of RFC 3667.  By submitting this Internet-Draft, each
   author represents that any applicable patent or other IPR claims of
   which he or she is aware have been or will be disclosed, and any of
   which he or she become aware will be disclosed, in accordance with
   RFC 3668.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as
   Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on June 29, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2004).

Abstract

   Most e-mail security protocols only protect the message body, leaving
   useful information such as the identities of the conversing parties,



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   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 David Chaum's mix-net concept.  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 unobservability of communications against a powerful adversary.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  The Mix-Net Protocol . . . . . . . . . . . . . . . . . . . . .  3
     2.1   Message Creation . . . . . . . . . . . . . . . . . . . . .  3
     2.2   Remailing  . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.3   Message Reassembly . . . . . . . . . . . . . . . . . . . .  5
   3.  Pool Behavior  . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1   Timed Dynamic Pool Mix Algorithm . . . . . . . . . . . . .  5
     3.2   Dummy Traffic  . . . . . . . . . . . . . . . . . . . . . .  6
   4.  Message Format . . . . . . . . . . . . . . . . . . . . . . . .  6
     4.1   Payload Format . . . . . . . . . . . . . . . . . . . . . .  6
     4.2   Cryptographic Algorithms . . . . . . . . . . . . . . . . .  7
     4.3   Packet Format  . . . . . . . . . . . . . . . . . . . . . .  7
       4.3.1   Header Section Format  . . . . . . . . . . . . . . . .  8
       4.3.2   Body Format  . . . . . . . . . . . . . . . . . . . . . 10
     4.4   Mail Transport Encoding  . . . . . . . . . . . . . . . . . 10
   5.  Key Format . . . . . . . . . . . . . . . . . . . . . . . . . . 11
   6.  Implementation Notes . . . . . . . . . . . . . . . . . . . . . 12
     6.1   Remixing . . . . . . . . . . . . . . . . . . . . . . . . . 12
     6.2   Administrative Commands  . . . . . . . . . . . . . . . . . 13
     6.3   Dummy Traffic  . . . . . . . . . . . . . . . . . . . . . . 13
     6.4   Key Rotation . . . . . . . . . . . . . . . . . . . . . . . 13
     6.5   Delivery of Anonymous Messages . . . . . . . . . . . . . . 14
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 16
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 18
       Intellectual Property and Copyright Statements . . . . . . . . 20











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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 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 unobservability 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.

2.  The Mix-Net Protocol

   The mix-net protocol [1] allows one to send messages while hiding the
   relation of sender and recipient from observers (unobservability).
   It also allows the sender of a message to remain anonymous to the
   recipient (sender anonymity).  If anonymity is not desired,
   authenticity and unobservability can be achieved at the same time by
   transmitting digitally signed messages.

   This section gives an overview of the protocol and messaging pattern.
   The mixing algorithm is specified in Section 3, and the message
   format is specified in Section 4.

   Viewed from a high level, Mixmaster is like a packet network, where
   each node in the network is known as a "remailer." The original
   content is split into pieces, and an independent path is determined
   for each piece, with the only requirement that all paths must end at
   the same remailer.  Each piece is multiply encrypted so that any
   intermediate remailer can only decrypt enough information to
   determine the next hop in the path.  When all pieces have arrived at
   the final remailer, the original content is re-created and sent to
   its final destination.

2.1  Message Creation

   In this section the terms "sender" and "user agent" are used
   informally.

   The user agent splits the original content into chunks of 10236



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   bytes; if the last chunk is shorter, random padding is added.  Each
   chunk has a four-byte length prepended, and the result is called the
   packet body.  If sender anonymity is desired, care should be taken to
   not include any identifying information (such as headers or unique
   content from the original plaintext message) in the packets.  The
   content may be compressed before splitting.

   The sender next chooses a chain of up to 20 remailers for each
   packet.  Each path is independent, and can be of a different length,
   but all paths must end at the same remailer.  This final remailer is
   responsible for detecting and discarding duplicate packets,
   reconstructing the message, and doing the final delivery.

   Each packet is next prepared as follows (the full details are in
   Section 4.3.1).  For a chain of "n" remailers, headers "n + 1"
   through 20 are filled with random data.  For headers "n" down to one,
   the sender generates a symmetric encryption key.  This key is used to
   encrypt the packet body and all the following headers.  The key, and
   other control information, is then encrypted with the public key of
   the "n"'th remailer in the chain.

   The process is repeated, working backward through the chain until the
   first packet has header information encrypted for the first remailer,
   and the packet body has been encrypted "n" times.  The packet is then
   sent to the first remailer on its chain.

2.2  Remailing

   When a remailer receives a message, it uses its private key to
   decrypt the first header section.  The Packet ID (see Section 4.3.1)
   can be used to detect duplicates.  The integrity of the message is
   verified by checking the packet length and verifying the message
   digest in the packet header.

   All header sections, as well as 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.

   The first header section is now removed, the others are shifted up,
   and the last section is replaced with random bytes.  Transport
   encoding is applied to the new message as described in Section 4.4.

   In order to prevent an adversary from determining the relationship
   between incoming and outgoing messages (i.e., traffic analysis), the
   remailer must collect several encrypted messages before sending the
   message it has just created; see Section 3.1.





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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 the complete message or if it is 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.  A final remailer may discard partial messages if all
   packets have not been received within a local time limit.

   Note that only the final remailer can determine whether packets are
   part of a specific message.  To all of other remailers, the packets
   appear to be completely independent.

3.  Pool Behavior

3.1  Timed Dynamic Pool Mix Algorithm

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

   The pooling scheme is a "Timed Dynamic Pool Mix" [6], which has the
   following three parameters:

   +--------------+----------------------------------------------------+
   | Name         | Description                                        |
   +--------------+----------------------------------------------------+
   | t            | Mixing interval                                    |
   | min          | Minimum number of messages in the pool             |
   | rate         | Percentage of messages to be send in one round     |
   +--------------+----------------------------------------------------+

    The following steps are implemented every "t" seconds:
   1.  Let "n" be the number of messages currently in the pool.
   2.  Let "count" be the smaller of "n - min" and "n * rate", or zero
       if "n - min" is negative.
   3.  Select "count" messages from the pool at random and send them.

   In its default configuration, Mixmaster has a mixing interval of 15
   minutes, a minimum pool size of 45 messages, and permits a maximum of
   65% of the pool to be sent in one round.




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3.2  Dummy Traffic

   Dummy messages (see Section 4.1) are multi-hop messages with four
   randomly selected remailers as the chain.  The chain must be selected
   such that no remailer will appear twice unless two other remailers
   separate them.

   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 parameters should be chosen so that on average the remailer
   creates one dummy for every 32 inbound messages and one every nine
   mixing rounds.

4.  Message Format

4.1  Payload Format

   The message payload can be an e-mail message [16], a Usenet message
   [8], or a dummy message.

   Mail and Usenet messages are prefixed with data specifying the
   payload type.  An additional, more restricted method of specifying
   message header lines is defined for reasons of backward
   compatibility.

   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              [ 2549K -  previous fields ]

   Each destination field consists 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.  The remailer will discard the message.
   post: Usenet message.  The remailer will post the message to Usenet.






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   post: [newsgroup] Usenet message.  The remailer will add a
      "Newsgroups" header with the specified content, and post the
      message to Usenet.
   [address] E-mail message.  The remailer will add a "To" header with
      the specified content, and send the message as e-mail.

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

   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:
   o  A compressed user data section begins with the GZIP identification
      header (31, 139).  This header contains an additional 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".
   o  An RFC 2822 user data section begins with the three bytes "##[CR]"
      (35, 35, 13).  It contains an e-mail message or a Usenet message.
   o  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 maximum 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 mechanism is RSA with 1024 bit RSA keys and
   the PKCS #1 v1.5 (RSAES-PKCS1-v1_5) padding format [13].  The
   symmetric encryption mechanism is EDE 3DES with cipher block chaining
   (24-byte key, 8-byte initialization vector) [7].  MD5 [9] 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 fields are all of fixed size.

   The packet header consists of 20 header sections (specified in



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   Section 4.3.1) 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.

   +-------------------+
   | Header section 1  |
   +- - - - - - - - - -+
   | Header section 2  |
   +- - - - - - - - - -+
   + ...               +
   +- - - - - - - - - -+
   | Header section 20 |
   +-------------------+
   |                   |
   | Payload           |
   |                   |
   |                   |
   |                   |
   |                   |
   |                   |
   +-------------------+

4.3.1  Header Section Format

   Packet layout

   [ 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 ]
   [ Random padding                31 bytes ]
    Total size: 512 bytes

   To generate the RSA-encrypted session key, a 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:








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   [ 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                      as needed ]
    Total size: 328 bytes

   The fields are defined as follows:
   Packet ID: randomly generated packet identifier.
   Triple-DES key: used to encrypt the following header sections and the
      packet body.
   Packet type identifier: The type identifiers are:

   +--------------+----------------------------------------------------+
   | Value        | Type                                               |
   +--------------+----------------------------------------------------+
   | 0            | Intermediate hop                                   |
   | 1            | Final hop, complete message                        |
   | 2            | Final hop, partial message                         |
   +--------------+----------------------------------------------------+

   Timestamp: A timestamp is introduced with the byte sequence (48, 48,
      48, 48, 0).  The following two bytes specify the number of days
      since January 1, 1970 (00:00 UTC), in little-endian byte order.  A
      random number between one and three, inclusive, may be subtracted
      from the number of days in order to obscure the origin of the
      message.
   Message digest: MD5 digest computed over the preceding elements of
      the encrypted header part.

   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 ]



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   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: Identifier unique to (all chunks of) this message.
   Chunk number: Sequence number used in multi-part messages, starting
      with one.
   Number of chunks: Total number of chunks.

   In the case of packet type zero, header sections two through twenty,
   and the packet body, each are decrypted separately using the
   respective initialization vectors.  In the case of packet types one
   and two, header sections two through twenty are ignored, and the
   packet body is decrypted using the given initialization vector.

4.3.2  Body Format

   The message payload Section 4.1 is split into chunks of 10236 bytes.
   Random padding is added to the last chunk if necessary.  The length
   of each chunk (not counting the padding), is prepended to the chunk
   as a four-byte little-endian number.  This forms 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 standard email messages [16].  The
   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 [9]
   of the packet prior to Base-64 encoding is encoded in Base-64.

   The packet itself is encoded in Base-64 encoding [10], with
   line-breaks every 40 characters.





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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, and are defined as
   follows:

   identifier: A human readable string identifying the remailer
   address: The remailer's Internet mail address
   key ID: Public key ID
   version: 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 be no
   more than eight characters in length.

   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
   implementations of the protocol specified here should prepend the
   software version with "2:".  Existing implementations 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 defined:








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   +--------------+----------------------------------------------------+
   | Flag         | Meaning                                            |
   +--------------+----------------------------------------------------+
   | 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 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).

   The version, capabilities, and date fields must each be no longer
   than 125 characters.

   The encoded key part 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 [10], with line-breaks every 40 characters.  Its
   length (258 bytes) is given as a decimal number.

   Digital signatures [14] should be used to ensure the authenticity of
   the key files.

6.  Implementation Notes

   This section discusses various implementation issues.

6.1  Remixing

   Some remailers may understand multiple remailer protocols.  In the
   interest of creating a unified anonymity set, remailers which speak
   multiple remailer protocols should attempt to remix messages that use
   the older protocols whenever possible.

   When a remailer receives a message in the older protocol format, it
   should determine if the message destination is another remailer which
   also speaks the Mixmaster protocol.  If the remailer knows the
   Mixmaster public key for the next hop, it should process the message
   normally, but instead of sending the message to its next hop, treat
   the processed message as opaque data which will comprise the body of
   a Mixmaster message.  The remailer should then create a Mixmaster



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   message with this body to be delivered to the next hop remailer.

   Ensuring that a remailer's keyring contains up to date copies of the
   public keys for other remailers is the responsibility of the given
   remailer's operator.  Utilities such as Echolot [2] can be used to
   assist in automating this task.

   If the remailer receives a Mixmaster message that, when decrypted,
   contains a message in an alternate protocol supported by the
   remailer, it should process the message as though it had initially
   been delivered in the alternate protocol format.

6.2  Administrative Commands

   The existing remailer software understands a number of specific
   administrative commands.  These commands are sent via the Subject:
   line of an e-mail to the email address of the remailer:
   remailer-help: Returns information about using the remailer.  The
      remailer may support a suffix consisting of a dash and a
      two-letter ISO 639 country code.  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.
   remailer-key: Returns the remailer's public key as described in
      Section 5.  It may also return the keys and attributes of other
      remailers it knows about.
   remailer-stats: Returns information about the number of messages the
      remailer has processed per day (again, a day starts at 00:00 UTC).
   remailer-conf: Returns local configuration information such as
      software version, supported protocols, filtered headers, blocked
      newsgroups and domains, and the attribute strings for other
      remailers the remailer knows about.
   remailer-adminkey: Returns the OpenPGP [14] key of the remailer's
      operator.

6.3  Dummy Traffic

   Older versions of Mixmaster (2.0.4 through 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.

6.4  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.




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   Keys are generated with a validity date and an expiration date.  User
   agents should only display valid keys which have not expired.

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

   Remailers must 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.

6.5  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 unauthorized Usenet
   control messages.  This 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.

   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 [3].

7.  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; [5] 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



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   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 at
   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 at 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.  To
   minimize the number of packet IDs that the remailer must retain,
   packets which bear a timestamp more than a reasonable number of days
   in the past may be discarded.  Implementors should consider that
   packets maybe up to three days younger than indicated by the
   timestamp, and select an expiration value which allows sufficient
   time for legitimate messages to pass through the network.  The number
   of packet IDs that the remailer must retain can be further minimized
   by discarding packet IDs for packets encrypted to a key which has
   expired more than a week in the past.

   The use of a link-level encryption protocol with an ephemeral key,
   such as STARTTLS with SMTP [15], provides for forward secrecy and
   further aids against replay attacks.  Remailer operators should be
   encouraged to deploy such solutions at the MTA level whenever
   possible.

   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.  Mixmaster versions 2.0.5 and greater in the 2.0.x tree
   as well as Mixmaster 3.0 in the 3.x tree do not permit processing of
   such packets.



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   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.
   Additional work has been done in the field of active flooding attack
   protection; future mix-net protocols may wish to take advantage of
   this work [4].

   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.

   Manipulation of the distribution of remailer keys, capabilities, and
   statistics can lead to powerful attacks against a remailer network.
   Sensitive information such as this should be distributed in a secure
   manner.

   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 6.5.

8.  Acknowledgments

   Several people contributed ideas and source code to the Mixmaster v2



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   software.

   "Antonomasia" <ant@notatla.org.uk>, Adam Back <adam@cypherspace.org>,
   Marco A.  Calamari <marcoc@dada.it>, Colin Tuckley
   <colin@tuckley.org>, Bodo Moeller <bmoeller@acm.org>, and Jon Orbeton
   <jono@networkcommand.com> suggested improvements to this document.
   Rich Salz <rsalz@datapower.com> contributed significantly to this
   document by XMLifying it and rewording many ambiguous sections.
   Myles Conley <miles@tenhand.com> also reworded many ambiguous
   sections and contributed important suggestions for improving clarity.

9  References

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

   [2]   Palfrader, P., "Echolot: A Pinger for Anonymous Remailers",
         2003, <http://http://www.palfrader.org/echolot/>.

   [3]   Mazieres, D. and F. Kaashoek, "The Design, Implementation and
         Operation of an Email Pseudonym Server", 1998,
         <ftp://cag.lcs.mit.edu/pub/dm/papers/mazieres:pnym.ps.gz>.

   [4]   Danezis, G. and L. Sassaman, "Heartbeat Traffic to Counter
         (n-1) Attacks", October 2003,
         <http://www.cl.cam.ac.uk/users/gd216/p125_danezis.pdf>.

   [5]   Moeller, U., "Anonymisierung von Internet-Diensten", January
         1998,
         <http://agn-www.informatik.uni-hamburg.de/people/3umoelle/st.ps
         >.

   [6]   Serjantov, A., Dingledine, R. and P. Syverson, "From a Trickle
         to a Flood: Active Attacks on Several Mix Types", October 2002,
         <http://freehaven.net/doc/batching-taxonomy/taxonomy.pdf>.

   [7]   Menezes, A., van Oorschot, P. and S. Vanstone, "Handbook of
         Applied Cryptography", CRC Press, 1996.

   [8]   Horton, M. and R. Adams, "Standard for interchange of USENET
         messages", RFC 1036, December 1987.

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

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



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   [11]  Deutsch, P., Gailly, J-L., Adler, M., Deutsch, L. and G.
         Randers-Pehrson, "GZIP file format specification version 4.3",
         RFC 1952, May 1996.

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

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

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

   [15]  Hoffman, P., "SMTP Service Extension for Secure SMTP over TLS",
         RFC 2487, January 1999.

   [16]  Resnick, P., "Internet Message Format", RFC 2822, April 2001.


Authors' Addresses

   Ulf Moeller
   Secardeo GmbH
   Betastr. 9a
   85774 Unterfoehring
   Germany

   EMail: ulf.moeller@secardeo.com


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

   EMail: loki@infonex.com


   Peter Palfrader
   The Mixmaster Project
   Hoettinger Auffahrt 1
   6020 Innsbruck
   Austria

   EMail: peter@palfrader.org
   URI:   http://www.palfrader.org/



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   Len Sassaman
   Nomen Abditum Services
   P.O. Box 900481
   San Diego, CA  92190-0481
   USA

   EMail: rabbi@abditum.com












































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