trunk/Docs/draft-sassaman-mixmaster-XX.xml

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<rfc ipr="full2026" docName="draft-sassaman-mixmaster-03.txt">

<front>

  <title abbrev="Mixmaster">Mixmaster Protocol Version 2</title>

  <author initials="U." surname="Moeller" fullname="Ulf Moeller">
    <organization>Secardeo GmbH</organization>
    <address>
      <postal>
        <street>Betastr. 9a</street>
        <city>85774 Unterfoehring</city>
        <country>Germany</country>
      </postal>
      <email>ulf.moeller@secardeo.com</email>
    </address>
  </author>

  <author initials="L." surname="Cottrell" fullname="Lance Cottrell">
    <organization>Anonymizer, Inc.</organization>
    <address>
      <postal>
        <street>5694 Mission Center Road #426</street>
        <city>San Diego</city> <region>CA</region> <code>92108-4380</code>
        <country>USA</country>
      </postal>
      <email>loki@infonex.com</email>
    </address>
  </author>

  <author initials="P." surname="Palfrader" fullname="Peter Palfrader">
    <organization/>
    <address>
      <postal>
        <street>Hoettinger Auffahrt 1</street>
        <city>6020 Innsbruck</city>
        <country>Austria</country>
      </postal>
      <email>peter@palfrader.org</email>
      <uri>http://www.palfrader.org/</uri>
    </address>
  </author>

  <author initials="L." surname="Sassaman" fullname="Len Sassaman">
    <organization>Nomen Abditum Services</organization>
    <address>
      <postal>
        <street>P.O. Box 900481</street>
        <city>San Diego</city> <region>CA</region> <code>92190-0481</code>
        <country>USA</country>
      </postal>
      <email>rabbi@abditum.com</email>
    </address>
  </author>

  <date month="May" year="2004"/>

  <abstract>
    <t>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. This document describes Mixmaster version 2, a mail
      transfer protocol designed to protect electronic mail against traffic
      analysis.</t>

    <t>Mixmaster is based on Dr. David 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 unobservability
      of communications against a powerful adversary.</t>
  </abstract>

</front>

<middle>

  <section anchor="introduction" title="Introduction">

    <t>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.</t>

    <t>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.</t>

    <t>This memo describes version 2 of the Mixmaster message format, as used
      on the Internet since 1995.</t>

  </section>

  <section anchor="protocol" title="The Mix-Net Protocol">

    <t>The mix-net protocol <xref target="Chaum81"/> 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.</t>

    <t>This section gives an overview of the protocol and messaging
      pattern.
      The mixing algorithm is specified in <xref target="pool-behavior"/>,
      and the message format is specified in
      <xref target="message-format"/>.</t>

    <t>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 independant
      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.</t>

    <section anchor="message-creation" title="Message Creation">

      <t>In this section the terms "sender" and "user agent" are
        used informally.</t>

      <t>The user agent splits the original content into chunks
        of 10236 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 the sender
        desires anonymity, care should be taken to not include
        any identifying information (such as from the original
        content) in the packets. The content may be compressed
        before splitting.</t>

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

      <t>Each packet is next prepared as follows (the full details
        are in <xref target="header-section"/>).  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.</t>

      <t>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.</t>

    </section>

    <section anchor="remailing" title="Remailing">

      <t>When a remailer receives a message, it uses its private key to
        decrypt the first header section. The Packet ID (see
        <xref target="header-section"/> 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.</t>

      <t>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.</t>

      <t>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 <xref
          target="transport-encoding"/>.</t>

      <t>In order to prevent an adversary from determining the relationship
        between incoming and outoing messages (i.e., traffic analysis), the
        remailer must collect several encrypted messages before sending the
        message it has just created; see <xref target="timed-pool"/>.</t>

    </section>

    <section anchor="message-reassembly" title="Message Reassembly">

      <t>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.</t>

      <t>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.</t>

    </section>

  </section>

  <section anchor="pool-behavior" title="Pool Behavior">

    <section anchor="timed-pool" title="Timed Dynamic Pool Mix Algorithm">

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

      <t>The pooling scheme is a "Timed Dynamic Pool
        Mix," which has the following three parameters:
        <texttable>
          <ttcol align='left' width='20%'>Name</ttcol>
          <ttcol align='left'>Description</ttcol>
          <c>t</c><c>Mixing interval</c>
          <c>min</c><c>Minimum number of messages in the pool</c>
          <c>rate</c><c>Percentage of messages to be send in one round</c>
        </texttable>
        The following steps are implemented every "t" seconds:
        <list style="numbers">
          <t>Let "n" by the number of messages currently in the pool.</t>
          <t>Let "count" is the smaller of "n - min" and "n * rate", or
            zero if "n - min" is negative.</t>
          <t>Select "count" messages from the pool at random and send
            them.</t>
        </list>
      </t>

      <!-- XXX: give the default Mix 3.0 parameters here for reference?
                - weasel -->

    </section>

    <section anchor="dummy-traffic" title="Dummy Traffic">

      <t>Dummy messages (see <xref target="payload-format"/>) are multi-hop
        messages with four randomly selected remailers as the chain. The
        chain must selected such that no remailer will appear twice unless
        two other remailers separate them.</t>

      <t>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.</t>

      <t>Similarly, prior to each execution of the mixing algorithm
        described in <xref target="timed-pool"/>, the remailer selects a
        random number from a different geometric distribution and adds
        that many dummy messages to the pool as well.</t>

      <t>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.</t>

    </section>

  </section>

  <section anchor="message-format" title="Message Format">

    <section anchor="payload-format" title="Payload Format">

      <t>The message payload can be an e-mail message <xref
          target="RFC2822"/>, a Usenet message <xref target="RFC1036"/>, or a
        dummy message.</t>

      <t>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.</t>

      <t>The payload format is as follows:<figure><artwork>
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]</artwork></figure>
      </t>

      <t>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:
        <list style="hanging">
          <t hangText="null:">Dummy message.  The remailer will discard the
            message.</t>
          <t hangText="post:">Usenet message.  The remailer will post the
            message to Usenet.</t>
          <t hangText="post: [newsgroup]">Usenet message. The remailer will
            add a "Newsgroups" header with the specified content, and post
            the message to Usenet.</t>
          <t hangText="[address]">E-mail message. The remailer will add a
            "To" header with the specified content, and send the message as
            e-mail.</t>
        </list>
      </t>

      <t>If no destination field is given, the payload is an e-mail
        message.</t>

      <t>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.</t>

      <t>There are three types of user data sections:
        <list style="symbols">
          <t>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".</t>
          <t>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.</t>
          <t>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.</t>
        </list>
      </t>

      <t>The payload is limited to a maximum size of 2610180 bytes.
        Individual remailers may use a smaller limit.</t>

      <t>Remailer operators can choose to remove header fields supplied by
        the sender and insert additional header fields, according to local
        policy; see <xref target="key-format"/>.</t>

    </section>

    <section anchor="crypto-algorithms" title="Cryptographic Algorithms">

      <t>The asymmetric encryption mechanism is RSA with 1024 bit RSA keys
        and the PKCS #1 v1.5 (RSAES-PKCS1-v1_5) padding format <xref
          target="RFC2437"/>. The symmetric encryption mechanism is EDE
        3DES with cipher block chaining (24-byte key, 8-byte initialization
        vector) <xref target="Schneier96"/>. MD5 <xref target="RFC1321"/>
        is used as the message digest algorithm.</t>
    </section>

    <section anchor="packet-format" title="Packet Format">

      <t>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.</t>

      <t>The packet header consists of 20 header sections (specified in
        <xref target="header-section"/>) 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.<figure><artwork>
+-------------------+
| Header section 1  |
+- - - - - - - - - -+
| Header section 2  |
+- - - - - - - - - -+
+ ...               +
+- - - - - - - - - -+
| Header section 20 |
+-------------------+
|                   |
| Payload           |
|                   |
|                   |
|                   |
|                   |
|                   |
+-------------------+</artwork></figure>
      </t>

      <section anchor="header-section" title="Header Section Format">

        <t>Packet layout<figure><artwork>
[ 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 ]</artwork></figure>
         Total size: 512 bytes
        </t>

        <t>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.</t>

        <t>The 328 bytes of data encrypted to form the encrypted header
          part are as follows:<figure><artwork>
[ 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 ]</artwork></figure>
         Total size: 328 bytes
        </t>

        <t>The fields are defined as follows:
          <list style="hanging">
            <t hangText="Packet ID:">randomly generated packet identifier.</t>
            <t hangText="Triple-DES key:">used to encrypt the following
              header sections and the packet body.</t>
            <t hangText="Packet type identifier:">The type identifiers
              are:
              <texttable>
                <ttcol align='left' width='20%'>Value</ttcol>
                <ttcol align='left'>Type</ttcol>
                <c>0</c><c>Intermediate hop</c>
                <c>1</c><c>Final hop, complete message</c>
                <c>2</c><c>Final hop, partial message</c>
              </texttable>
            </t>
            <t hangText="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.</t>
            <t hangText="Message digest:">MD5 digest computed over the
              preceding elements of the encrypted header part.</t>
          </list>
        </t>

        <t>The packet information depends on the packet type identifier,
          as follows:<figure><artwork>
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 ]</artwork></figure>

          <list style="hanging">
            <t hangText="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.</t>
            <t hangText="Remailer address:">E-mail address of next hop.</t>
            <t hangText="Message ID:">Identifier unique to
              (all chunks of) this message.</t>
            <t hangText="Chunk number:">Sequence number used in multi-part
              messages, starting with one.</t>
            <t hangText="Number of chunks:">Total number of chunks.</t>
          </list>
        </t>

        <t>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 20 are ignored, and the packet body
          is decrypted using the given initialization vector.</t>

      </section>

      <section anchor="body-format" title="Body Format">

        <t>The message payload <xref target="payload-format"/> 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.</t>

        <t>A message may consist of up to 255 packets.</t>

      </section>

    </section>

    <section anchor="transport-encoding" title="Mail Transport Encoding">

      <t>Mixmaster packets are sent as standard email messages <xref
          target="RFC2822"/>. The message body has the following format:
        <figure><artwork>
##
Remailer-Type: Mixmaster [version number]

-----BEGIN REMAILER MESSAGE-----
[packet length ]
[message digest]
[encoded packet]
-----END REMAILER MESSAGE-----
      </artwork></figure></t>

      <t>The length field always contains the decimal number "20480", since
        the size of Mixmaster packets is constant. An MD5 message digest
        <xref target="RFC1321"/> of the (un-encoded) packet is encoded in
        base64.</t>

      <t>The packet itself is encoded in Base-64 encoding <xref
          target="RFC1421"/>, with linebreaks every 40 characters.</t>

    </section>

  </section>

  <section anchor="key-format" title="Key Format">

    <t>Remailer public key files consist of a list of attributes and a public
      RSA key:<figure><artwork>
[attributes list]

-----Begin Mix Key-----
[key ID]
[length]
[encoded key]
-----End Mix Key-----</artwork></figure>
    </t>

    <t>The attributes are listed in one line separated by spaces. Individual
      attributes must not contain whitespace, and are defined as follows:</t>
    <t>
      <list style="hanging">
        <t hangText="identifier:">A human readable string identifying the
          remailer</t>
        <t hangText="address:">The remailer's Internet mail address</t>
        <t hangText="key ID:">Public key ID</t>
        <t hangText="version:">Software version number</t>
        <t hangText="capabilities:">Flags indicating additional remailer
          capabilities</t>
        <t hangText="validity date:">Date from which the key is valid</t>
        <t hangText="expiration date:">Date of the key's expiration</t>
      </list>
    </t>

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

    <t>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.</t>

    <t>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.</t>

    <t>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:
      <texttable>
        <ttcol align='left' width='20%'>Flag</ttcol>
        <ttcol align='left'>Meaning</ttcol>
        <c>C</c><c>Accepts compressed messages</c>
        <c>M</c><c>Will forward messages to another mix when used as final hop</c>
        <c>Nm</c><c>Supports posting to Usenet through a mail-to-news gateway</c>
        <c>Np</c><c>Supports direct posting to Usenet</c>
      </texttable>
    </t>

    <t>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).</t>

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

    <t>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 <xref target="RFC1421"/>, with linebreaks every 40 characters.
      Its length (258 bytes) is given as a decimal number.</t>

    <t>Digital signatures <xref target="RFC2440"/> should be used to ensure
      the authenticity of the key files.</t>

  </section>


  <section anchor="implementation" title="Implementation Nodes">

    <t>This section discusses various implementation issues.</t>

    <section anchor="remixing" title="Remixing">

      <t>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.</t>

      <t>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
        message with this body to be delivered to the next hop remailer.</t>

      <t>Ensuring that a remailer's keyring containing the public keys for
        other remailers in the network remains up-to-date is the
        responsibility of the given remailer's operator.</t>

      <t>If the remailer receives a Mixmaster message that, when decrypted,
        reveals a message of another protocol which the remailer speaks, it
        should process the message as though it had been delivered in the
        other protocol format initially.</t>

    </section>

    <section anchor="admin-commands" title="Administrative Commands">

      <t>The 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:
        <list style="hanging">
          <t hangText="remailer-help:">Returns information about using the
            remailer. The remailer may support a suffix consisting of a
            dash and a two-leter 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.</t>
          <t hangText="remailer-key:">Returns the remailer's public key as
            described in <xref target="key-format"/>. It may also return the keys and attributes
            of other remailers it knows about.</t>
          <t hangText="remailer-stats:">Returns information about the number
            of messages the remailer has processed per day (again, a day
            starts at 00:00 UTC).</t>
          <t hangText="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.</t>
          <t hangText="remailer-adminkey:">Returns the OpenPGP <xref target="RFC2440"/>
            key of the remailer's operator.</t>
        </list>
      </t>

    </section>

    <section anchor='impl-dummy-traffic' title="Dummy Traffic">

      <t>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.</t>

    </section>

    <section anchor="key-rotation" title="Key Rotation">

      <t>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.</t>

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

      <t>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.</t>

      <t>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.</t>

    </section>

    <section anchor="anon-delivery" title="Delivery of Anonymous Messages">

      <t>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 unauthorized. This is a problem because many news servers
        accept control messages automatically without any authentication.</t>

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

      <t>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.</t>

      <t>If the recipient does not wish to receive anonymous messages,
        unobservability of communications and authenticity can be achieved at
        the same time by the remailer verifying that the message is
        cryptographically signed <xref target="RFC2440"/> by a known
        sender.</t>

      <t>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 <xref target="Mazieres98"/>.</t>

    </section>

  </section>

  <section anchor="security-consideration" title="Security Considerations">

    <t>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; <xref
        target="Moeller98"/> contains a more detailed analysis of these
      attacks.</t>

    <t>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.</t>

    <t>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.</t>

    <t>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.</t>

    <t>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.</t>

    <t>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.</t>

    <t>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.</t>

    <t>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.</t>

    <t>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.</t>

    <t>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.</t>

    <t>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 <xref
        target="anon-delivery"/>.</t>

  </section>

  <section anchor="acknowledgements" title="Acknowledgements">

    <t>Several people contributed ideas and source code to the Mixmaster v2
      software.</t>

    <t>"Antonomasia" &lt;ant@notatla.org.uk&gt;,
      Adam Back &lt;adam@cypherspace.org&gt;,
      Marco A. Calamari &lt;marcoc@dada.it&gt;,
      Bodo Moeller &lt;bmoeller@acm.org&gt;,
      Jon Orbeton &lt;jono@networkcommand.com&gt;, and
      Myles &lt;myles@tenhand.com&gt; suggested improvements to this
      document.  Rich Salz &lt;rsalz@datapower.com&gt; contributed
      significantly to this document by XMLifying it and rewording
      many ambiguous sections.</t>

  </section>

</middle>

<back>

  <references title="References">

    <reference anchor="Chaum81">
      <front>
        <title>Untraceable Electronic Mail, Return Addresses, and Digital
          Pseudonyms</title>
        <author initials="D." surname="Chaum" fullname="David Chaum">
          <organization/><address/>
        </author>
        <date year="1981"/>
      </front>
      <seriesInfo name="Communications of the ACM" value="24:2"/>
    </reference>

    <reference anchor="Mazieres98"
      target="ftp://cag.lcs.mit.edu/pub/dm/papers/mazieres:pnym.ps.gz">
      <front>
        <title>The Design, Implementation and Operation of an Email Pseudonym
          Server</title>
        <author initials="D." surname="Mazieres" fullname="D. Mazieres">
          <organization/><address/>
        </author>
        <author initials="F." surname="Kaashoek" fullname="F. Kaashoek">
          <organization>5th ACM Conference on Computer and Communications
            Security</organization>
          <address/>
        </author>
        <date year="1998"/>
      </front>
    </reference>

    <reference anchor="Moeller98"
      target="http://agn-www.informatik.uni-hamburg.de/people/3umoelle/st.ps">
      <front>
        <title>Anonymisierung von Internet-Diensten</title>
        <author initials="U." surname="Moeller" fullname="Ulf Moeller">
          <organization>Studienarbeit, University of Hamburg</organization>
          <address/>
        </author>
        <date month="January" year="1998"/>
      </front>
    </reference>

    <reference anchor="Schneier96">
      <front>
        <title>Applied Cryptography (second edition)</title>
        <author initials="B." surname="Schneier" fullname="Bruce Schneier">
          <organization>Wiley</organization>
          <address/>
        </author>
        <date year="1996"/>
      </front>
    </reference>

    <?rfc include="reference.RFC.1036.xml" ?>
    <?rfc include="reference.RFC.1321.xml" ?>
    <?rfc include="reference.RFC.1421.xml" ?>
    <?rfc include="reference.RFC.1952.xml" ?>
    <?rfc include="reference.RFC.2311.xml" ?>
    <?rfc include="reference.RFC.2437.xml" ?>
    <?rfc include="reference.RFC.2440.xml" ?>
    <?rfc include="reference.RFC.2822.xml" ?>

  </references>

</back>

</rfc>
