Network Working Group U. Moeller
Internet-Draft Secardeo GmbH
Expires: November 23, 2004 L. Cottrell
Anonymizer, Inc.
P. Palfrader
The Mixmaster Project
L. Sassaman
Nomen Abditum Services
May 25, 2004
Mixmaster Protocol Version 2
draft-sassaman-mixmaster-02.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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 November 23, 2004.
Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
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.
Moeller, et al. Expires November 23, 2004 [Page 1]
Internet-Draft Mixmaster May 2004
Mixmaster is based on Dr. 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
Moeller, et al. Expires November 23, 2004 [Page 2]
Internet-Draft Mixmaster May 2004
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
Moeller, et al. Expires November 23, 2004 [Page 3]
Internet-Draft Mixmaster May 2004
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.
Moeller, et al. Expires November 23, 2004 [Page 4]
Internet-Draft Mixmaster May 2004
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.
Moeller, et al. Expires November 23, 2004 [Page 5]
Internet-Draft Mixmaster May 2004
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.
Moeller, et al. Expires November 23, 2004 [Page 6]
Internet-Draft Mixmaster May 2004
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
Moeller, et al. Expires November 23, 2004 [Page 7]
Internet-Draft Mixmaster May 2004
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:
Moeller, et al. Expires November 23, 2004 [Page 8]
Internet-Draft Mixmaster May 2004
[ 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 ]
Moeller, et al. Expires November 23, 2004 [Page 9]
Internet-Draft Mixmaster May 2004
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.
Moeller, et al. Expires November 23, 2004 [Page 10]
Internet-Draft Mixmaster May 2004
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:
+--------------+----------------------------------------------------+
| Flag | Meaning |
+--------------+----------------------------------------------------+
| C | Accepts compressed messages |
| M | Will forward messages to another mix when used as |
Moeller, et al. Expires November 23, 2004 [Page 11]
Internet-Draft Mixmaster May 2004
| | 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
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
Moeller, et al. Expires November 23, 2004 [Page 12]
Internet-Draft Mixmaster May 2004
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.
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
Moeller, et al. Expires November 23, 2004 [Page 13]
Internet-Draft Mixmaster May 2004
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
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
Moeller, et al. Expires November 23, 2004 [Page 14]
Internet-Draft Mixmaster May 2004
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.
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
Moeller, et al. Expires November 23, 2004 [Page 15]
Internet-Draft Mixmaster May 2004
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
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
Moeller, et al. Expires November 23, 2004 [Page 16]
Internet-Draft Mixmaster May 2004
<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.
[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.
Moeller, et al. Expires November 23, 2004 [Page 17]
Internet-Draft Mixmaster May 2004
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/
Moeller, et al. Expires November 23, 2004 [Page 18]
Internet-Draft Mixmaster May 2004
Len Sassaman
Nomen Abditum Services
P.O. Box 900481
San Diego, CA 92190-0481
USA
EMail: rabbi@abditum.com
Moeller, et al. Expires November 23, 2004 [Page 19]
Internet-Draft Mixmaster May 2004
Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
Full Copyright Statement
Copyright (C) The Internet Society (2004). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assignees.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
Moeller, et al. Expires November 23, 2004 [Page 20]
Internet-Draft Mixmaster May 2004
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
Moeller, et al. Expires November 23, 2004 [Page 21]