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Re: Lurkers: First draft: call for comments (was Re: Paper deadlines)
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Roger Dingledine wrote:
> But the reason we don't currently have much discussion about the details
> of mixmaster is because they're not documented anywhere and we don't know
> them. So we have no idea what we specifics we're reusing from mixmaster.
There was a draft-of-an-Internet Draft describing Mixmaster 2 posted to
sci.crypt by Ulf Möller (attached).
- --
David Hopwood <david.hopwood@zetnet.co.uk>
Home page & PGP public key: http://www.users.zetnet.co.uk/hopwood/
RSA 2048-bit; fingerprint 71 8E A6 23 0E D3 4C E5 0F 69 8C D4 FA 66 15 01
Nothing in this message is intended to be legally binding. If I revoke a
public key but refuse to specify why, it is because the private key has been
seized under the Regulation of Investigatory Powers Act; see www.fipr.org/rip
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From: 3umoelle@informatik.uni-hamburg.de (Ulf =?iso-8859-1?Q?M=F6ller?=)
Subject: Re: Mixmasters encrypt how?
Date: 08 Mar 2000 00:00:00 GMT
Message-ID: <8a62ku$m7j$1@rzdspc3.informatik.uni-hamburg.de>
References: <88pr7v$cq$1@nnrp1.deja.com> <38B168F9.8108E3F@zks.net> <20000224223443.984@a.earth.net> <894u11$u78$1@eskinews.eskimo.com>
X-Complaints-To: usenet@news.uni-hamburg.de
X-Trace: rzsun03.rrz.uni-hamburg.de 952536340 1371 134.100.8.63 (8 Mar 2000 17:25:40 GMT)
Organization: HR13
NNTP-Posting-Date: 8 Mar 2000 17:25:40 GMT
Newsgroups: sci.crypt,alt.privacy.anon-server
William Rowden <rowdenw@eskimo.com> wrote:
>The second article mentions that Mixmaster messages are 3DES encrypted
>with headers containing the destination address, packet ID, and 3DES
>key encrypted by a 1024-bit RSA key. This sheds some light on the
>first question.
>
>I have yet to find documentation with enough detail to explain the
>Mixmaster key format. Maybe I need to dig through the source.
This is an unfinished draft, but it should contain most of the
information you need.
Mixmaster Protocol
Version 2
Abstract
Most e-mail security protocols only protect the message body, leaving
useful information such as the the identities of the conversing
parties, sizes of messages and frequency of message exchange open to
adversaries. This document describes Mixmaster (version 2), a mail
transfer protocol designed to protect electronic mail against traffic
analysis.
Mixmaster is based on D. Chaum's mix-net protocol. A mix (remailer)
is a service that forwards messages, using public key cryptography to
hide the correlation between its inputs and outputs. Sending messages
through sequences of remailers achieves anonymity and
unobserveability of communications against a powerful adversary.
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Table of Contents
1. Introduction
2. The Mix-Net Protocol
2.1 Message Creation
2.2 Remailing
2.3 Message Reassembly
3. Message Format
3.1 Payload Format
3.2 Cryptographic Algorithms
3.3 Packet Format
3.3.1 Header Section Format
3.3.2 Body Format
3.4 Mail Transport Encoding
4. Key Format
5. Delivery of Anonymous Messages
6. Security Considerations
7. Acknowledgements
8. References
9. Authors' Addresses
1. Introduction
This document describes a mail transfer protocol designed to protect
electronic mail against traffic analysis. Most e-mail security
protocols only protect the message body, leaving useful information
such as the the identities of the conversing parties, sizes of
messages and frequency of message exchange open to adversaries.
Message transmission can be protected against traffic analysis by the
mix-net protocol. A mix (remailer) is a service that forwards
messages, using public key cryptography to hide the correlation
between its inputs and outputs. If a message is sent through a
sequence of mixes, one trusted mix is sufficient to provide anonymity
and unobserveability of communications against a powerful adversary.
Mixmaster is a mix-net implementation for electronic mail.
This memo describes version 2 of the Mixmaster message format, as
used on the Internet since 1995. An improved protocol is described in
a separate document.
2. The Mix-Net Protocol
The mix-net protocol [Chaum 1981] allows to send messages while
hiding the relation of sender and recipient from observers
(unobserveability). It also provides the sender of a message with the
ability to remain anonymous to the recipient (sender anonymity). If
anonymity is not desired, authenticity and unobserveability can be
achieved at the same time by transmitting digitally signed messages.
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This section gives an overview over the protocol used in Mixmaster.
The message format is specified in section 3.
2.1 Message Creation
To send a message, the user agent splits it into parts of fixed size,
which form the bodies of Mixmaster packets. If sender anonymity is
desired, care should be taken not to include identifying information
in the message. The message may be compressed.
The sender chooses a sequence of up to 20 remailers for each packet.
The final remailer must be identical for all packets.
The packet header consists of 20 sections. For a sequence of n
remailers, header sections n+1, ... , 20 are filled with random data.
For all sections i := n down to 1, the sender generates a symmetric
encryption key, which is used to encrypt the body and all following
header sections. This key, together with other control information
for the remailer, is included in the i-th header section, which is
then encrypted with the remailer's public key. The resulting message
is sent to the first remailer in an appropriate transport encoding.
To increase reliability, redundant copies of the message may be sent
through different paths. The final remailer must be identical for all
paths, so that duplicates can be detected and the message is
delivered only once.
2.2 Remailing
When a remailer receives a message, it decrypts the first header
section with its private key. By keeping track of a packet ID, the
remailer verifies that the packet has not been processed before. The
integrity of the message is verified by checking the packet length
and verifying message digests included in the packet. Then the first
header section is removed, the others are shifted up by one, and the
last section is filled with random padding. All header sections and
the packet body are decrypted with the symmetric key found in the
header. This reveals a public key-encrypted header section for the
next remailer at the top, and removes the old top header section.
Transport encoding is applied to the resulting message.
The remailer collects several encrypted messages before sending the
resulting messages in random order. Thus the relation between the
incoming and outgoing messages is obscured to outside adversaries
even if the adversary can observe all messages sent. The message is
effectively anonymized by sending it through a chain of independently
operated remailers.
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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 a complete message or part of a multi-part message.
If the packet contains the entire message, the packet body is
decrypted and after reordering messages the plain text is delivered
to the recipient. For partial messages, a message ID is used to
identify the other parts as they arrive. When all parts have arrived,
the message is reassembled, decompressed if necessary, and delivered.
If the parts do not arrive within a time limit, the message is
discarded.
Only the last remailer in the chain can determine whether packets are
part of a certain message. To all the others, they are completely
independent.
3. Message Format
3.1 Payload Format
The Mixmaster message payload can be an e-mail message, a Usenet
message or a dummy message.
The messages use the formats specified in [RFC 822] and [RFC 1036]
respectively, prepended with data specifying the payload type. An
additional, more restricted method of specifying message header lines
is defined for reasons of backward compability.
The payload format is as follows:
Number of destination fields [ 1 byte]
Destination fields [ 80 bytes each]
Number of header line fields [ 1 byte]
Header lines fields [ 80 bytes each]
User data section [ up to ~2.5 MB]
Each destination field consist of a string of up to 80 ASCII
characters, padded with null-bytes to a total size of 80 bytes. The
following strings are defined:
null: dummy message
post: Usenet message
post: [newsgroup] Usenet message
[address] e-mail message
If no destination field is given, the payload is an e-mail message.
If the destination field is "post: [newsgroup]", a "Newsgroups:
[newsgroup]" field is added to the header of the resulting message.
If the destination field is of the fourth type, a "To: [address]"
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field is added to the header of the resulting message. [address] and
[newsgroup] are strings of ASCII characters.
Message headers can be specified in header line fields. Each header
line field consists of a string of up to 80 ASCII characters, padded
with null-bytes to a total size of 80 bytes.
There are three types of user data sections:
A compressed user data section begins with the GZIP identification
header (31, 139). It contains another user data section. The data are
compressed using GZIP [RFC 1952]. The GZIP operating system field
must be set to Unix, and file names must not be given. Compression
may be used if the capabilities attribute of the final remailer
contains the flag "C".
An RFC 822 user data section begins with the identification "##<CR>"
(35, 35, 13). It contains an e-mail message or a Usenet message as
specified in [RFC 822] and [RFC 1036]. This type cannot be used if
the final remailer uses a Mixmaster software version prior to 2.0.4.
A user data section not beginning with one of the above
identification strings contains only the body of the message. When
this type of user data section is used, the message header fields
must be included in destination and header line fields.
The payload is limited to a maximal size of 2610180 bytes. Individual
remailers may use a smaller limit.
Remailer operators can choose to remove header fields supplied by the
sender and insert additional header fields, according to local policy
(see section 5).
3.2 Cryptographic Algorithms
The asymmetric encryption operation in Mixmaster version 2 uses RSA
with 1024 bit RSA keys and the PKCS #1 v1.5 (RSAES-PKCS1-v1_5)
padding format [RFC 2437]. The symmetric encryption uses EDE 3DES
with cipher block chaining (24 byte key, 8 byte initialization
vector) [Schneier 1996]. MD5 [RFC 1321] is used as the message digest
algorithm.
3.3 Packet Format
A Mixmaster packet consists of a header containing information for
the remailers, and a body containing payload data. To ensure that
packets are indistinguishable, the size of these encrypted data
fields is fixed.
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The packet header consists of 20 header sections (specified in
section 3.3.1) of 512 bytes each, resulting in a total header size of
10240 bytes. The header sections -- except for the first one -- and
the packet body are encrypted with symmetric session keys specified
in the first header section.
3.3.1 Header Section Format
Public key ID [ 16 bytes]
Length of RSA-encrypted data [ 1 byte ]
RSA-encrypted session key [ 128 bytes]
Initialization vector [ 8 bytes]
Encrypted header part [ 328 bytes]
Padding [ 31 bytes]
Total size: 512 bytes
To generate the RSA-encrypted session key, a random 24 byte
Triple-DES key is encrypted with RSAES-PKCS1-v1_5, resulting in 128
bytes (1024 bits) of encrypted data. This Triple-DES key and the
initialization vector provided in clear are used to decrypt the
encrypted header part. They are not used at other stages of message
processing.
The 328 bytes of data encrypted to form the encrypted header part are
as follows:
Packet ID [ 16 bytes]
Triple-DES key [ 24 bytes]
Packet type identifier [ 1 byte ]
Packet information [depends on packet type]
Timestamp [ 7 bytes] (optional)
Message digest [ 16 bytes]
Random padding [fill to 328 bytes]
The possible packet type identifiers are:
Intermediate hop 0
Final hop 1
Final hop, partial message 2
The packet information depends on the packet type identifier, as
follows:
Packet type 0 (intermediate hop):
19 Initialization vectors [152 bytes]
Remailer address [ 80 bytes]
Packet type 1 (final hop):
Message ID [ 16 bytes]
Initialization vector [ 8 bytes]
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Packet type 2 (final hop, partial message):
Chunk number [ 1 byte ]
Number of chunks [ 1 byte ]
Message ID [ 16 bytes]
Initialization vector [ 8 bytes]
Packet ID: randomly generated packet identifier.
Triple-DES key: used to encrypt the following header sections and the
packet body.
Initialization vectors: For packet type 1 and 2, the IV is used to
symmetrically encrypt the packet body. For packet type 0, there is
one IV for each of the 19 following header sections (Note: This is
solved more efficiently in later versions of the protocol). The IV
for the last header section is also used for the packet body.
Remailer address: e-mail address of next hop.
Message ID: randomly generated identifier unique to (all chunks of)
this message.
Chunk number: Sequence number used in multi-part messages, starting
with 1.
Number of chunks: Total number of chunks.
Timestamp: A timestamp is introduced with the byte sequence (48, 48,
48, 48, 0). The following two bytes specify the number of days since
Jan 1, 1970, given in little-endian byte order. A random number of up
to 3 may be subtracted from the number of days in order to obscure
the origin of the message.
Message digest: MD5 digest computed over the preceding elements of
the encrypted header part.
In the case of packet type 0, header sections 2 .. 20 and the packet
body each are decrypted separately using the respective
initialization vectors. In the case of packet types 1 and 2, header
sections 2 .. 20 are ignored, and the packet body is decrypted using
the given initialization vector.
3.3.2 Body Format
The message payload (section 3.1) is split into chunks of 10236
bytes. To each chunk, its length is prepended as a 4 byte
little-endian number to form the body of a Mixmaster packet.
A message may consist of up to 255 packets.
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3.4 Mail Transport Encoding
Mixmaster packets are sent as text messages [RFC 822]. The RFC 822
message body has the following format:
::
Remailer-Type: Mixmaster [version number]
-----BEGIN REMAILER MESSAGE-----
[packet length ]
[message digest]
[encoded packet]
-----END REMAILER MESSAGE-----
The length field always contains the decimal number "20480", since
the size of Mixmaster packets is constant. An MD5 message digest [RFC
1321] of the (un-encoded) packet is encoded as a hexadecimal string.
The packet itself is encoded in base 64 encoding [RFC 1421], broken
into lines of 40 characters (except that the last line is shorter).
4. 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:
identifier: a human readable string identifying the remailer
address: the remailer's Internet mail address
key ID: public key ID
version: the Mixmaster software version number
capabilities: flags indicating additional remailer capabilities
The identifier consists of alphanumeric characters, beginning with an
alphabetic character. It must not contain whitespace.
The encoded key packet consists of two bytes specifying the key
length (1024 bits) in little-endian byte order, and of the RSA
modulus and the public exponent in big-endian form using 128 bytes
each, with preceding null bytes for the exponent if necessary. The
packet is encoded in base 64 [RFC 1421], and broken into lines of 40
characters each (except that the last line is shorter). Its length
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(258 bytes) is given as a decimal number.
The key ID is the MD5 message digest of the representation of the RSA
public key (not including the length bytes). It is encoded as a
hexadecimal string.
The capabilities field is optional. It is a list of flags represented
by a string of ASCII characters. Clients should ignore unknown flags.
The following flags are used in version 2.0.4:
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.
Digital signatures [RFC 2440] should be used to ensure the
authenticity of the key files.
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 Usenet control
messages [RFC 1036] unauthorized, which is a problem because many
news servers accept control messages automatically without any
authentication.
For these reasons, remailer software should allow the operator to
disable certain types of message headers, and to insert headers
automatically.
Remailers usually add a "From:" field containing an address
controlled by the remailer operator to anonymous messages. Using the
word "Anonymous" in the name field allows recipients to apply scoring
mechanisms and filters to anonymous messages. Appropriate additional
information about the origin of the message can be inserted in the
"Comments:" header field of the anonymous messages.
If the recipient does not wish to receive anonymous messages,
unobserveability of communications and authenticity can be achieved
at the same time by the remailer verifying that the message is
cryptographically signed [RFC 2440] by a known sender.
Anonymous remailers are sometimes used to send harassing e-mail. To
prevent this abuse, remailer software should allow operators to block
destination addresses on request. Real-life abuse and attacks on
anonymous remailers are discussed in [Mazieres 1998].
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6. 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 ([Moeller 1998] contains
a more detailed analysis of these attacks).
Passive adversaries can observe some or all of the messages sent to
mixes. The users' anonymity comes from the fact that a large number
of messages are collected and sent in random order. For that reason
remailers should collect as many messages as possible while keeping
the delay acceptable.
Statistical traffic analysis is possible even if single messages are
anonymized in a perfectly secure way: An eavesdropper may correlate
the times of Mixmaster packets being sent and anonymized messages
being received. This is a powerful attack if several anonymous
messages can be linked together (by their contents or because they
are sent under a pseudonym). To protect themselves, senders must mail
Mixmaster packets stochastically independent of the actual messages
they want to send. This can be done by sending packets in regular
intervals, using a dummy message whenever appropriate. To avoid
leaking information, the intervals should not be smaller than the
randomness in the delay caused by trusted remailers.
There is no anonymity if all remailers in a given chain collude with
the adversary, or if they are compromised during the lifetime of
their keys. Using a longer chain increases the assurance that the
user's privacy will be preserved, but in the same time causes lower
reliability and higher latency. Sending redundant copies of a message
increases reliability but may also facilitate attacks. An optimum
must be found according to the individual security needs and trust in
the remailers.
Active adversaries can also create, suppress or modify messages.
Remailers must check the packet IDs to prevent replay attacks.
Message integrity must be verified to prevent the adversary from
performing chosen ciphertext attacks or replay attacks with modified
packet IDs, and from encoding information in an intercepted message
in a way not affected by decryption (e.g. by modifying the message
length or inducing errors). This version of the protocol does not
provide integrity for the packet body. Because the padding for header
section is random, in this version of the protocol it is impossible
for a remailer to check the integrity of the encrypted header
sections that will be decrypted by the following remailers. Chosen
ciphertext attacks and replay attacks are detected by verifying the
message digest included in the header section.
The adversary can trace a message if he knows the decryption of all
other messages that pass through the remailer at the same time. To
make it less practical for an attacker to flood a mix with known
messages, remailers can store received messages in a reordering pool
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that grows in size while more than average messages are received, and
periodically choose at random a fixed fraction of the messages in the
pool for processing. There is no complete protection against flooding
attacks in an open system, but if the number of messages required is
high, an attack is less likely to go unnoticed.
If the adversary suppresses all Mixmaster messages from one
particular sender and observes that anonymous messages of a certain
kind are discontinued at the same time, that sender's anonymity is
compromised with high probability. There is no practical
cryptographic protection against this attack in large-scale networks.
The effect of a more powerful attack that combines suppressing
messages and re-injecting them at a later time is reduced by using
timestamps.
The lack of accountability that comes with anonymity may have
implications for the security of a network. For example, many news
servers accept control messages automatically without any
cryptographic authentication. Possible countermeasures are discussed
in section 5.
7. Acknowledgements
Several people contributed ideas and source code to the Mixmaster v2
software. "Antonomasia" <ant@notatla.demon.co.uk>, Adam Back
<aba@dcs.ex.ac.uk> and Bodo Moeller <bmoeller@acm.org> suggested
improvements to this document.
8. References
[Chaum 1981] Chaum, D., "Untraceable Electronic Mail, Return
Addresses, and Digital Pseudonyms", Communications of the ACM 24
(1981) 2.
[Mazieres 1998] Mazieres, D., and Kaashoek, F., "The Design,
Implementation and Operation of an Email Pseudonym Server", 5th ACM
Conference on Computer and Communications Security, 1998. <URL:
ftp://cag.lcs.mit.edu/pub/dm/papers/mazieres:pnym.ps.gz>.
[Moeller 1998] Moeller, U., "Anonymisierung von Internet-Diensten",
Studienarbeit, University of Hamburg, 1998. <URL:
http://agn-www.informatik.uni-hamburg.de/people/3umoelle/st.ps>.
[RFC 822] Crocker, D., "Standard for the Format of ARPA Internet Text
Messages", STD 11, RFC 822, August 1982.
[RFC 1036] Horton, M., and Adams, R., "Standard for Interchange of
USENET Messages", RFC 1036, December 1987.
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Internet Draft Mixmaster Protocol 2000
[RFC 1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992.
[RFC 1421] Linn, J., "Privacy Enhancement for Internet Electronic
Mail: Part I -- Message Encryption and Authentication Procedures",
RFC 1421, February 1993.
[RFC 1952] Deutsch, P., "GZIP file format specification version 4.3",
RFC 1952, May 1996.
[RFC 2311] Dusse, S., Hoffman, P, Ramsdell, B, Lundblade, L., and
Repka, L., "S/MIME Version 2 Message Specification", RFC 2311, March
1998.
[RFC 2437] Kaliski, B., and Staddon, J., "PKCS #1: RSA Cryptography
Specifications, Version 2.0", RFC 2437, October 1998.
[RFC 2440] Callas, J., Donnerhacke, L., Finney, H., and Thayer, R.:
"OpenPGP Message Format", RFC 2440, November 1998.
[Schneier 1996] Schneier, B., "Applied Cryptography", 2nd Edition,
Wiley, 1996.