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Anonymous Remail with Mixmaster MESSAGES FROM NOWHERE www.photocase.de
Anonymous remail protects the sender’s identity against potential eavesdroppers. The Mixmaster protocol gives users a mature technology for anonymous remail, and the text-based Mixmaster client is an example of a free remailer application. BY JENS KUBIEZIEL
hen Johan Helsingius started messages and installed the software on messages were not covered by the mail up an anonymization service his server. secrecy act and thus facilitated eaves- Wfor email back in 1993, he The address of this server anon.penet. dropping and the identification of the could hardly have anticipated the kind fi soon became known, and it is still spo- users. This, in turn, prompted of trouble he would be in for. Despite, or ken of with awe today. To use the ser- Helsingius, to switch off the server [1]. maybe because of, the hostile reactions, vice, users had to send an email message Johan is now renowned for his pioneer- with a special entry in the header to the Cypherpunk and Mixmaster ing work. address. The server replaced the sender By the time Johan Helsingius switched In the early 90s, mailing lists and address with an address in the form of off his anonymous mail server, develop- USENET discussion groups had left the [[email protected]] (where XXXX ment was progressing at lightning speed. phase in which they mainly concen- is a combination of numbers) and for- The Cypherpunks, a group that focused trated on scientific and computer-related warded the email to the address speci- on protecting privacy and the use of subjects. USENET also supported a num- fied in the additional header line. cryptography, developed a number of ber of highly controversial political and The service was easy to use, and that remailer models that did not rely on a religious discussions. Because these dis- attracted many users. By 1996, the soft- central server. Their work was based on cussions were of interest to secret ser- ware was handling around 10,000 mes- a paper published back in 1981 by David vices and employers, users were looking sages a day. This was the year that the Chaum [2], describing mix networks for a way of expressing themselves Scientology movement sued the opera- that had been implemented with the anonymously. Johan Helsingius devel- tor, demanding the release of email idea of protecting the anonymity of the oped software for depersonalizing email addresses. A Finnish court decided email parties in email exchanges.
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The principle is comparable with sen- ding a letter in a number of envelopes. If Ralf Penn wants to send an anony- mous letter, he originally addresses the letter to the recipient, but instead of sending the letter directly, he then adds a number of intermediate stations. He puts the letter in another envelope and writes the address of one of these stations on the envelope. The letter gets a new envelope for each of these stations. The letter is then sent to the first inter- mediate address, where the external envelope is opened. The envelope is destroyed and the letter is sent to the address on the next envelope until, finally, the last intermediate station sends the letter to the actual recipient. Figure 1: Mixmaster client start up screen. The recipient can only trace the letter back to the last intermediate station, tity of the sender and recipient. Also, an for a potential investigator to identify as all the other envelopes have been attacker could intercept a message and messages by their size, the remailer also destroyed. This process guarantees the repeatedly reinsert it into the remailer makes all messages a uniform size. If a anonymity of the sender. chain. message is too small, Mixmaster adds Because each message is handled in random fill characters; if a message is First Generation Remailers exactly the same way, it takes exactly the too big, Mixmaster splits that message The first remailer model to be based on same route. These were the weaknesses into blocks of the same size. This tech- this principle was the Cypherpunk that Lance Cottrell identified in 1995 in nique makes it impossible for attackers Remailer, also known as the Type I “Mixmaster & Remailer Attacks” [3]; he to associate incoming packets with out- remailer. In contrast to Helsingius’ also proposed a few changes, which led going packets. model, there are a number of servers to the Type II Remailer, the Mixmaster. Also, each message packet is assigned involved, all of which operate indepen- a packet ID. Mixmaster checks if the ID dently of one another. If one server is How Mixmaster Works is already registered, and drops the mes- not accessible, users can fall back on Mixmaster does not forward incoming sage if it is. Dropping registered message one of the others. As the servers are messages immediately. Instead Mixmas- packets protects the server against rein- located in different countries, with dif- ter waits until enough messages have sertion attacks. These steps remove ferent legal systems, attackers would been added to the queue. When the some of the weaknesses of the Cypher- find it difficult to do anything about this message pool is full, the server sends the punk remailer. Additionally, Mixmaster kind of remailer. messages to the next station in the chain remailers use symmetric encryption, Cryptographic techniques are used to in random order. To make it impossible which accelerates message processing. wrap the message, as described earlier. This process involves the sender encrypting the message with the public key of each remailer in the chain. Users can request the key via email (Listing 1) or via the website of the server. Each remailer in the chain can only decrypt the part of the message intended for its use. The decrypted part contains the address to which the server has to forward the message. The remailer setup removes some of the weaknesses of Helsingius’ service, but it still leaves a few problems. For example, each remailer forwards emails as soon as they arrive. This allows an attacker to deduce relationships between incoming and outgoing messages, and thus to draw conclusions about the iden- Figure 2: The Mixmaster client displaying an overview of available remailers.
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In fact, the Mixmaster has a lot of out the RFC draft for the Mixmaster pro- volunteer programmers as an open advantages over the Cypherpunk tocol [4]. source project. Development work is remailer. hosted by Sourceforge [5]. Users can A detailed description of the way Daily Operations download the program sources from this works is far beyond the scope of Mixmaster is also the name of a client Sourceforge and build the program from this article. Readers might like to check software package that was developed by the sources. Debian also has precom-
Listing 1: Retrieving a Remailer Key 01 From: Jens Kubieziel
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piled binaries of the Mixmaster client for selects a chain of four remailers, tions. People tend to think of denuncia- its users [6]. although users can type [C] to define a tions, bomb threats, spamming, or ille- After installing the software, users chain of remailers themselves (Figure 2). gal material. However, anonymous should download the public keys and Figure 2 shows remailers and their remailers simply comply with a require- availability statistics for the remailers. reliability values. These statistics are ment for a safe IT infrastructure; that is, Many remailer operators publish this only snapshots, and some variation is they hide the fact that communication is data on their websites [7]. The Debian expected, so use these values as a rough taking place. There are many legitimate Mixmaster package includes a Perl script guide only. After selecting a chain, you reasons for wanting to hide communica- called mixmaster-update. The script can send the message to the message tions from public view. For example, a automatically downloads the required pool by pressing [M] and then go on to sudden increase in the volume of email files and is designed to run as a cron job compose another message if needed. between two companies may give listen- or ip-up script. After downloading the When enough messages have accumu- ers a clue that the companies are consid- files and storing them in /var/lib/mix- lated, or if a user issues a command to ering a partnership, even though the master/stats/, you can go on to type mix- this effect, the program sends the mes- content of the messages might be master and launch the program (see Fig- sages to the other stations in the chain. encrypted. Members of radical groups, ure 1). All in all, Mixmaster is very easy to reform advocates in authoritarian coun- Within the program, users can com- use and has a self-explanatory user tries, or people with serious, socially pose, read, and send messages. For interface. Newcomers should have no stigmatized illnesses also may wish to example, if you need to send an email problem getting accustomed to using the protect their anonymity. message, you are prompted to enter the software and sending anonymous mes- On the other hand, there is no denying recipient and subject line of the message sages whenever they need to do so. the potential for misusing the anony- when you press the [M] key. Pressing [E] mous remailer. Lobbyists and email in the send menu allows you to compose Pros and Cons of authorities are quick to point out the the message; you are returned to the Anonymous Mail potential for abuse, and the resulting menu after completing the message. By In a pluralistic society, anonymous com- contraversy has provoked calls to ban default, the program automatically munication often has seedy connota- anonymity services. Johan Helsingius, the man behind the remailer, claims Box 1: Email Delivery via Cypherpunk Remailers never to have used the service he invented. It was, however, important for 1. Compose message and add a header. a9BfsGTel1DjnHeLWypdW4XuPNn The message is addressed to the CiNA8fwdnu him to develop the technology to sup- port anonymity, which allows users to recipient first. Two lines are inserted at 09 C58rmBo2B8XTjcc1eGjD+SayRn/ the start of the message: exercise their right to freedom of speech. F3eGc3rdGw3EkwWpRxwgcXU/Sv And this access to anonymous email is :: HwE6vnOnTwE still available to remailers around the Anon-To: [email protected] 10 +9fWwweS+WUFRCBNPqaUZkXqZ6j world today. ■ These lines give the last remailer the BpVe5fRAUZDRhqOhUcEA0nvRHn9 information it needs to send the mes- D7QMJuqV9R INFO sage to its final destination. 11 7CPEAb/+Dd2+hxqqezeXpTHOqJK [1] Press release on the closing down of 2. Encrypt message and add the iUiE8SqGnBBAw5uOpMffuGG120b anon.penet.fi: http://www.fitug.de/ encrypted header. The message is LPEDfuM7yF news/1997/penet.html now encrypted with the remailer’s 12 xaXWu6TQ94GTV/+2Inw9LufUPNs public key. Another line is inserted in [2] David L. Chaum, “Untraceable Elec- aTfrWWRxFNphWvTh9a+MRIIKb7a tronic Mail, Return addresses and Dig- front of the cipher text: Encrypted: bSCee4qcwP ital Pseudonyms”: http://world.std. PGP. This line tells the remailer that com/~franl/crypto/chaum-acm-1981. any following lines need to be 13 vjJsDM2f html decrypted. 14 =7HnR [3] Lance Cottrell, “Mixmaster & 01 :: 15 -----END PGP MESSAGE----- Remailer Attacks”: http://riot.eu.org/ 02 Encypted: PGP anon/doc/remailer-essay.html [4] RFC Draft for the Mixmaster protocol, 03 3. Repeat these steps for the given num- ber of remailers. If the user wants to Version 2: http://www.ietf.org/ 04 -----BEGIN PGP MESSAGE----- add another remailer, a new Anon-To: internet-drafts/ draft-sassaman-mixmaster-03.txt 05 Version: GnuPG v1.2.5 line is added at the start of the mes- (GNU/Linux) sage. Then step 2 is repeated. These [5] Mixmaster project homepage: http:// mixmaster.sourceforge.net 06 steps are repeated for the number of remailers in the chain. [6] Information on the Debian Mixmaster 07 hQEOA1gu3H8UQS6IEAP/UgB5ZbyRS5 4. Send message. The message is sent package: http://packages.qa.debian. Kkmi/mD4Vi4PHBg6XOOoS8BL/t6HGa to the first remailer in the chain, which org/m/mixmaster.html CkMc forwards the information described [7] Statistics for the Noreply.org remailer: 08 BHAB4YCnQGz1IEzxhrMnYxeFl0C above. http://www.noreply.org/echolot/
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