3. Remailer Support

3.1 Remailer Introduction

One solution to this is to use chains of remailers that send encrypted messages. For example, suppose Bill wishes to send a message to Louis using a chain of remailers A, B, and C. He writes the message (possibly encrypting it for Louis), then encrypts the result (including the fact that Louis is the recipient) using a public key supplied by remailer C. Then he encrypts this result using a public key supplied by remailer B. Then he encrypts this result using a public key supplied by A and sends the message to A.

When A receives the message, it decrypts the message with its key to produce something encrypted for B, learns that the next remailer in the chain is B, strips off the information that the message came from Bill, and sends the message on to B. B then decrypts, learns that the next remailer in the chain is C, strips off the information that the message came from A, and sends the result to C. C then decrypts, learns that the destination is Louis, strips off the information that the message came from B, and sends the result to Louis. With this arrangement, only A knows that the original message came from Bill, and only C knows that the intended recipient is Louis. In general, the sender and recipient can both be known only to someone who has compromised all remailers in the chain.

If Bill wishes, he can include an encrypted "response block" in his message to Louis, which defines a remailer chain that Louis can use to reply to Bill. Louis can use this chain without knowing who Bill is -- only the last remailer in the chain need know the final recipient. Bill can also establish a pseudonym for use in signing his anonymous messages.

Mailcrypt includes facilities for sending messages via remailers, for defining chains of remailers, for generating response blocks, and for using pseudonyms.

3.2 Remailer Quick Start

To use Mailcrypt's remailing facilities, you need to configure them first. Begin with the following steps:

1. Do `finger rlist@mixmaster.shinn.net > ~/.remailers'. This will create a Levien-format list of remailers in the file `.remailers' in your home directory (see the variable mc-levien-file-name). Mailcrypt will parse this the first time you access a remailer function. `rlist@noisebox.remailer.org' is another good place to get this list.

2. Look over the `.remailers' file and find the ones you want to use.

3. Add their PGP public keys to your keyring. You can finger remailer-keys@mixmaster.shinn.net for an armored keyring full of remailer public keys. Note that Mailcrypt requires that you have the public keys of all the remailers you want to use, and therefore that the remailers support PGP encryption.

Note: These steps need only be done once, although repeating them from time to time is probably a good idea, since remailers come and go.

Now test the remailer functions. First compose an outgoing Email message (using C-x m, for example) addressed to yourself. Type C-c / r. Choose a remailer; use TAB to get completion on its name. The buffer will be rewritten for anonymous mailing through that remailer.

Note that you can only select a single remailer when you rewrite the message. To send the message through multiple hops, either rewrite the message multiple times, or define a chain of remailers that can be referred to like a single remailer. Remailer chains are described in the next section.

The remailer list and keyrings can also be obtained via HTTP. http://anon.efga.org/Remailers/Settings and http://mixmaster.shinn.net/stats/settings/index.html have lists of URLs from which these can be retrieved.

3.3 Remailer Chains

mc-write-mode binds the function mc-remailer-encrypt-for-chain to the key C-c / r. This function rewrites the message for a remailer or chain. The resulting buffer is just a new Email message, so it can itself be rewritten for another remailer; this is one way to manually construct a remailer chain.

Mailcrypt also has powerful facilities for defining automatic chains. We will start with an example. Suppose you have put the following into your `.emacs' file:

(setq mc-remailer-user-chains
      '(("Foo" "alumni" "robo")
        ("Bar" (shuffle-vector ["replay" "flame" "spook"]))
        ("Baz" "Foo" "Bar" "rahul" "Bar")
        ("Quux" 4)))

This code defines four chains. The first is named "Foo" and consists of "alumni" and "robo", in that order. The second is named "Bar" and consists of "replay", "flame", and "spook" in some random order (a different order will be chosen each time the chain is used). The third is named "Baz" and consists of 9 remailers: The two from "Foo", followed by a permutation of the three from "Bar", followed by "rahul", followed by another permutation of the three from "Bar". Finally, the fourth is named "Quux" and consists of a random permutation of the four best remailers as ordered in the `~/.remailers' file.

Now whenever you are prompted for a "remailer or chain", the chains "Foo", "Bar", "Baz", and "Quux" will be available, including TAB completion on their names. By capitalizing their names, you guarantee they will show up near the top of the completion list if you type TAB on an empty input.

Now for the gritty details. mc-remailer-user-chains is a list of chain definitions. A chain definition is a list whose first element is the name (a string) and whose remaining elements form a remailer list. Each element of a remailer list is one of the following:

1. A raw remailer structure. This is the base case, but you will probably never want nor need to deal with these directly.

2. A string naming another remailer chain to be spliced in at this point.

3. A positive integer N representing a chain to be spliced in at this point and consisting of a random permutation of the top N remailers as ordered in the `~/.remailers' file.

4. An arbitrary Emacs Lisp form, which should return another remailer list which will be spliced in at this point and recursively evaluated. Mmmm, Lisp.

So where do the definitions for "replay" etc. come from?

There is another variable, mc-remailer-internal-chains, which has the same format as mc-remailer-user-chains. In fact, the concatenation of the two is always used internally when resolving chains by name. The "internal chains" are normally generated automatically from a Levien-format remailer list, which lives in `~/.remailers' by default and is parsed at startup time. The parser creates several chains, each containing a single remailer, and names each chain after the respective remailer.

Thus "replay" (for example) is actually the name of a chain whose single element is the remailer at <remailer@replay.com>. So "replay" is a valid name of a chain to include in the definition of another chain, as was done above in the definition of "Bar".

3.4 Response Blocks

Mailcrypt can generate a response block for you. Just type C-c / b in an outgoing mail buffer. That will prompt you for a chain to use, and will insert the response block at point. Note that you can use any chain you want for your response block; it need not be related to the chain you (later) use to remail the message.

If instead you type C-u C-c / b, you will be dropped into a recursive edit of the innermost part of the response block. This text is what you will see at the top of the message when the response block is used. This text is the only way to identify the response block, since it will be used to mail you through anonymous remailers.

You probably won't need to use the C-u feature, since by default the response block contains the date, `To' field, and `From' field of the message you are composing. However, if you want your response block to point to a USENET newsgroup instead of your Email address, you may edit the innermost part of the response block to have a `Newsgroups' line instead of a `To' line.

Inserting a response block also updates the `Reply-to' hashmark header field. So, when your recipient replies to your message, the reply will automatically be addressed properly. This only works if the last remailer in the chain used to encrypt the message supports hashmarks (the response block chain doesn't matter). If the last remailer does not support hashmarks, Mailcrypt will generate an error when you try to use the chain.

Note that you should insert your response block before you encrypt the message for remailing. Also, see 3.8 Remailer Security.

3.5 Pseudonyms

Mailcrypt supports pseudonyms. Type C-c / p in an outgoing message buffer and you will be prompted for a pseudonym to use. Your pseudonym will show up in the `From' line that the recipient sees. Your pseudonym may either be a complete `From' line (including an Email address), or just a full name (with no Email address). In the latter case, the Email address will automatically be set to <x@x.x>, an invalid address designed to prevent sendmail from going rewrite-happy.

If you have one or more pseudonyms which you normally use, and you aren't afraid of revealing them if your account is compromised, you can set up a default list of pseudonyms with lines like the following in your `.emacs' file:

(setq mc-remailer-pseudonyms
      '("Elvis Presley" "Vanna White" "Charles Manson"))

Then those names will be available for completion when you are prompted for your pseudonym.

You should insert your pseudonym before you insert a response block, so that the response block will contain the `From' line as well as the `To' line. That way you can tell who you were pretending to be when you get a reply to your message.

Note: Many remailers do not support pseudonyms. In addition, the Levien format does not (yet) indicate which do and which do not, so Mailcrypt can't warn you when your pseudonym isn't going to work. The only way to be sure is to send yourself a test message, and to try different remailers until you find one or more which work. On the bright side, only the last remailer in the chain needs to provide such support; none of the others matter.

3.6 Remailing Posts

Mailcrypt will generate an error if the last remailer in the chain does not have both the post and hash (hashmarks) properties. The hashmarks are used to preserve `References' and similar headers, so your anonymous or pseudonymous followups will thread properly. The variable mc-remailer-preserved-headers controls which headers are preserved when rewriting a message, but you should not need to change it since the default value is reasonable.

Before rewriting, you can use C-c / p to insert your pseudonym, and C-c / b to insert your response block, just like when composing mail. In this case, the response block will include the `From' line and the `Newsgroups' line (which is the news analogue to the `To' line).

3.7 Mixmaster Support

Mixmaster is a new kind of remailer which provides excellent security against traffic analysis and replay attacks. (For more information on these attacks and Mixmaster, see Lance Cottrell's home page at http://www.obscura.com/~loki/.

If you do not use Mixmaster, you may skip this section entirely; Mailcrypt's default configuration treats Mixmaster as if it did not exist.

If you have the Mixmaster executable installed, you can tell Mailcrypt to use it by placing lines like the following into your `.emacs' file:

(setq mc-mixmaster-path "mixmaster")
(setq mc-mixmaster-list-path "/foo/bar/baz/type2.list")

mc-mixmaster-path is a string representing the Mixmaster executable. mc-mixmaster-list-path is the complete path to the type2.list file.

Once these variables are defined, Mailcrypt will automatically try to use the Mixmaster executable whenever possible. Specifically, when you rewrite a message for a chain, Mailcrypt will find maximal length sub-chains which have the mix property and will use the Mixmaster executable to rewrite for those sub-chains.

This allows arbitrary intermingling of Mixmaster and normal (also called Type 1) remailers, but you should note that this is not recommended. The recommended procedure is to have a single Mixmaster sub-chain which is most or all of the whole chain.

There are advantages and disadvantages to having the Mixmaster sub-chain at the end of the whole chain. The primary advantage is that Mixmaster remailers support multiple recipients. The primary disadvantages are that they do not support pseudonyms nor posting.

So here, as always, it is the last element of the chain which needs to support the special features you want. In general, the remaining elements do not matter, and the superior security of Mixmaster remailers is a good argument for using them for the bulk of your chains.

Mixmaster remailers also have a "Type 1 compatibility mode" which you might want to invoke to use a pseudonym or make a post. You can do this with the function mc-demix. Here is an example of its use:

(setq mc-remailer-user-chains
      '(("Foo" "vishnu" "spook")
        ("Bar" "Foo" (mc-demix "replay"))))

This makes "Bar" a chain of three remailers, and guarantees that the last one ("replay") will be used in compatibility mode.

Note that Mixmaster remailers cannot be used for response blocks. Mailcrypt will ignore the mix property when generating a response block.

3.8 Remailer Security

Keep in mind that there is only one person fully qualified to protect your privacy: you. You are responsible for obtaining a list of remailers and their public keys; you are responsible for choosing which of them to use and in what order. There are public lists of remailers and keys (the Quick Start section above relies on them), but you pay for the convenience by putting your trust in a single source. This is one reason Mailcrypt does not access these public lists automatically; you need to get into the habit of watching what goes on behind the scenes. You should also try to learn something about the remailers themselves, since you are relying on them to help protect your privacy.

How many remailers should you include in your chain, and how should you choose them? That depends on whom you perceive as a threat. If the threat is your ex-spouse or your boss, even a single remailer is probably adequate (more won't hurt, but will cost in latency). If the threat is the Church of Scientology, you probably want to use a fair number of remailers across multiple continents. If the threat is a major world government, well, best of luck to you.

Also, there is a huge difference between chains suitable for regular messages and chains suitable for response blocks. Some remailers don't even keep mail logs (at least, their operators claim they do not), so it may be literally impossible to trace a message back to you after the fact if you chain it through enough remailers. Response blocks, on the other hand, have your identity buried in there somewhere. In principle, at least, it is possible to compromise the keys of all the remailers in the chain and decrypt the response block. So you should either use very long and strong chains for your response blocks, avoid using response blocks at all, or only use response blocks which themselves ultimately point to a newsgroup.

3.9 Verifiable Pseudonyms

1. You create a public/private PGP key pair. You give it a User ID which is your pseudonym. You upload the public key to the key servers or otherwise distribute it. (Be aware that anyone who compromises your account can read the IDs on your secret keyring, thus discovering your verifiable pseudonyms.)
2. You compose an Email message, Email reply, news post, or news followup.

3. You insert your pseudonym with C-c / p.

4. (Optional) You insert your response block with C-c / b.

5. You type C-c / s to sign the message. The mc-sign function understands pseudonyms.

6. You type C-c / r to rewrite the message for remailing. (Or use C-u C-c / r to view each step of the rewriting as it happens.)

7. You type C-c C-c to send the message.

Now the recipient(s), reading your message through mail or news, can verify your pseudonymous signature; thus you have started to create a verifiable pseudonymous identity. If you use it consistently, it will develop a reputation of its own. With Mailcrypt, using a pseudonym is almost as easy as using your real name (and your followups in news will even thread properly). Welcome to the new age of letters...

3.10 Remailer Tips

This is a collection of tips for using Mailcrypt's remailer support.

• Read and understand the `.remailers' file. Other sources for this list include http://www.chez.com/frogadmin/MyCypSta1.txt and http://anon.efga.org/Remailers/rlist. If all of these servers have gone away by the time you read this, track down a comparable service elsewhere. (Do a web search for "anonymous remailer list", ask around in news:alt.privacy.anon-server, or, as a last resort, news:alt.security.pgp.) Check the documentation (C-h v) for the variable mc-levien-file-name for a description of Levien format.

• Mailcrypt needs to be able to encrypt a message to each remailer in the chain, so it needs access to their public keys, in a keyring usable by the currently selected backend. Keyrings containing keys for all the well-known remailers are usually available from the same places as the remailer lists above: http://www.chez.com/frogadmin/Keys/dsskeys.asc, http://anon.efga.org/Remailers/TypeIList/pubring.asc.

• The relevant remailer properties are pgp (required), hash (required if you use hashmark headers), and post (required for posting to USENET). Remailers which do not support PGP won't even show up in the completion list.

• The only remailer which needs special properties (e.g., posting, hashmarks, pseudonym support) is the last one in a chain. Any remailer can be used at the beginning or in the middle. So if you find a few remailers which support the feature(s) you require, and you always use them at the end of your chains, then you can be confident that even the longest chains will work.

• If you update your `~/.remailers' file, you can reread it with M-x mc-reread-levien-file.

• Remember the natural order of operations. First you compose your message. Then you insert your pseudonym with C-c / p. Then you insert your response block with C-c / b. Then you sign (C-c / s) or sign and encrypt (C-c / e) the message. Then you rewrite it for a remailer or chain (C-c / r). Then you send it. All but the first and last two of these are optional. (Well, strictly speaking, they are all optional, but you get the idea.)

• Find and read some of the excellent remailer documentation available on the Internet. For some good starting points, see 9. References.

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