n anonymous remailer is a server computer which receives messages with embedded instructions on where to send them next, and which forwards them without revealing where they originally came from. There are Cypherpunk anonymous remailers, Mixmaster anonymous remailers, and nym servers, among others which differ in how they work, in the policies they adopt, and in the type of attack on anonymity of email they can (or are intended to) resist. Remailing as discussed in this article applies to emails intended for particular recipients, not the general public. Anonymity in the latter case is more easily addressed by using any of several methods of anonymous publication. There are several strategies which contribute to making the e-mail so handled (more, or less) anonymous. In general, different classes of anonymous remailers differ with regard to the choices their designers/operators have made. These choices can be influenced by the legal ramifications of operating specific types of remailers. It must be understood that every data packet traveling on the Internet contains the node addresses (as raw IP bit strings) of both the sending and intended recipient nodes, and so no data packet can ever actually be anonymous at this level. However, if the IP source address is false, there will be no easy way to trace the originating node (and so the originating entity for the packet). In addition, all standards-based email messages contain defined fields in their headers in which the source and transmitting entities (and Internet nodes as well) are required to be included. However, since most users of email do not have very much technical expertise, the full headers are usually suppressed by mail reading software. Thus, many users have never seen one. Some remailers change both types of address in messages they forward, and the list of forwarding nodes in email messages as well, as the message passes through; in effect, they substitute ‘fake source addresses’ for the originals. The ‘IP source address’ for that packet may become that of the remailer server itself, and within an email message (which is usually several packets), a nominal ‘user’ on that server. Some remailers forward their anonymized email to still other remailers, and only after several such hops is the email actually delivered to the intended address. There are, more or less, four types of remailers:

- Pseudonymous remailers

A Pseudonymous remailer, simply takes away the email address of the sender, gives a pseudonym to the sender and sends the message to the intended recipient (that can be answered via that remailer).

- Cypherpunk remailers, also called type I

A Cypherpunk remailer sends the message to the recipient stripping away the sender address on it. You can not answer a message sent via a Cypherpunk remailer. You can usually encrypt the message sent to the remailer, and the remailer will decrypt it and send it to the recipient address hidden inside the encrypted message. In addition, you can chain 2 or 3 remailers, so each remailer can’t know who is sending a message to whom. Cypherpunk remailers do not keep logs of transactions.

- Mixmaster remailers, also called type II

Mixmaster remailers require use of a program on your computer to write your messages. Such programs are not supplied as a standard part of most operating systems or mail management systems.

- Mixminion remailers, also called type III

A Mixminion remailer attempts to address the following challenges in Mixmaster remailers: replies, forward anonymity, replay prevention and key rotation, exit policies, integrated directory servers, dummy traffic. They have been implemented for the Linux and Windows platforms and are currently available. Some implementations are open source.

Some remailers establish an internal list of actual senders and invented names such that a recipient can send mail to invented name AT some_remailer.net. When receiving traffic addressed to this user, the server software consults that list, and forwards the mail to the original sender, thus permitting anonymous — though traceable with access to the list — two way communication. The famous “penet.fi” remailer in Finland did just that for several years. Because of the existence of such lists in this type of remailing server, it is possible to break the anonymity by gaining access to the list(s), by breaking into the computer, asking a court (or merely the police in some places) to order that the anonymity be broken, and/or bribing an attendant. This happened to penet.fi as a result of some traffic passed through it about Scientology. The Church claimed copyright infringement and sued penet.fi’s operator. A court ordered the list be made available. Penet’s operator shut it down after destroying its records (including the list) to retain identity confidentiality for its users; though not before being forced to supply the court with the real email addresses of two of its users. More recent remailer designs use cryptography in an attempt to provide more or less the same service, but without so much risk of loss of user confidentiality. These are generally termed nym servers or pseudonymous remailers. The degree to which they remain vulnerable to forced disclosure (by courts or police) is and will remain unclear, since new statutes/regulations and new cryptanalytic developments proceed apace. Multiple anonymous forwarding among cooperating remailers in different jurisdictions may retain, but cannot guarantee, anonymity against a determined attempt by one or more governments, or civil litigators. If users accept the loss of two-way interaction, identity anonymity can be made more secure. By not keeping any list of users and corresponding anonymizing labels for them, a remailer can ensure that any message which has been forwarded leaves no internal information behind which can later be used to break identity confidentiality. However, while being handled, messages remain vulnerable within the server (e.g., to Trojan software in a compromised server, to a compromised server operator, or to mis-administration of the server), and traffic analysis comparison of traffic into and out of such a server can suggest quite a lot — far more than almost any would credit.

The Mixmaster strategy is designed to defeat such attacks, or at least to increase their cost (i.e., to ‘attackers’) beyond feasibility. If every message is passed through several servers (ideally in different legal and political jurisdictions), then attacks based in legal systems become considerably more difficult, if only because of ‘Clausewitzian’ friction amongst lawyers, courts, different statutes, organizational rivalries, legal systems, etc. And, since many different servers and server operators are involved, subversion of any (i.e., of either system or operator) becomes less effective also since no one (most likely) will be able to subvert the entire chain of remailers. Random padding of messages, random delays before forwarding, and encryption of forwarding information between forwarding remailers, increases the degree of difficulty for attackers still further as message size and timing can be largely eliminated as traffic analysis clues, and lack of easily readable forwarding information renders ineffective simple automated traffic analysis algorithms. There are also web services that will let you send anonymous email messages. These services do not provide the anonymity of real remailers, but they are easier to use. When using a web based anonymous email or anonymous remailer service you should first analyze the reputation of this service, since the service stands in the middle between you and the email recipient. If the object is identity anonymity, nothing sent via a remailer can ever include identifying information in content available to an outside observer. Thus, “From: anon(At)remailer.net Hey dude, send me that new comic to 123 Maple Street, Wherever, Country, Postal Code. Thanx” is evidently entirely unsecure.

Encrypting such a message with an adequately secure cryptosystem would help, and some remailers are set up to do so. In general cleartext messages are likely to include such information even if inadvertently, and user anonymity when sending cleartext messages is accordingly likely to be lost. Less obviously, some software (eg, recent versions of Microsoft Office components — Microsoft Word, Microsoft Excel, etc) includes (ordinarily invisible) identifying information in each formatted file it saves. The information might be name / organization / email address (collected at ‘product registration’ and retained internally), or product copy serial number, or computer ID (eg, CPU serial number, or interface hardware address (eg, Ethernet MAC address, a unique in the world ID), or … One software program which claims to remove such information from files notes that there are about 30 different kinds in Word format files. Those interested in anonymity should limit themselves to plain text messages (ASCII text only) produced by plain text editors (eg, vi, emacs, Notepad, …) as they don’t include such hidden information. Alternatively, users should take great care to inspect files (eg, text, images, sound files, …) to ensure they contain no identifying information. Note however, that even byte-by-byte inspection will not necessarily uncover such information since it can be easily concealed by encryption, steganography, or simple unfamiliarity. Anonymity, once lost, can almost never be regained as those interested in breaching it will often keep (and have often kept) records of such discoveries. Such records have typically had very long lives, particularly if those keeping them have long planning horizons (eg, governments, or groups with social or political interests). For some opinions or speech, this may have, or come to have, serious consequences. Not all anonymous remailers are identical, even when all work as intended. Close attention to operational standards and intent, locations, and reliability records is needed before choosing one. Among the criteria which should be considered are:

- Class: (two-way or one-way, encrypted message content or cleartext only, mixmaster style or one hop forwarding, …)

- Location: Some offshore jurisdictions permit seizure of equipment, data, or operating records) Geographical Mapping

- History: Operators who maintain and administer hardware and software in better condition than others; paying particular attention to security configuration issues)

- Security: Some operating systems have worse security histories than others, even when properly configured, maintained, and administered)

- Operator: At worst, a remailer run by some infamous Secret Police Department. an operator may be ominously inattentive)

- Privacy and operating policies: If stated, better than not stated. If stated, sensible, and observed, better still. However, recourse (legal or otherwise) has almost never been available against operators, software developers, operating system suppliers, especially in cases of loss of anonymity and/or consequent damages, regardless of operating policies)

-Software: Some remailer software is widely used (and live tested), some is not.

-Record and reputation: Consult remailer statistics sites, and check (Google search, news group postings, blogs, …

There is no way to ensure that a particular remailer server will never cause problems for its users (loss of identity or confidentiality). A remailer system not under one’s own (expert level) control will always remain unknown. In most cases, remailers are owned and operated by individuals, and are not as stable as they might ideally be. In fact, remailers can, and have, gone down without warning. It is important to use up-to-date statistics when choosing remailers.