Anonymous Remailers


An anonymous remailer is specialized kind of mail server designed to send e-mail messages without identifying the sender. Many of them are provided as a free service.

Imagine being in protective custody or being a journalist and you want to get an e-mail out without being discovered. You decide to use an anonymous remailer to remedy that issue. These systems strip all headers from the message making it next to impossible to trace where the message originated. Some systems gave you an anonymous address that other people can send mail to, which will be forwarded on to your regular e-mail account. This is sometimes known as a “pseudonymous server”. However, the majority of remailers use encryption. In addition to remailer technology, ther eare a variety of attacks that remailers face.

Types of Remailers

Pseudo-Anonymous Remailers

With a pseudo-anonymous remailer, the operator of the remailer knows the user’s true e-mail ID. The user’s messages are sent with an untraceable (and “anonymised”) return address. These remailers are easy to use, but you are anonymous only because the operator promises to keep your identity secret. While these remailers are committed to protecting your identity, courts occasionally force these remailer-operators to reveal particular clients (based on crimes ranging from harassment to child pornography).

Pseudo-anonymous remailers are probably sufficient for most people’s needs (e.g., posting comments or sending a resume).

Anonymous Remailers

True anonymous remailers provide a much higher guarantee of privacy and anonymity. They are essentially computers designed as specialized e-mail servers. While traditional e-mail servers track and preserve all incoming and outgoing sender, receiver, and message information, anonymous remailer servers are designed to strip out all identifying and traceable information. While there are a number of steps involved, truly anonymous remailers use encryption to disguise the origin of messages. There two basic types.

Cypherpunk remailers were a big advance over the pseudo-type of remailers. They provide anonymity without even using an anonymized return address, in large part through encryption

Mixmaster remailers are the latest generation of anonymous remailers. It requires a special message format and a specific software package. This type of remailer uses advanced techniques and strong cryptography to make most methods of traffic analysis next to impossible.

The structure of current remailer messages is a nested set or encrypted messages. The message contains the instructions for each remailer and the message to be forwarded. Each remailer removes a layer of encryption, accompanying instructions, takes any requested actions, and sends the message on. An example of this is Figure 1 (shown below).

 This figure shows a remailer that gets sent through three remailers (A,B,C) and finally to Bob. The boxes represent encryption, with the name of the person to whom the message is encrypted, outside the upper left corner of the box. One thing to note is that the message shrinks after each hop.

Mixmaster remailers are also known as cyberphunk remailers. These do several things:
·        They send a message to another e-mail address or post it to a news group.
·        They accept encrypted messages with instructions for processing hidden inside the encrypted envelope.
·        They strip all mail headers.
·        They add new headers such as subject lines.
·        They remove some information from the end of the message.
·        They encrypt part of a message using a key specified in the message.


Attacks are actions taken against a remailer when a message is trying to be sent. I will explain six very serious attacks that happen to remailers when they are not careful.

Threat Model

An attacker is able to record the contents of all messages into and out of all remailers, along with the times they arrive and depart. All messages are monitored as they leave the sender’s machine and as they arrive at the destination.

The attacker is also able to send an unlimited number of messages through the remailers, including previously intercepted messages. The attacked can also prevent messages from arriving at their destinations.

Trivial Attacks

This can happen to anyone when the message that is sent is unencrypted. This can also happen when the message is encrypted, only if the remailer is compromised because the operator knows both the originating and final addresses.


Messages can be traced through chains of remailers with encryption because incoming messages are forwarded directly after processing. When a message arrives, another one leaves immediately. The attacker knows that these are the same messages despite all precautions and can even be known by using mail logs.

The first fix to this problem was to delay incoming messages for some random length of time before sending it out. However it depends on the traffic. If there are many messages arriving in the average holding time, then the identity of the message is reasonably well disguised. If there is very little traffic, due to network outages, etc., there is little or no protection.

Reordering always involves keeping some number of messages in the remailer at all times. These messages are called the message “pool”. The most efficient way to do this is to have N messages in the pool and send out one of the (N+1) messages in the pool, which is chosen at random. Unfortunately, this scheme is susceptible to “spam” attacks.

To keep that from happening, another idea was proposed. It was decided that instead of sending out one message, all messages should be sent out at once periodically and at a random time.

Size and Distinguishability

Your message can also be tracked by size. Default messages automatically decrease in size by a small amount at each hop. Padding is removed from each message by the remailer, at each hop however, this is only to decrease the size of the message. Extremely large messages also stand out and will also be decreased yet will remain bigger than other messages. Because each comes in different sizes, attackers can pinpoint a message and “attack” it.

The solution that was found to be that each message should be the same size.

Replay Attack

This type of attack can be used to follow a message to its final destination or backtrack from the end to its original sender. To trace a message forward, the attacker captures your message and sends many copies to the first remailer. Many identical messages will then emerge from the remailer and move on to the next remailer. This bump in the remailer traffic will show the route of the message.

To prevent this attack, remailers must refuse to send a message more than once. This can be done by including a random ID number for each hop, which the remailer records. This also has the benefit of making spams expensive.

Man-in-the-Middle Attack

This attack deals with the initial public key exchange by intercepting the very first message to a new correspondent and substituting a genuine public key with a fake one. In order to exchange messages, you need the other person’s public key and they need yours. Once you get it, you put it on your Public Key Ring. It is the very first message with the public key that is vulnerable. If precautions are taken, there is no more threat.

This attack is not limited to the Internet either. This could have been done a thousand years ago with plain old postal mail (snailmail). For example, suppose to merchants in England and Germany, who never met in person, wanted to communicate over mail. The Englishman approaches the German with a business proposal and he signs his letter. How could the German know the signature is authentic? A crooked postal clerk in France could intercept the message, rewrite it himself, pretending to be the Englishman. If the German admits the signature the French postal clerk would intercept all future correspondence, replace it with rewritten letters and contracts, and keep pretending to be the Englishman.

In the digital age, an attacker can get access to one of the Internet nodes through which your message hops. Then:
1.      Malice intercepts your message.
2.      They will remove your public key and save it for themselves.
3.      They generate their own private + public key pair.|
4.      They attach this newly generated public key to your message, instead of the original.
5.      You send the tampered message to IDS.

If IDS accepts this bogus key, the following would happen:
1.      IDS writes a confidential message to you.
2.      IDS uses the bogus public key to encrypt the message.
3.      IDS sends the encrypted message via e-mail.
4.      Malice intercepts the message.
5.      Malice deciphers it using the private key they created.

To prevent the attack from happening, you should use a digital signature certificate from a qualified company, exchange public keys in person and on disk, and/or download public keys over a secure website.

How a Remailed Message is Constructed

After going on to, you can go to an access page that will generate a list of remailers at random for you. This list shows their “screenname”, the configuration options, and special features. It also shows its twelve-day history, average latency, and uptime for each remailer. An example of the first part is below:

$remailer{"anonmtq"} = " cpunk mix hybrid pgp latent ek ekx esub cut hash post repgp remix reord ext max test inflt75 rhop5 klen500";

To decipher this:
·        “anonmtq” is the screen name
·        cpunk – a major class of remailers, supports-request remailing
·        mix – can accept messages in Mixmaster format
·        pgp – remailer supports encryption with PGP – a short name instead of the full e-mail address will be shown.
·        Latent – is a late time option
·        Ek – encrypt responses in reply blocks using encrypt-key: header

These are just a few of the many configuration options used. It is almost like deciphering a code. An example of the second part:

noisebox ++++++*++++* 19:15 99.99%

·        The first part is the fake e-mail address used.
·        The second part is the response time over a 12-day period.
·        (The + stands for less than four hours and the * stands for less than one hour.)
·        The time is the “uptime” that is has been in use.
·        The percentage is the average latency.

Related Links

Anonymous Remailer FAQs

White Paper – Anonymous Remailers

Anonymous Remailer Information – Electronic Frontiers Georgia

Anonymous Remailer Information – Riot Anonymous Remailer

Anonymous Remailers, General Information on the Concept of Anonymity on the Web

New Order Anonymity/Privacy Tools

Andre Bacard’s Privacy Page

Anonymizer Privacy Products

Free Anonymous Remailers You Can Use

The Offshore MixMaster Anonymous Remailer

Riot Anonymous Remailer

W3 Anonymous Remailer

a joint project of the George Mason Society and the Global Internet Liberty Campaign