Editing Penet remailer (section)

==Implementation==
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Julf's remailer worked by receiving an e-mail from a person, stripping away all the technical information that could be used to identify the original source of the e-mail, and then remailing the message to its final destination. The result provided Internet users with the ability to send e-mail messages and post to Usenet newsgroups without revealing their identities.

In addition, the Penet remailer used a type of “post office box” system in which users could claim their own anonymous e-mail addresses of the form ''an''xxxxx''@anon.penet.fi'', allowing them to assign pseudonymous identities to their anonymous messages, and to receive messages sent to their (anonymous) e-mail addresses.

While the basic concept was effective, the Penet remailer had several vulnerabilities which threatened the anonymity of its users. Chief among them was the need to store a list of real e-mail addresses mapped to the corresponding anonymous e-mail addresses on the server. A potential attacker needed only to access that list to compromise the identities of all of Penet's users. The Penet remailer was on two occasions required by the legal system in [[Finland]] (the country where the Penet server hardware resided) to turn over the real e-mail address that was mapped to an anonymous e-mail address. Another potential vulnerability was that messages sent to and from the remailer were all sent in [[cleartext]], making it vulnerable to electronic eavesdropping.

Later anonymous remailer designs, such as the [[Cypherpunk anonymous remailer|Cypherpunk]] and [[Mixmaster anonymous remailer|Mixmaster]] designs, adopted more sophisticated techniques to try to overcome these vulnerabilities, including the use of encryption to prevent eavesdropping, and also the technique known as [[onion routing]] to allow the existence of pseudonymous remailers in which no record of a user's real e-mail address is stored by the remailer.

Despite its relatively weak security, the Penet remailer was a hugely popular remailer owing to its ease of anonymous account set-up and use compared to more secure but less user-friendly remailers, and had over 700,000 registered users at the time of its shutdown in September 1996.