At the centre of it all are private computer services commonly known as 'anonymous remailers', believed to have make the WTC attacks possible with negligible possibilities of detection.

In a nutshell, anonymous remailers -- like pretty good privacy, anon.penet.fi and anonymizer -- are programs that accept outgoing email on a service provider's Web site, strip identifying information (such as sender's name, email address, ISP number), and then send it to its destination devoid of anything that could link it to its source.

Good remailers route messages through several other remailers before delivering it, to further conceal the source. Better ones pause for a random amount of time (anywhere from a couple minutes to days) during each hop, to remove correlations between the time a message was sent and received.

This is unlike conventional email, which is notoriously difficult to hide behind especially if a pursuer is determined to track down a sender. All email messages contain routing data - where it originated and how it got to its destination - that can be used, in conjunction with log files, timing information and even law enforcement subpoenas, to track down its origin.

Similarly, every computer on the Internet has an Internet Protocol (IP) address, that lets email servers and sites know where to send the data. Therefore, even if users visit a site without giving their name or company affiliation, the visit can still be traced to their computer or network.

None of this can happen if data is routed through anonymous remailers: an effective way of sending anonymous, totally untraceable information over the Internet.

One of the first signs that the terrorists were tech savvy enough to exploit this utility came from an FBI document obtained by the German magazine Der Spiegel, which disclosed how the hijackers purchased their airline tickets. Several used a pay-per-use public Internet terminal at a store in Florida, while some chose paperless electronic tickets.

The modus operandi in both cases was the same - routing all requests and other communication through anonymous remailers, thereby making their digital footprints impossible to trace.

The second hint came soon after the FBI announced that a man named Abdulaziz Alomari with a birth date of December 24, 1972 was on the flight that hit the North Tower. Immediately, a man by that name and with that birth date informed a Saudi Arabian newspaper that he was alive and well.

Later, as more names released by the FBI corresponded to living people, remailer-aided identity theft (which was seen as an irritating consequence of modern life before September 11) stood exposed as a fatal breach of national security.

This, added to the claim of US officials that Osama bin Laden had turned to data-hiding steganography software to communicate with operatives, tolled the final death knell for anonymous remailers all over the world.

It doesn't end here. "I am not concerned whether the remailer network is, or was, used by the actual terrorists," one operator wrote on the politech mailing list. "What concerns me are the inevitable bogus threats and tips to various newsgroups, federal offices and officials".

Detractors, however, see the shutting down of these services as an invasion of privacy and threat to the freedom of speech, since anonymous remailers are also invaluable tools for legitimate uses - such as providing a safe way for victims of abuse to participate in support forums without revealing their identities. Another operator posted: "Shutting down freedom of speech would be to do the terrorists' job for them. I will not surrender to terror. I will not carry out the will of these cowards. They will have to do it themselves: I'm keeping my remailer up".

Such sentiment is still limited. When asked to make a choice between security and "freedom of speech", a majority veer towards the former. The popular opinion is that like everything else on the Internet, anonymous remailers are a privilege that must be respected if they are to be preserved.

Thus, the future of privacy and security - both online and in society at large - depends upon users not abusing the protection technology can provide. A conclusion best summed up by French President Jacques Chirac at the 23rd International Conference of Data Protection Commissioners, where he told delegates to "respect freedom of thought, but don't let the Internet become the tool of the enemies of liberty and human dignity".

September 11 made one thing clear - no one will underestimate the keyboard ever again.

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