Wired on Friday: Like many young men looking for an outlet for his discontent, Zouhair Yahyaoui set up a discussion forum online. It was called TuneZine, and one of its common threads was to poke fun at the government in Zouhair's native Tunisia. In 2001, posting under the pen-name Ettounsi, he asked his readers to vote on whether Tunisia was a) a republic, b) a kingdom, c) a zoo or d) a prison.

The authorities didn't appreciate the joke, and in May of that year, he was arrested without a warrant in a Tunis cybercafe. While in custody, he was tortured and revealed the access codes to his website, allowing the authorities to remove the site from the internet.

Zouhair communicated under an internet pseudonym, but his postings were easy to track down to the computer he used to communicate. The net often provides the reassuring feel of anonymity to those who communicate over it, but truly protecting your identity online is not easy - especially when you have the government or a well-financed enemy against you. Every message you pass on the net (whether that message is an e-mail or a request to a webserver to send a page to your browser) gets broken down into packets.

Each of those packets are marked with their destination and source address. That's vital to how the net directs its data from place to place, but the source and destination, if intercepted, can give away a great deal. Both are IP addresses - and IP addresses can be traced to individual network connections. That tiny, unavoidable, snippet of information can pose a real hazard for people in tense situations.

Dissidents like Zouhair aren't the only ones who might benefit from disguising their tracks online. Governments, too, are keen to provide net access to soldiers and agents in the field, but are loathe to do so when enemy forces can locate their source by listening in.

Tor is a voluntary network of computers which acts as an anonymising intermediary for its users. It concentrates not just on concealing the contents of the messages you send via it, but hiding their source. Any internet message can be sent through it, not just e-mail.

Tor works rather like a series of go-betweens in a forbidden, romantic liaison. Rather than send your message directly to Romeo, a budding Juliet hands it to a local Tor router. This router might pass it directly to Romeo, but is far more likely to send it to another router. Eventually, the go-betweens pass the packet onto Romeo, with no evidence as to the original source of the message. Romeo, of course, can reply using the same path.

Between routers, the message is encrypted, and if one or more of the routers is compromised, the chances of the attacker being able to reconstruct the message, or determine even who it was from or to, is small indeed.

For soldiers in the field that's vital - it's often their location they are trying to keep secret as much as the message. Government agents monitoring Zouhair's website would have seen updates appearing from all around the world.

As well as acting as a courier for secret messages, Tor can also provide "hidden" services - anonymous web services like websites and forums which can only be accessed by users of Tor. Their names are not catchy - http://6sxoyfb3h2nvok2d.onion/ is the main portal - but that string of unmemorable numbers is Tor's way of guaranteeing the identity of the anonymous site. Only one person has the key to that site and no one else (unless they obtain the access code), can create a site with the same code word in its stead.

Tor's .onion sites are likely to be stuffed with controversy. Already, alongside the Declaration of Human Rights, and an archive of Noam Chomsky are pictures of two undercover Swiss police officers, previous pulled off the public net by authorities in Switzerland, Italy and the US. Of course, anonymity isn't just for dissidents and government agents. The history of other anonymous communities suggest that social nasties like child porn may already be living under .onion, hard to find with a browser and harder still to find in the real world.

That's one of the penalties of having an anonymising network open to everybody. Prior systems were so hard to use that only the most highly motivated people with access to good IT tools could be found using them - mainly recreational cryptographers and child pornographers.

The government dissidents and Enron-style whistleblowers the system's idealistic designers dreamt of never materialised. Sometimes this was because the people that needed the systems couldn't find them or use them; or because those people couldn't afford to be associated with recreational cryptographers and child pornographers.

What may be Tor's saving grace is that it is the first anonymous system that has paid attention to being usable. Tor can be up and running on most computers in less than 10 minutes, including a reboot. From there, a few other tools like anonymous mail from Hushmail.com can have the blogging in minutes, in relative safety. If Tor pays attention to these factors, that may be due to its curious history. The anonymising network has been sponsored by civil liberties groups and the US department of defence.

In March, Zouhair Yahyaoui died of a heart attack at age 36. That's uncommon, but not so much for people who have spent years in prison starving and being tortured, as was the case for Yahyaoui.

Many people interested in both blogging and Tunisian reform are grieving the loss of his voice. Perhaps services like Tor, and those that might follow it, might prevent such painful losses in the future.