By Clark Boyd
Technology correspondent in Tunis
Tunisia has again come under fire for its attitude towards media freedom.
President Ben Ali has been in power since 1987
The North African country has been condemned for the way it filters websites in an academic study released to coincide with a UN net summit there.
Human rights groups have already questioned the UN's decision to hold an information conference in Tunisia.
In a separate development, the head of the media freedom group, Reporters Without Frontiers, said Tunisia had blocked him from entering the country.
The secretary general of the group, Robert Ménard told the BBC that he had been prevented by Tunisian security officials from leaving his aircraft seat, following his arrival from Paris.
The Tunisian authorities said they had refused entry to Mr Ménard because of pending legal action against him in France.
'Effective and deceptive'
The head of Reporters Without Borders (RSF) had been planning to take part in the World Summit on the Information Society under way in Tunis.
The conference is looking at ways of combating poverty through increased use of information and communication technologies.
But the event has been dogged by reports of the harassment of journalists and civil society groups by the Tunisian authorities.
At the summit, the RSF unrolled a banner condemning 15 countries it called "enemies of the internet" because they filtered out access to websites.
The group listed Belarus, China, Cuba, Iran, Libya, the Maldives, Myanmar, Nepal, North Korea, Saudi Arabia, Syria, Turkmenistan, Tunisia, Uzbekistan and Vietnam.
The government of President Zine El Abidine Ben Ali rejects any suggestion that it violates human rights or limits legitimate access to traditional or electronic media.
But a study by the OpenNet Initiative found that nearly 10% of the 2,000 sites it had tested from within the country were blocked.
They were mostly sites devoted to political opposition, human rights, pornography and tools to circumvent the country's controls.
The OpenNet Initiative is university collaboration between Toronto, Harvard and Cambridge.
"Tunisia's internet filtering is focused, effective, and deceptive," said the report's co-author Derek Bambauer, who is a fellow at Harvard Law School's Berkman Center for Internet and Society.
"These practices run counter to the goals espoused by the World Summit on the Information Society and highlight the challenges to freedom of expression from states determined to limit it."
Tunisian authorities, the report states, use aggressive filtering tactics, not just to block pornography, but also the websites of political opposition groups, Western and Tunisian human rights groups, and the sites of Tunisians living abroad that are critical of the government of President Ben Ali.
The Tunisians are also, according the OpenNet Initiative, blocking sites that tell people how to get around internet filtering.
Nart Villeneuve, director of technical research for the Citizen Lab at the University of Toronto, was involved in compiling the report. He has been in Tunis since Monday, monitoring the online situation.
"I actually expected the Tunisians to lift the filtering during WSIS, and I was surprised they decided to keep it in place," he said.
But it seems to be selective. Surfers in the UN-controlled area of the summit site are not experiencing many filtering problems at all.
Robert Ménard spoke to reporters in Tunis via a mobile phone
Just 100 metres away, in the Tunisian-controlled area of the summit site, people are having problems accessing certain sites.
"Just walk out that door," said Mr Villeneuve, "and its filtered internet."
Certain rights groups here have found their websites blocked since the start of the summit earlier this week.
The OpenNet Initiative Report notes that Tunisia uses a commercially available product called SmartFilter to block access to certain websites. SmartFilter is made by a US company called Secure Computing.
'Tolerance and moderation'
A Secure Computing spokesman said the firm sells to service providers around the world without making any distinctions about whether they are government-owned or how they use its products.
Saudi Arabia, among other Middle Eastern countries, uses the same product according to the report.
Mr Villeneuve said Tunisia uses SmartFilter differently. In Saudi Arabia, an attempt to access banned online material will produce a "Forbidden Page" message.
"Tunisia doesn't do that. Instead, it creates a fake error message telling you the page couldn't be found."
In the end, Mr Villeneuve said, Tunisia's filtering does not have to be perfect.
"But when it's in place, it reminds the local population that there are consequences for trying to access certain types of content. So the filtering doesn't have to be 100% effective, it just has to get the message across for people to start self-censoring."
In his opening speech yesterday, Tunisian President Ben Ali highlighted the great strides his country has made in making information technology, and internet access, available to the people.
"Tunisia," said the president, "has always been a land of dialogue, tolerance and moderation."
Clark Boyd is technology correspondent for The World, a BBC World Service and WGBH-Boston co-production