Image caption President Obama with members of his team in a mobile war room in a tent

A rare photo, released by the White House, shows Barack Obama fielding calls from a tent in Brazil, to keep up with events in Libya. The tent is a mobile secure area known as a Sensitive Compartmented Information Facility, designed to allow officials to have top secret discussions on the move.

They are one of the safest places in the world to have a conversation.

Designed to withstand eavesdropping, phone tapping and computer hacking, Sensitive Compartmented Information Facilities - also known as SCIFs - are protected areas where classified conversations can be held.

They can be permanent enclosures within a building, or mobile areas set up when a world leader is on the move, to allow them to view sensitive documents or have secret conversations without any outsiders listening or hacking in.

When operations in Libya commenced at the weekend, President Obama was in Brazil on a pre-arranged trip.

Ring of Steel

In order to keep abreast of events a mobile war room was set up in his hotel so he could hold a secure conference call with Secretary of State Hillary Clinton, National Security Advisor Tom Donilon and Secretary of Defense Robert Gates, among others.

A photo released by the White House showed the president and advisers gathered around a video phone, inside what looked like a standard blue tent, erected on the hotel's floral carpets.

Image caption President Obama can also make secure calls aboard Air Force One.

Making sure no-one can intercept the activities in the room requires detailed planning, says Mark Pfeifle, who worked as a deputy National Security Adviser in the White House under George W Bush.

"When a president travels in the US or internationally one thing a team is doing in advance of the visit is locating and securing a certain area where the tent could be placed," explains Mr Pfeifle.

It has to be positioned carefully in relation to windows, and concentrations of people.

Hotel rooms, as used by President Obama in Brazil, are a popular place to locate them, but it can depend on the nature of the trip and where the president is.

Once you have found a place for the tent, the next issue is making sure it is completely secure.

Windowless rooms

This can mean creating a self sufficient pod with its own air supply, says Phil Lago, who is one of the founders of Command Consulting Group, a company which provides SCIFs to government agencies.

"We have to make sure that any kind of emissions don't get out. That could be from your laptop, your radio, your telephone," he explains.

Rather than a ring of steel around a secure complex, he likens it to a "ring of electronic waves" which prevents signals from getting in and out of the tent. The only signal which can get out is the encrypted communications, which are made through a secure and encrypted phone line, which sends conversations through a satellite, he says.

Nothing in a SCIF is allowed to operate on a remote control because that's a frequency that can be tapped. Michael Creasey, Director, CSG Partners

"We never knew if there was someone in the building with a long range listening device," he says. "If we put up a tent in a secure area we knew the president could go in and feel fairly confident that a conversation is private."

Mr Lago recalls travelling with George W Bush to Kennebunkport, where the president used his mobile SCIF to conduct discussions with Tony Blair, who was in Downing St, about Afghanistan and Iraq.

The exact specifications of a mobile pod are top secret, but a public document (The intelligence community directive 705) states that a SCIF, mobile or not, needs not only to be totally soundproofed, but built with an "Intrusion Detection System" to detect any break-ins.

The tent itself is windowless and is made from a secret material which is designed to keep emissions in and listening devices out.

Only those specially authorised can go inside a SCIF, with entry usually requiring a combination of pin numbers, access badges and biometric data.

The perimeters of the tent might be controlled by guards, but there would also be people monitoring outside to see if any data gets out. "You have a line of defence for everything," says Mr Lago.

Carrying a mobile SCIF around is an important part of any presidential trip, he says, adding that they are getting easier to transport.

"You can usually fit them into two large foot lockers and that's most of the equipment you need. In the old days you had to put them in the back of a trailer," he says.

'Not infallible'

As well as mobile tents used to hold conversations, many permanent SCIFs are used to hold secure meetings, in offices and embassies. They are typically constructed with bomb proof walls and similarly tight security measures, explains Michael Creasey, director of development at CSG partners, another company which provides SCIFs.

He says most of his customers are government or defence contractors and departments, but might also be companies who are working on a new design for a plane or a ship and want to hold secure conversations.

Image caption George W Bush used SCIFs to call Tony Blair

When creating SCIFs for clients Mr Creasey says he can use a range of equipment, from a fence which alerts you when someone is touching it to cameras, or devices which can tell when someone has just received an e-mail.

"Nothing in a SCIF is allowed to operate on a remote control because that's a frequency that can be tapped," he says. "Much of what is distributed is done on fibre, not copper as fibre as yet can't be hacked into."

Mr Creasey says demand for SCIFs is particularly high in the Baltimore and Washington area, where many US government agencies are based, and where the cost of setting one up can vary widely from $200 to $5,000 a square foot.

While they are designed to be as impenetrable as possible, and are constantly becoming more sophisticated, Mr Lago says they are not 100% infallible.

"We call them 99.9% infallible."