'This goes no further...'
Following revelations about bugging at the United Nations, is there any way of ensuring that your private conversations stay that way?
News that Kofi Annan and other senior UN figures may have been routinely bugged by US or British security services has caused a huge political row around the world. But it will also have caused alarm among other people in the public eye who deal with sensitive information - or anyone, indeed, who values their privacy.
If the secretary general of the United Nations cannot prevent his private conversations from being listened to by all and sundry, who can?
It seems if someone wants to listen to what you are saying badly enough, there is very little you can do to stop it.
"Technological advances, particularly in the fields of power supply and miniaturisation, mean that its now possible to bug almost anywhere and anything," says Charles Shoebridge, a former counter-terrorism intelligence officer.
"Similar advances have enormously improved anti-bugging capabilities too, and an enormous effort has gone into making communications secure - particularly those of governments and even large commercial organisations.
"However, if security is absolutely critical, it will always pay to assume that a conversation is at least capable of being monitored."
According to security experts, the most common listening device remains the electronic bug.
But government agencies such as the CIA and MI5 have far more advanced systems at their disposal.
Powerful uni-directional microphones can pick up conversations through open windows.
If the window is closed, radio waves or a laser beam can be bounced off the glass. The vibrations detected can be translated into speech.
But potentially the most powerful tool for the modern spy is the mobile phone.
Mobiles that double as listening devices can be bought over the internet.
But today's spies are also able to convert conventional phones into bugs without the owners' knowledge.
Experts believe this is the most likely method used to gather information in the UN building.
Mobiles communicate with their base station on a frequency separate from the one used for talking. If you have details of the frequencies and encryption codes being used you can listen in to what is being said in the immediate vicinity of any phone in the network.
According to some reports, intelligence services do not even need to obtain permission from the networks to get their hands on the codes.
So provided it is switched on, a mobile sitting on the desk of a politician or businessman can act as a powerful, undetectable bug.
The technology also exists to convert land line telephones into covert listening devices.
According to one security expert, telephone systems are often fitted with "back doors" enabling them to be activated at a later date to pick up sounds even when the receiver is down.
Telephone conversations are also routinely intercepted by spy satellites. The potency of key word recognition technology is often overstated, but it is still used to scan millions of conversations a day for potentially juicy information.
Encryption devices, which clip on to the base of mobile phones and scramble the voice data being sent from your phone, are available.
But those listening in may well be able to crack the codes.
Intelligence is a constant battle between the bugger and the bugged, says Michael Marks, of surveillance-equipment supplier Spymaster, and "at the moment the buggers probably have the upper hand".
Mr Marks' advice to anyone who thinks they may be under surveillance is to ensure their office is swept regularly for bugs, buy an encrypted phone and make sure no one in a meeting has a mobile phone on them.
Inside the tent
Another way of making sure you are not being bugged is to use a Faraday cage or shielded tent, which prevents radio waves entering or leaving.
Mobile phone calls are impossible from inside the tent, but no-one will be able to listen to your conversations using bugs or radio wave listening devices.
It will also prevent anyone intercepting radio emissions from computers, preventing them from seeing what you have on screen.
"[A Faraday cage] will stop you doing anything other than having a conversation. It is a very crude, but very secure, way of talking," says Michael Marks.
A more sophisticated - and expensive - method is to build a "clean room", of the type used by the military, to shield radio waves and electromagnetic signals.
But the hardest part, according to counter-surveillance consultant William Parsons, is trying to convince diplomats and politicians that there is a threat.
"They think you are trying to cramp their style. Talking is what they do.
"The fact that someone might be listening doesn't actually come into their mind. It is not something that they actually comprehend."
There are a few simple steps anyone can take, Mr Parsons says, to throw would-be eavesdroppers off the scent.
Don't hold sensitive conversations in your office or boardroom. Or rather, give anyone listening enough to think they are getting the full picture and then save anything truly top secret for conversations in unusual locations, such as the basement.
It is better to use the office phone for secret conversations, Mr Parsons says, rather than a home phone, because with 20 or more lines leaving most buildings they are much harder to bug.
The big outdoors
Switching on the shower while you talk in the bathroom - a favoured method of celluloid spies - is also unlikely to work, as constant volume noise can easily be filtered out.
In fact, the only way to truly guarantee privacy, according to most security experts, is to take a walk in the park.
Charles Shoebridge says: "It remains the case today as it has always been, that probably the best way to avoid being eavesdropped is to pass information during a long, unpredictable and unannounced walk in the big outdoors.
"Word of mouth is always preferable to any form of electronic communication - assuming the information's recipient is entirely trustworthy, of course."