Big Brother is listening to you...

The phrase, "For reasons of security, this conversation may be recorded," will never sound the same again...

In 1992 a team of high-ranking Civil Guard officers, university professors and interns started work on a tool that has revolutionized the world of phone-tapping. It is now used in 35 countries across the world, primarily by government security and defense departments. The tool is a biometric software system comparable to the biosonar capability of bats. Christened the Batvox, the system is capable of identifying a suspect by his or her voice with the same reliability as a fingerprint.

Its latest victim is former French Junior Minister for the Budget, Jérôme Cahuzac.

"We had to find a way to identify a suspect by voice, independently of what was said and the language it was said in," says Lieutenant-Colonel Lucena of the Civil Guard's Image and Acoustics Department. "On the other hand, we had to make sure the level of error was small enough that prosecutors could use it evidence."

The search for technicians able to bring the Batvox to fruition led to young university professors Javier Ortega and Joaquín González-Rodríguez. Today, 1.2 million euros later, it is the leading voice-recognition system on the planet. "It is a case not of what somebody said but who said it, and being able to prove it irrefutably," says González-Rodríguez.

Monsieur Cahuzac did not bank on such technology when on March 19 he bluntly denied tax evasion. "None of the material elements produced are convincing," he said. But one was: his own voice, which he had left on an answering machine talking about an undeclared bank account in Switzerland. He subsequently resigned his post, admitted the existence of the account and was expelled from François Hollande's ruling Socialist Party.

Batvox can  identify a suspect by their voice as reliably as by a fingerprint

"We have created software capable of comparing two voices and showing the level of probability that they are the same," says Javier Castaño, who worked with Ortega and González-Rodríguez and now heads Agnitio - Latin for "recognition" - the company that patented and marketed Batvox.

"Listen carefully please: I am calling you in the name of ETA to tell you there is a large explosive device in the parking lot in Terminal 4 of Madrid Barajas Airport and that it will go off at 9am, one hour from now." This message was delivered to the Madrid fire service on December 30, 2006, by Igor Portu, a member of ETA's "comando Elurra" cell and brought to an end a nine-month ceasefire. The device exploded at the allotted hour, bringing down the entire five-story parking lot, killing two people and damaging over a thousand cars and the terminal building itself. The two fatalities were Ecuadorian nationals Carlos Alonso Palate and Diego Armando Estacio, who were sleeping in their cars waiting to pick up friends and relatives and not evacuated.

The voice, which sounded nervous on the Civil Guard recording, was matched to Portu after his arrest and subsequent interrogation. "It was enough to compare both voices [...] - the message left with the fire service and that recorded when the suspect was in custody," says Lieutenant Ricardo Nieto of the Civil Guard. "In Portu's case a positive LR was returned."

The LR is the quotient that measures authenticity: the level of probability that a voice corresponds without doubt to a suspect. "In a range of hundreds, the probability is high. In the thousands, it is extremely high," says Antonio Moreno, the Agnitio technician who works directly with "clients" - mainly police forces and forensic laboratories across the world.

The result of the voice comparison is two horizontal curves. The red one is equivalent to a police line-up. "We take recordings of other bad guys from the archives whose voices are similar - in the same way that if our suspect were a blond, we would do the line-up with more blonds," says Moreno. The blue line is the suspect's voice recorded in the police interview. A third line, green, measures authenticity. The closer the blue line is to the green - and the further away it is from the red - the more likely an exact match.

A Batvox costs around 50,000 euros and demand for voice archives is increasing, especially among applications for iPhones and other mobile devices.

The Mexican police - which Moreno says started out archiving the voices of its own officers - has asked for a capacity increase: "At the moment it is 25,000, but they want 100,000."

In Spain, there are around 5,000 voices digitally archived "and rising." The database has brought about some unexpected results. "A common smuggler who was recorded in Algeciras was, some time later, revealed to be the voice of a contact in Algeria for a Salafist jihadist terrorist cell in Spain," says Lucena.

The Batvox has also found a use in South Africa. "The Social Security system uses it to check if people are alive," says Niko Brummer, a "biometrics guru" linked to Agnitio. "Families often don't tell the authorities when a relative dies so they can carry on picking up their pension. Now they record the voices of senior citizens, phone the house and see if they are still alive."