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A major security flaw could leave scores of car models at risk of being stolen, according to a paper published after a two-year legal battle.

Volkswagen had used its lawyers to keep the research under wraps, reports the Birmingham Mail.

It was compiled by University of Birmingham computer scientist Flavio Garcia and his colleagues Baris Ege and Roel Verdult from Radboud University Nijmegen in the Netherlands.

They discovered more than 100 models of cars produced by 26 car manufacturers are at risk of theft by hackers who could crack codes to produce fake keys - thanks to flaws in a device designed to prevent vehicles from being stolen.

(Image: Unisex Association)

Among the makes of car at risk are Audi, Honda, Skoda, Citroen, Fiat and Volvo - as well as top of the range sports cars produced by Porsche and Ferrari.

During a lengthy legal wrangle, Volkswagen sued both the universities and researchers.

But the manufacturer has now agreed to the publication of the paper after the researchers agreed to omit a single line from their report - a pivotal detail which could allow a nontechnical person to work out the hack.

In the past, thieves could hot-wire a car to get it to start.

But now, there are computer chips inside the key fob and car ignition switch aimed at making car theft more difficult.

A car only starts if the chips are near each other and send just the right code.

(Image: BPM)

However, the researchers claimed a flaw lies in a chip called a Megamos Crypto transponder - widely used in the car manufacturing industry.

The transponder “talks” to the key fob wirelessly to check its identity - and if it can’t find the correct code, it immobilises the engine.

In theory there are billions of possible combinations for the code, making it all but impossible to happen upon the right one by chance.

But the hackers discovered that by listening in to the wireless communication between the car and the transponder just twice, they could narrow the number of possible combinations down to just 200,000.

Then an automated ‘cracking’ programme could try each one of those 200,000 codes - allowing it to find the right combination in just half an hour.

And once the right combination has been found, it would be child’s play for the hackers to make a fake key that will be recognised by the car as the real deal.

It is feared a hacker could potentially become a valet driver and steal a fleet of cars - or steal a rental vehicle long after returning it.

Mr Garcia said: “It’s a bit like if your password was ‘password’.

“We want to emphasise that it is important for the automotive industry to migrate from weak proprietary ciphers like this to community-reviewed ciphers and use it according to the guidelines.”

Mr Garcia and his fellow researchers presented their findings in August at a prestigious conference in Washington.

They said they gave the Swiss firm EM Microelectronic, which produces the Megamos Crypto transponder, nine months to fix the problem in late 2012 before they planned on going public with their discovery.

The firm was unavailable for comment.

A Volkswagen spokesman said the hack takes “considerable, complex effort that’s unlikely to be used - except by tech-savvy, organized crime syndicates”.

He added: “We have an interest in protecting the security of our products and customers.

“In this connection, Volkswagen does not make available information that might enable unauthorised individuals to gain access to its vehicles.

“In all aspects of vehicle security, be this mechanical or electronic, Volkswagen goes to great lengths to ensure the security and integrity of its products against external malicious attack.”