Criminals are increasingly using the internet telephone service Skype to prevent their conversations being monitored, Swiss law enforcement officials have warned.
Investigators are currently pursuing dozens of drug-related cases where Skype has been used for communications purposes. But the firm is keeping quiet over the issue.
Skype software lets computer users talk to each other for free and make cheap calls to mobiles and landlines. But unlike other voice-over-internet services, its calls are heavily encrypted using complex mathematical operations that make them very hard to snoop on.
"Criminals know that the police have difficulties monitoring Skype," said Christoph Winkler, a Zurich prosecutor in charge of drug-related and organised crime in the canton.
The fact that drug dealers use Skype is no accident, he added, as the police frequently have to resort to wiretapping in such cases.
For Bernhard Weder, who heads a federal working group examining how to monitor internet telephone calls, that criminal elements use the service is no great surprise either.
"It's a problem for law enforcement agencies around the world," he said.
Skype is not the first application for encrypted communications on the internet. But its system is particularly complex with calls passing through different servers dotted around the globe.
Skype communications zip around the internet encrypted with "keys" – essentially very long numbers. These numbers are 256 bits long – twice as long as the 128-bit keys used to send credit card numbers over the internet.
In theory Skype's keys would take much longer to crack than 128-bit keys, which are themselves regarded as practically impossible to break.
According to Weder, the programming has resisted every attempt to be deciphered using a so-called "Reverse Engineering" programme.
"The software is extremely cleverly built," he admitted.
Weder believes the company has the possibility of decoding calls and handing them over to the authorities, but claims it is fairly uncooperative. In one particular blackmail case he sought support from Skype, but "never heard back from them".
The firm, meanwhile, rejects the accusation that it is unhelpful.
"The company is doing everything to cooperate at the legal and technical level with law enforcement agencies," Skype claimed in a statement by its PR agency.
But cooperation is still difficult, said Stéphane Esposito, a chief examining magistrate in Geneva.
"Skype has good intentions, but they are outweighed by stronger commercial interests," he added.
The Geneva authorities deal with around a dozen cases a year where criminals do business using the internet service – especially drug deals.
So for now the investigators' only weapons to monitor the criminals are traditional tried-and-tested techniques to eavesdrop conversations – even if they are not via Skype.
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Skype's software lets computer users talk to each other for free and make cheap calls to mobiles and landlines.
Using Voice over Internet Protocol (VoIP) services, computer users can communicate with each other via a headset or microphone and speakers.
Exact numbers of Skype users are hard to come by but the company claims that its software has been downloaded about 200 million times and it is thought about 50 million people regularly use it.
The firm was founded in 2003 by the Swedish and Estonian entrepreneurs who created the Kazaa file-sharing network. Internet auction firm eBay bought Skype in September 2005 for $2.6 billion (SFr3 billion).
Other players in the online phone market include computer giants such as Google, Microsoft, AOL and Yahoo.