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By | John Leyden 23rd November 2007 12:01

Skype crypto stumps German cops

Police boss attempts to justify Trojan plan

German police have expressed frustration about their inability to decipher the encryption used by Skype in order to tap into the VoIP calls of suspected terrorists.

Lawful interception (or wiretapping) of telephone calls has happened since before the time of rotary phones. In many countries, telecos must promise to allow wiretapping to be granted a license. VoIP services provided by software firms independent of ISPs complicate this picture.

"The encryption with Skype telephone software... creates grave difficulties for us," said Joerg Ziercke, president of Germany's Federal Police Office (BKA) at an annual gathering of security and law enforcement officials. "We can't decipher it. That's why we're talking about source telecommunication surveillance - that is, getting to the source before encryption or after it's been decrypted."

Ziercke's comments are an attempt to justify controversial German plans, yet to be legally approved, to develop "remote forensic software" (AKA a law enforcement Trojan). Proposals to give explicit permission for law enforcement officials to plant malware stem from a Federal Court ruling earlier this year declaring clandestine searches of suspects' computers to be inadmissible as evidence, pending a law regulating the practice.

The idea of a law enforcement Trojan has sparked a fierce civil liberties debate, as well as objections from the IT security community. German police are reportedly looking to hire two "specialists" to develop "white hat" malware. Ziercke's comments provide an insight into the sort of capabilities, such as capturing the raw output of microphones on compromised PCs, that these law enforcement Trojans ought to have.

Ziercke told reporters that it was not asking Skype to divulge its encryption keys or leave "back doors open" for law enforcement authorities, arguing that such requests would leave the eBay-owned VoIP firm at a competitive disadvantage to other services. "There are no discussions with Skype. I don't think that would help. I don't think that any provider would go for that," he said.

Uncrackable Skype?

Skype provides end to end encryption for connections between users, which is not to say that calls are guaranteed to be absolutely confidential.

Security chiefs at the firm talk about providing a "safe communications experience" rather than the rather more robust claims of the likes of, for example, Phil Zimmermann's Zfone project.

Skype uses widely trusted encryption techniques, such as Advanced Encryption Standard to encrypt conversations and RSA for key negotiation. But unlike Zfone, its source code has not been publically released.

In a presentation (pdf) at Black Hat Europe 2006 Philippe Biondi and Fabrice Desclaux argued that without access to the source code we can't be sure if Skype is secure. The researchers also expressed concerns that Skype has the keys to decrypt calls or sessions, a contention the firm itself denies.

Often law enforcement agencies are just as interested in who someone is talking to and for how long. Skype offers confidentiality, but not anonymity.

Last year, however, a fugitive chief exec was tracked down to Sri Lanka after a Skype call. Quite how he was tracked down remains unclear, beyond the availability of papers on tracking anonymous peer-to-peer VoIP traffic.

Spys 'R Us

Ziercke argued that the ability to plant "Trojan horse" spyware on the PCs of suspected terrorists would be an important tool in the armoury of law enforcement officials.

The former East Germany, and the country as a whole before the war, has a dark history of official surveillance. Ziercke said civil rights concerns about the law enforcement Trojan plan were overblown because online searches would be needed only on rare occasions.

"We currently have 230 proceedings related to suspected Islamists. I can imagine that in two or three of those we would like to do this," he said.

Ziercke suggested the ability to conduct online searches of computer hard drives of suspected terrorists are especially important in cases where suspects suspect they are monitored and leave dodgy material on their hard drives. These comments bug the question, not answered by Ziercke, of how effective a tactic emailing custom-built Trojan to paranoid suspects might be in these circumstances.

Would-be terrorists need only use Ubuntu Linux to avoid the ploy. And even if they stuck with Windows their anti-virus software might detect the malware. Anti-virus firms that accede to law enforcement demands to turn a blind eye to state-sanctioned malware risk undermining trust in their software, as evidenced by the fuss created when similar plans for a "Magic Lantern" Trojan for law enforcement surfaced some years ago. ®

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