The Swiss government has been testing a "white hat" Trojan that can listen in to phonecalls made using VoIP services such as Skype.

Swiss newspaper SonntagsZeitung , reports that the Department of the Environment, Transport, Energy and Communications (UVEK) hired ERA IT Solutions some time ago to come up with a program capable of infecting PCs and tapping conversations without the need to crack VoIP PC-to-PC encryption.

The program - which would have to find its way onto a target computer using some form of Trojan-like deception - would be able to record and save conversations, sending them to a remote server hidden inside normal network traffic.

If the PC was turned off before transmission had been completed, the program is would resend missing parts after the machine was next powered up.

Under Swiss law, using such a program would require judicial permission, but it is still not clear whether the country’s Federal Post and Telecommunications Surveillance Act would make use of VoIP taps submissible as evidence.

The effectiveness of the program will depend on it not being detectable by conventional anti-virus scanners, a style of subversion that is uncomfortably close to how conventional criminal Trojans work. The authorities have already indicated that it will be for official use only and they will not offer details of its operation to security companies.

This does raise an important question about the effectiveness of conventional security systems. If the scanners can’t see this program, can they necessarily see other programs with a similar design? There is growing evidence that a specially-crafted Trojans can beat most scanners, most of the time.

It is also highly unusual for a government to admit it is creating a program that subverts PC security, albeit for what is claimed to be legitimate reasons. The Swiss are the first to admit to such a thing, but they are highly unlikely to be the first government to have used the technique.

Finally, where does this leave VoIP security? Many suggestions have been put forward as to how such software, which employs tight encryption, might be stopped. The Swiss appear to have come up with a neat answer.

So VoIP will from now on offer a high degree of confidentiality only if the host PC in question can be guaranteed not to have been hacked by a Trojan, or can somehow detect the extra traffic that results from it.