Optical fibre is a lot easier to tap than most people imagine. There is no need to break or splice the fibre now - a relatively shallow bend can be enough.

The technique works because the light in the cable propagates by bouncing off the insides of the fibre. Unsheath the cable, and a detector can pick up the tiny amount of light that escapes through the fibre's coating, explained Thomas Meier, the CEO of Swiss company Infoguard.

He demonstrated the technique on a fibre carrying a VoIP phone call over Gigabit Ethernet. A section of fibre from inside a junction box was looped into a photodetector called a bend coupler, and the call was recorded and then played back on a laptop.

"People claim optical fibre is harder to tap than copper, but the opposite is true - you don't even have to break the insulation, as you would with copper," Meier said. "You can read through the fibre's cladding with as little as half a dB signal loss."

He claimed that suitable bend couplers can be bought off the shelf - or from eBay - for a few hundred dollars, and connected to the extra fibre that is typically left coiled up in junctions boxes for future splicing needs.

The demo was to promote Infoguard's encryption devices which it said are 10Gig-capable with a latency of just 5us. The Eur 29,000 (£20,000) boxes encrypt at the network level, Layer 1, so are suitable for point-to-point links only, but Meier said that it means they can encode any protocol, not just IP.

He added that the risk is not imaginary or theoretical - optical taps have been found on police networks in the Netherlands and Germany, and the FBI investigated one discovered on Verizon's network in the US. Networks used by UK and French pharmaceutical companies have also been attacked, probably for industrial espionage, he said.