Securing Fiber Using Light Polarization

screenbert writes: "A new and novel way of communicating over fiber optics is being developed by physicists supported by the Office of Naval Research. Rather than using the amplitude and frequency of electromagnetic waves, they're using the polarization of the wave to carry the signal. Such a method offers a novel and elegant method of secure communication over fiber optic lines. This press release has more information. Of course I always thought that fiber was always pretty secure anyway since it's a lot harder to tap than copper."

Comments

More Secure it ain't

[Paul Johnson]

You cannot [measure] the polarization of a wave of light with out changing it.

Thats the theory behind quantum encryption, in which single photons are used to create a shared key by playing tricks with polarisation. The important point is the words "single photon".

However QE cannot work over long distances because photons get lost (i.e. attenuation). General purpose signalling sends a lot of photons so that at least a few get through (I think the detection level for general purpose detectors without special cooling is around 70 photons). They also get amplified. I'm not sure if fibre amplifiers maintain polarisation. If not then this technique is just an interesting novelty.

So tapping would be easy. Just put the signal through a splitter (e.g. a bend in the fibre) and route your half of the signal to a decoder that works in the same way as the official one. The other end sees a 3dB drop in signal, but thats probably too small to be noticed.

Where this might be important is increased bandwidth. At the moment fibre transmission uses binary keying: send photons for 1, no photons for 0. Polarisation modulation means that you could use several different angles, and hence encode more than one bit per light pulse.

But don't get too excited about the bandwidth either. The limiting factor on bandwidth at the moment is the routers at the end of the fibre. We can pump terabits down a fibre in the lab, and 100 Gbit is pretty straighforward to do in the field. But put ten 100Gbit links into a router and you have to have a machine that can switch 1 Tbit. If the average packet is 1.5kbytes (Ethernet frame) then thats around 83 million packets per second. Even with hardware assist thats an awful lot of address table lookups per second.

Paul.