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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."
...until polarizaton-based recievers become widespread, anyway.
Security through exclusivity ("It'll be secure, because we're the ONLY PEOPLE who have the hardware to read it!") doesn't work for very long.
Not that it's easy to tap fibers, anyway... Even if you have the equipment, you have to figure out which fiber out of 288 or more is the one you need, and the documentation is usually kept locked up tight.
How do you secure a physical medium from interception? If you intercept a signal, can't you just rebroadcast the same signal back out as long as it was read correctly in the first place? Isn't the real security in the encryption of the data being transmitted over the medium?
"It provides a definite advantage over direct encoding of polarization, leaving an eavesdropper only chaotic static, and no means to extract the signal."
Why the extra security? There's already the depths of the ocean, the difficulty of trying to tap a fiber line, not to mention whatever encryptation they have on their data. They must be looking at some questionable pr0n to go to these lengths.
Job? I don't have time to get a job! Who will sit around and bitch about being broke and unemployed then?
Quantum cryptography uses the polarization of light to transmit provably secure information. The trick is that when you receive polarized light, if you pick the wrong polarization there's a 50% chance that the light will spontaneously flip to that polarization. Thus, unless you know the correct polarization sequence (the key), as you receive the light, you will not be able to intercept the communications under even the best of circumstances.
Q C. htmlr ypto/q uantum1.htm
This isn't exactly new either. Its been around since at least the 70's.
More info:
http://www.cs.mcgill.ca/~crepeau/CRYPTO/Biblio-
http://www.cyberbeach.net/~jdwyer/quantum_c
Because it was harder to tap than string between cans.
Is this the promised end? Or image of that horror? KING LEAR
Of course I always thought that fiber was always pretty secure anyway since it's a lot harder to tap than copper
Boy did you think wrong. The USS Jimmy Carter is being retrofitted just for the purpose of tapping fiber optic cable.
Why go through the trouble of intercepting it at the fibre level when we can just intercept it near their WiFi stations?
The central issue is that in most of the inexpensive single mode fibers, there are random rotations of the polarization state as you transmit light down the fiber.
Moreover those random shifts are time-dependent on account of the physical fluctuations in environment of the fiber optic channel.
That makes traditional polarization modulation difficult to do since the receiver has to dynamically track the unknown polarization matrix correpsonding to the transformation, and that is not easy or inexpensive.
This new method obviates the issue by doing polarization modulation in a distinctly new way, wherein the modulation is in the feedback arm of a chaotic erbium doped fiber ring laser. Changes in the modulation (i.e. message being transmitted) is thus fed back into the dynamics of the transmitter somewhat akin to the state of a cypher (though these schemes are not designed or analyzed to resist cryptanalytic attacks)
There are a few things combined as one then: the production of light in high power (EDRFL), chaotic signal masking by transmitting a high dimensional chaotic state, modulation based on dynamical polarization differences. Also, detection methods for polarization usually require "coherent detection" i.e. interferometry with a coherent source (local laser)---those detectors are much more expensive and difficult than amplitude detectors that measure the short term intensity. Greg has previously shown a technique to use the ampltitude only detectors to nevertheless extract the instantaneous (and not time averaged) polarization state on the Poincare sphere so I expect such techniques to be used in this paper as well.
Just polarization differences via time-delay doesn't work either if you don't have a chaotic underlying carrier as too many things cancel.
I previously collaborated with the two of them on chaotic communication in fiber ring lasers; we derived simulations of the equations of motion and amplitude modulation in the chaotic state. They published experimental results on amplitude modulation in a similar setup before.
Well, er, not exactly.
The technique described in the press release describes a technique for hiding a polarization modulation signal in the polarization state noise inherent in the ring laser system the experimenters used. It's clever, but it's very much not quantum encryption. In principle, it would be possible to siphon a few photons off the fiber and squeeze information out of them, though it would be very difficult. Quantum encryption, as described in the article referenced in the parent post, is a very different technique. It relies on measurements of the polarization states of single photons, not continuous beams. It is immune to (undetected) interception, because tapping the beam irretrievably loses some data (hooray for quantum mechanics.) It is not well-suited to fibre systems--it's difficult to push single photons down a fibre and reliably measure and retain their polarization. It would excel, however, for communcations that could take place over line-of-sight spans, even very long ones.
~Idarubicin
According to Northern
Telecom, tapping a FO cable requires stripping the cable's plastic outer
sheathing and gaining access to the glass fibers within. "When we enter a
fiber bundle, we have instruments that detect whether a given fiber is carrying
a signal before we cut it," North Telecom stated. "A tap could be
accomplished in much the same way."Tapping an optical fiber relies on a macrobending effect. Bending a
fiber 180 degrees around an 1/8-inch radius forces the contained light signal
to go around a tighter bend than it's capable of traversing without some loss
of light. This light loss can be detected and, given the right equipment,
demultiplexed and decoded.
Get it?
HTTP/1.1 400
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.
You are lost in a twisty maze of little standards, all different.