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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."
This article (did you read it?) doesn't have anything to do with security through exclusivity. The "signal" is encoded in the chaotic "noise" that occurs in a light "circle" and that noise is subtracted from the total received communication at the receiving end to come up with the "signal" again. The researchers have come up with what I would call a type of quantum interference encryption using light (instead of quantum particles). The encryption exists in the chaos of the system rendering the signal received by an eaves dropper useless.
There's alot more fiber out there than you may think. Any cable TV system newer than 1995 or so consists of more fiber than copper by the distance the signal travels.
Very new systems are quite literally fiber to the curb.
Were it not for the expense involved in termination (and the precision required), fiber into the home would be feasible.
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
This method is secure because you cannot intercept the signal and still. With standard light techniques it is possible to place yourself as the "man in the middle", intercept the stream of light and re-broadcast it though the fiber. Using polarization as the encoding technique this is not possible because the system can be designed so that you cannot guess exactly what is the exact polarization of the bit you just received, and so you cannot re-broadcast it adequately.
Simon Singh in its book "The Code Boob" has a interesting explanation of one such system; it is tool lengthy to quote here (and I don't have the book with me now) but I highly recommend reading it.
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.
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
"50% chance of picking the wrong polarization"
Who said anything about a 50% chance? If your detector can have a semicircle resolution of, say, 100 degrees, then you only have a 1% chance of guessing the right polarization. 1% * 50% = 0.5%, and as other posters stated, if you don't know the sequence, that means that you have a 0.5% chance of getting EACH bit right, so your entire chances of getting a complete message are almost nil.
And as time marches on, the resolution can only increase...