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The Science Behind Unhackable Quantum Communication

For many years, we've been able to communicate via electronic messages really, really securely. Thank asymmetric key cryptography for this, a scheme for encoding messages that is nigh unbreakable. The widely-used and depended-upon secrecy scheme rests...
August 4, 2012, 2:00pm

For many years, we’ve been able to communicate via electronic messages really, really securely. Thank asymmetric key cryptography for this, a scheme for encoding messages that is nigh unbreakable. The widely-used and depended-upon secrecy scheme rests upon a particular relationship between two different keys, one public and one private, and the fact that the relationship between the two is based on math that our present-day computers can’t handle very well (crazy factorizations, specifically).

How it works is that a public key is transmitted between two people, a sender and receiver, and and at its destination, that key is combined with a private key (already in the receiver’s possession) to unlock some message. The only way to unlock that message is to have both keys or a supercomputer that doesn’t exist to crunch some very difficult numbers. This method is absolutely crucial to our present-day information way-of-life and it’s not at all perfect. Enter quantum encryption, the secure communication promised-land.

Perhaps the need for a supercomputer is an overstatement. It’s possible to bust through an asymmetric key algorithm if the key is short enough. The idea, however, is that you make the key very long and, thus, taking possibly hundreds or thousands of years to break through (or a lot less, depending on the message and how time-sensitive it is). Requiring a whole lot less effort than toiling over the math is just getting a hold of/stealing the private key. In some schemes, like online certificate validation, private keys are used over and over again. And the longer one key gets used, the chances of it being compromised go up. And there’s no reason for you the message-sender to have to know about that compromise (and change your private key); that hack remains invisible to you.

And then, of course, there is the specter of even faster computing and quantum computing cutting dramatically the time and effort needed to crack the key. So, for a wide variety of reasons, we need to push encryption tech further.

You simply can’t look at the key with the key remaining intact. It’s kind of perfect.

The future of secure communication involves quantum theory, quantum key distribution (QKD), and the promise of unbreakable encryption. The basic idea is that instead of regular old bits of information sent around via electrons, we’re sending out encryption keys via photons, the particles that make up light and allow most of the interactions between particles that allow us to exist.

Conceptually, the idea is to send out single photons occupying different spin states encoding information, which all combines to make a key. The basic principle that makes this special is that you cannot intercept this message without disturbing it, changing it in some way. It’s that neat-o idea of waveform collapse, that once you observe a quantum system, a collection of probabilities, it collapses into one solution. You simply can’t look at the key with the key remaining intact. It’s kind of perfect.

Imagine you’re hurling a blob of something through zero gravity. It’s just sailing through emptiness without a shape. You need a container to catch the thing and you can’t very well have a blob-shaped container because the blob doesn’t have a static shape. It’s just a blob. So you have a cup or something else that has a definite shape because that’s the only way we really know how to make containers. Just by virtue of being static, that cup is going to alter the shape of the blob. That’s fine; the blob is intended for you and doesn’t have a purpose anymore outside of that cup. Now, imagine you’re hanging out there in zero gravity waiting for the arrival of your blob. It arrives but, what in the hell?, it’s not a blob anymore, but this cup-shaped thing. Someone got to it first.

Sadly, my metaphor doesn’t account very well for encryption and key delivery — the cup seems to imply you’re defining the key at receipt, which is not the case — but hopefully it helps with the general quantum communication concept. And we use this concept everyday and have been for several years but, rather than individual photons, we can only do this with lasers. That’s cool and all, but it’s not perfect. When we use lasers to fire our photos around, we’re really sending beams of photons, which are basically big swarms of particles. One problem with this is the need for “rather bulky laser systems,” in the words of a new paper in the open-access New Journal of Physics that describes for the first time a potential single-photon encryption system.

Via IOP, a schematic of the SPS system

The laser form of QKD is also vulnerable to what’s known as a beam-splitting attack. This is simply where an attacker wishing to read your message shaves off some photons from your laser, not enough to change the luminosity of the laser, but enough to gain access to your encryption key. Obviously, you can’t do this with a single photon system (SPS). You shave anything off and you’ve disrupted the message for the receiver. The researchers, led by Tobias Heindel, describe a system in which two devices, semiconductor nanostructures, give up a single photon upon receiving an electrical pulse. They then used these photon-emitting diodes to send keys across a span of 40 centimeters in a laboratory.

Forty centimeters isn’t a terrible long distance and scaling this technology up poses a big problem for widespread implementation. This would require the installation of repeating towers throughout the communication network to amplify the message and keep it intact. Nonetheless, the team ends its paper saying, “We believe that this successful QKD proof of concept of two different electrically operated semiconductor SPSs can be considered to be a major step toward practical and efficient QKD scenarios.” And now you know a whole lot more about this communications technology about to change everything than 99.9 percent of the world affected by it.

Hopefully this technology will even be legal by the time it deploys. Possibly not.

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