# Quantum cryptography leaves the lab

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• April 30th, 2007, 11:00 PM
Drakain Zeil
So the real question is how many ways can we make imprints on a photon? xD
Brute force is probably not going to be added off-the-bat. You heard it here first.
• May 1st, 2007, 09:08 AM
Aardpsymon
Ok, the basic principle for quantum encryption relies on polarisation. A single photon vibrates in a single axis. Thus you can encode binary data on to photons by changing the axis of vibration. E.g. Vertical vibration is 1 and horizontal vibration is 0. You then detect the data by passing it through a vertical (or a horizontal) polaroid filter. If the photon is "1" it will pass through the filter and be recieved, if not it will be absorbed by the filter. Thus you build up a sequence of 1's and 0's.

Now the encryption bit. You change between encrypting on + axis and X axis at "random". Using | and / as 1, - and \ as 0. The receiving end knows your sequence of X and +, the key, and so can receive the data. Malice intercepting your data has to play a 50-50 guessing game with his polaroid filters. if he puts in a | filter and recieves a / photon there is a 50-50 chance that the photon will pass through the filter. Thus, every filter he gets wrong gives a 50% chance of receiving the wrong data. Additionally, when he sends the photon on he will encode the photon using the same axis as his filter, giving a 50-50 chance that Bob the recipient will receive the wrong data. In this way not only does Malice not intercept the correct message but Bob is very much aware of Malice's interference since the received message is corrupted.

Perhaps some day we will discover a way of identifying the vibration of a photon without destroying it, I don't deny that. The key point here is that the message is UNINTERCEPTABLE not UNDECRYPTABLE. Once you receive the message its no problem to decrypt it. In fact, its in the clear. But first you have to get the key (so, weakness right there....simply get the key).

Other points:
1) this requires a dedicated fibre line. If you have a dedicated fibre line between two points its difficult to evesdrop anyway. This is not something that gets sent over the net for anyone with a traffic sniffer in the right place to catch.

2) Its a bloody slow method of communicating. Sending single photons is slow especially allowing time for filter changes.

So yes, I do believe it is unbreakable encryption. I don't count getting the key as breaking the encryption. There is no way to "brute force" this. Its a laws of physics thing not a "10,000,000,000 years with the fastest PC ever" thing.
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