Mysteries Of Science -- Explained!

2005-01-20 18:06:28

I just love this -- from slashdot, it's the clearest explanation I've ever read of "Quantum Encryption" (reproduced w/o permission and I've cleaned up some typos etc):

Easy explanation of Quantum Encryption -- by tonywestonuk (Score: 5, Informative)

Alice sends Bob a stream of photons. Each photon that is sent, Alice encodes a state of '1' or '0' on each photon.

Unfortunately, due to Quantum Mechanics, Bob only has a 50% chance of actually reading the state of the photon. 50% of the time he gets '0' or '1', and 50% of them time he gets 'Unknown', and the photon is destroyed. This is ok, because after receiving 1 million bits, Bob phones up Alice on an unsecured line and says "I managed to read photon numbers 5, 6, 9, 12, 13, 16 . . . (+ approx 500,000 more), so I will use the state of these photons as a one time pad." Alice looks up the states she sent these photons, and now both parties have a one time pad to encrypt data.

Now, lets say there was an Intruder attempting to intercept the key exchange. The Iintruder is also constrained by QM, and can only read 50% of the photons, with the other 50% destroyed. Because the 50% of the photons the Intruder would receive would be different than the 50% Bob had read, it is impossible for the Intruder to use the information sent by Bob to Alice, via the unsecured phone call, to build an equivalent one time pad.

Also, the Intruder is only able to forward an exact copy of just 50% of the photons to Bob, with the other 50% now destroyed. He could replace this 50% of photons with his own set of random state photons, but this will be detected by Bob and Alice, as the one time pads would be different on this 50%, and the transmitted data using the pads would be corrupted.