X-Message-Number: 8665 Date: Fri, 10 Oct 1997 11:12:18 -0700 (PDT) From: John K Clark <> Subject: TM's, NN;s, QC's, many-worlds -----BEGIN PGP SIGNED MESSAGE----- In #8664 On Thu, 09 Oct 97 Mike Perry <> Wrote: >anyone, I'm looking for some information on the quantum encryption >channel. Even if everyone had a Quantum Computer in their pocket we could still communicate privately because there is a way to make Cryptography as secure as the laws of Physics, and the really amazing thing is that it's practical too. Recently 2 banks exchanged financial information with Quantum Cryptography through a cable under Lake Geneva. The idea is to use Quantum Cryptography to send a One Time Pad of numbers to the person you want to talk to, the message can be detected but NOT without you knowing it was detected. With a random One Time Pad you can generate a perfect encryption scheme, even if to everybody's surprise it turns out that P=NP, even if your opponent has a quantum computer, even if your opponent has a computer of infinite power, he can never read your message. Despite its perfect security it has severe practical limitations, how do you distribute the One Time Pad to the person you want to talk with? You can't send the pad electronically, if your electronic channel is not secure then an eavesdropper can tap your line and get a copy of the pad, if your channel is already secure then you don't need the pad. The only secure method is to physically hand a disk with the pad on it to the person you want to talk to and then hand him another one when that one gets used up. That's not practical in most cases, what is practical is to use Quantum Cryptography, and unlike Quantum Computers we know for sure this will work because it's already been done. This is how: I send you a bunch of photons, each photon is polarized in one of 4 directions, horizontal, vertical, left-diagonal and right diagonal, - | \ / . In this example I send you 10 photons polarized as follows | | / - - \ - | - / . You have a polarization detector, you can set your detector to measure the horizontal and vertical photons (+) OR you can set it to measure the left-diagonal and right diagonal photons (x). The laws of physics do not allow you to measure one photon both ways, because measuring one destroys all information about the other. You set your detector at random, let's say you set it to find rectilinear photons and let's say you have guessed correctly and it really is a rectilinear photon. If you can detect the photon after it passes through your polarized material, you will correctly deduce that it is a horizontal photon. If you can not detect a photon after it hits your polarizing material you will correctly deduce that the photon is vertical. What if you guessed incorrectly when you set your detector, what if you set it to detect a rectilinear photon but I send you a diagonal polarized photon? Then the photon will hit your polarizing material at a 45 degree angle so there is a 50% chance the photon will get through, a 50% chance it will not. In other words you get a random result. I send you 10 photons polarized as follows | | / - - \ - | - / At random you set your polarization detector as follows x + + x x x + x + + So you might claim the photons were polarized as follows / | - \ / \ - / - | Now you tell me over an insecure channel how you set your detector for each photon, Big Brother is free to listen in, it won't help him. I tell you over the same channel which settings on you polarization detector were correct, in this example settings number 2,6,7 and 9 were correct * | * * * \ - * - * We only use those readings and junk the others, and we agree that horizontal and right diagonal photon means 1, and vertical and left diagonal means 0. So we have sent the number 0011 and we can be as certain as we want to be that there has been no eavesdropping. An eavesdropper can not know what type of photon is being sent, and just like you must guess what direction to set his polarization detector. He will be wrong 50% of the time and when he is he will change the polarization of the photon and give himself away. We compare N bits in the string of numbers sent over an insecure channel, if there are no discrepancies then there is only one chance in 2^N that somebody is eavesdropping, so we can use the remaining bits as a one time pad. As I said this has already been done and messages have been sent about 35 miles in this way. I learned about this stuff mostly from Bruce Schneir's wonderful book "Applied Cryptography". >This (the secure channel) is cited by David Deutsch in his recent >book *The Fabric of Reality* Another wonderful book. John K Clark -----BEGIN PGP SIGNATURE----- Version: 2.6.i iQCzAgUBND5qkX03wfSpid95AQHs4QTuPDBRbfhQ2VqsnW4TOMtpJBKrU6jgjUBA WzVqBoAaB1Y6S783iYAuPNfPyAISES7PlyxZlwJp5qoTo7Cp+LSsN3QFuPNfqxUO Xm0pf3ZzStfr6cX16Qiq21vAraMOgegvTxYeeSY2vPJ6omGyR/gUcp7DqkZzWNhi fyqppX8taZfn7ZoFnsEOb4PngqkAlth1wIoCuRJceXRNWYSEEtQ=SuoE -----END PGP SIGNATURE----- Rate This Message: http://www.cryonet.org/cgi-bin/rate.cgi?msg=8665