Quantum Cryptography: 100km Barrier Broken

jdfox writes "Toshiba Research Europe have just demonstrated quantum crypto over 100km fibre links. Sounds like there's still a fair bit of work to be done before it leaves the lab, but it's amazing that they've got as far as they have. There's another article about it, though still not much technical detail, here on the BBC and here on The Register."

Comments

What is so good about it..

[Gortbusters.org]

Communication with quantum cryptography is inherently secure because it takes advantage of the physical properties of single photons. In the technique, each transmitted bit of a cryptographic key is encoded upon a single photon.

The sender and recipient each have a key to decode the photon stream, but any attempt to hack into the link and capture the key is doomed to failure as it alters the quantum state of the intercepted photons. These changes are easily detectable, revealing the presence of the hacker.

a bit unprecise ...

[Anonymous Coward]

In the technique, each transmitted bit of a cryptographic key is encoded upon a single photon.

Actually it is not completely true, you cannot guarantee that you send out a single photon. Indeed, you don't. You try to approximate a single photon source by using weak laser pulses, but this does not mean you always send out a single photon (sometimes you send out more, sometimes you do not send out any at all). But every security proof consider the fact that you are able to send single photons (which is highly not trivial)

Actually this fact makes most implementations of quantum crypto protocols insecure to a class of attacks (PNS), even though they would take place in a very unrealistic framework (but you have to consider them).

Re:put in a repeater

[aliens]

If I remember my research correctly, you can't sample the photons without changing their state. Thus it's not possible to generate new ones. If it were possible the entire idea would goto shit as a man in the middle could just intercept everything and regenerate new ones without being caught.

Re:That's a big lab!

[mrand]

> That must be a big lab! Or maybe they had 100km of fibre
> and they just looped it round and round and round. ;)

Fiber without the colored "protective insulation" takes up surprisingly little space, and weighs next to nothing. 100km of fiber could be picked up by with one hand if mounted on single spool.

In our lab, we have four fiber spools (two 20km and two 40km) that can be connected together to create various distances. Each is mounted in a plastic case that is about a foot in diameter and 4 inches wide.

Key Distribution

[Luk Fugl]

A description of quantum cryptography resides at Dartmouth (http://www.cs.dartmouth.edu/~jford/crypto.html). The real advantage of quantum cryptography is in the generation of a secret key for use in secret-key encryption (128- or 256-bit or whatever). From the above mentioned site:

"In secret-key encryption, a k-bit ``secret key'' is shared by two users, who use it to transform plaintext inputs to an encoded cipher. . . A key of 128 bits used for encoding results in a key space of two to the 128th (or about ten to the 38th power). Assuming that brute force, along with some parallelism, is employed, the encrypted message should be safe: a billion computers doing a billion operations per second would require a trillion years to decrypt it. . .

"The main practical problem with secret-key encryption is determining a secret key. . . A possible solution is to agree on a key at the time of communication, but this is problematic: if a secure key hasn't been established, it is difficult to come up with one in a way that foils eavesdroppers. In the cryptography literature this is referred to as the key distribution problem. . .

"Quantum encryption provides a way of agreeing on a secret key . . ."

Through the use of random quantum polarizations of the photons and public (unencrypted) discussion of these measurements and their accuracy, the two communicants can determine a shared secret key without an eavesdropper knowing the same info. They then use this key to do standard encryption. A demo of this process can be found here (http://www.cs.dartmouth.edu/~jford/crypto.html).