Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It only takes a minute to sign up.
Sign up to join this community
Why does quantum cryptography give us uncrackable codes? What makes it 'uncrackable'? Articles in for example pop science magazines always claim QC produces uncrackable coded, however I highly doubt these claims.
p.s. - Just type in 'quantum cryptography uncrackable codes' and you'll find a ton of hits.
Quantum cryptography ("QC") isn't a method of encryption; it's a method of generating a random shared secret (random bits known to Alice and Bob but not to the eavesdropper). Actually, if Alice and Bob don't already have a shared secret, or at least some way of authenticating each other, then they are vulnerable to a man-in-the-middle attack even if they use QC, so sometimes QC is said to be a way of expanding an existing shared secret to a longer one. You can use the long shared secret as a one-time pad, which is a (classical) provably secure encryption technique, to send the actual message over a conventional communication line.
We already have a way of expanding a short shared secret to a long one, and using it as a "one-time pad", without QC: it's called a stream cipher. The reason people are interested in QC, despite its being vastly slower and more expensive, is that the bits you get from it are true quantum randomness, whereas stream cipher bits are pseudorandom, and no one has ever managed to prove that a one-time pad with pseudorandom numbers is secure (this is related to P =? NP).
There are proofs that an eavesdropper can't learn the random bits produced by QC. However, these proofs rely not only on the correctness of quantum mechanics but also on far less plausible assumptions about the QC hardware and limitations on the actions that the eavesdropper is allowed to take. Real-world QC systems have been successfully attacked by violating those assumptions. So QC is "unconditionally secure if certain conditions are met", and I'm not sure that those conditions can ever be met in reality.
Because the very act of viewing the code changes it. Maybe not uncrackable but any sniffing will be evident because it will be changed. From RSA Laboratories:
Quantum cryptography has a special defense against eavesdropping: If an enemy measures the photons during transmission, he will use the wrong basis half the time, and thus will change some of the polarizations.
As a previous answer mentioned, XORing with a one-time pad is uncrackable, with a high-quality source of randomness. It's not even necessary to use a block cipher. In fact, this is how SIGSALY worked during World War II. It's also true that quantum cryptography is still subject to man-in-the-middle attacks, as are all public-key cryptography systems. There's no guaranteed way to know who you are talking to without a physical exchange from the outset. However, it's entirely incorrect to suggest that quantum systems produce pure randomness. All quantum mechanics guarantees is uncorrelated variables. It's mathematically impossible to actually prove randomness. Classical random fields can very plausibly produce the same behaviors. Since you can cheaply and securely transport an SD card with a small military escort and have a MORE secure system, I find quantum cryptography to be an utterly foolish waste of money.
