What is quantum encryption

Quantum encryptionRead out the light of the data

Kloiber: Until four years ago, quantum encryption was considered a safe bet. But then it gradually became known how secret services and cryptography experts of organized crime themselves leveraged this encryption, which was considered unbreakable. Methods to make quantum cryptography secure again are now being discussed on the 31C3. Where do the code breakers start when they want to eavesdrop on quantum-encrypted data, Peter Welchering?

Which ring: Similar to hackers who steal passwords, transaction numbers and other account data in online banking - with the "man-in-the-middle attack". They simply catch the light particles and read them out.

Kloiber: But anyone who intercepts and reads out light particles during quantum encryption changes them and attracts attention.

Which ring: In the quantum cryptographic process, the transmitter sends light particles, known as photons, as quantum bits over the line. Photons have four different polarization states. The receiver measures the polarization states of the light particles, derives a bit sequence from it and compares this bit sequence with the quantum bits originally sent by the transmitter. If a data spy wants to eavesdrop on the line, he has to intercept individual light particles and measure their polarization. This measurement actually changes the light particles.

Kloiber: And what trick do the photon thieves use to deceive the recipient so that he does not notice that something has been bugged?

Which ring: So that the recipient does not notice that the photons have been intercepted and manipulated, the NSA spies really blind the recipient detectors. They send out a very bright glare pulse. Due to the bright glare pulse, the photodiode in the receiving device is under a kind of continuous bombardment. This has made it insensitive to individual photons. It can also no longer recognize the quantum properties of individual photons. The detector in the receiving device is blinded and only works as a normal light sensor. The data spies take advantage of this. They intercept individual photons from the transmitter, reconstruct the quantum key and then send the photons on to the receiving device. And nobody notices that they have been changed because the receiving device can no longer recognize individual photons. It therefore considers the quantum bits sent by the data spies to be the original quantum bits of the transmitter. This means that the data spies have the same key as the sender and receiver, who exchange encrypted data with quantum cryptography, and can read along directly.

Kloiber: And how should that be prevented now?

Which ring: One approach discussed on the 31C3 is to send some kind of negotiation sequence of photons to the receiving device. A test routine then evaluates whether individual photons and their polarization states can be recognized. Then come the encrypted user data. But they are not all sent away one after the other, but are repeatedly interrupted by test data, which checks that no glare pulse has been sent. It is important that the test photons are not sent regularly. Then a data spy could always send a glare pulse when such a sequence of test photons has just been processed. The test photons must be sent and processed in a completely random order.

Kloiber: Has such a security concept been tested with test photons?

Which ring: In two laboratories in Germany and Japan. But that is actually still at the stage of laboratory development. From the idea to the device with the first test setups, so to speak.