Quantum Cryptography – One time pad

Alice would like to send Bob a message – say, this picture, for example. But watch out! Unencrypted messages are not secured on the Internet against eavesdropping!

How can data be securely transmitted? There are various methods – the safest, which takes a lot of effort with current technology and is only rarely used yet, is the following: we encrypt the picture in 0 and 1, and add a random pattern Z. The random pattern is the key – it turns every bit around if Z=1, otherwise not. What emerges is a random pattern: Z+B no longer contains any information. Z+B can thus be publicly transmitted without any problem. Only those who know the key Z can turn garbled bits again back to its original form again.

Secure data transmission is therefore possible if Alice and Bob share a secret random pattern Z, the so-called one-time pad. Thus, Alice needs to transmit the random pattern to Bob. It could be said that this would be the starting problem again – so the spy actually needs to eavesdrop on the random pattern in order to be able to decrypt the message.

However, nothing can create random patterns better than quantum physics! In the next slide, we will explain how we can outsmart the spy through the quantum dimension.