The BB84 Quantum Key Exchange Protocol Explained

#cryptology, #cryptography, #cryptanalysis

The BB84 is a key exchange protocol for establishing a secret key between two parties (e.g. Alice and Bob) securely. In contrast to other (common) protocols like Diffie-Hellman-Merkle or RSA, the BB84 protocol's security is not based on hard mathematical problems. Its security is based on actual physical properties of light (quantums). Due to that, Alice and Bob exchange a key using light and they can measure if an attacker (Eve) is also listening and receiving the key while they exchange it. If they detect and attacker, they change their communication channel and restart the protocol.

In this video, we explain how the BB84 protocol works. We also have a look at its security, so what happens when an attacker is present. Finally, we have a look at the implementation of a simulation of the BB84 protocol in CrypTool 2.

[dilrubasharmeenahmed]

Outstanding effort and explanation! Thank you

[eswarilekhyajarajapu]

How does Bob get exactly half the no of bits right? Why not more or less than that? We know there’s half half of chance for the photons to pass through but there’s no rule that anything with half probability will take place exactly half the no.of times. For example, if we a toss a coin 2n times there’s no rule that I would get exactly n heads and n tails even though if the chance is 50 percent each. Please explain

[notknown]

we see here that only few parts of information has been received by Bob so how shall we ensure that the complete information that Alice wants to send will reach at its destination

[hanxuanchen]

Great Video. While I still got one problem - How can Alice and Bob calc their error ratio? If they need to calc the ratio, then both of them need to send the final key they got to compare and see the difference, but then it may also lead to the key leak?

[viaprenestina]

Maybe horizontal is 0 degree and vertical 90?