BB84protokollassa

BB84protokolla is a quantum key distribution protocol developed by Charles Bennett and Gilles Brassard in 1984. It is a foundational protocol in the field of quantum cryptography, demonstrating the feasibility of securely distributing cryptographic keys using quantum mechanical principles. The protocol leverages the properties of photons, such as superposition and entanglement, to detect any eavesdropping attempts.

The BB84 protocol typically uses polarized photons. Alice, the sender, encodes bits of a secret key onto photons using two different bases: rectilinear (horizontal/vertical polarization) and diagonal (45°/135° polarization). She randomly chooses which basis to use for each photon. Bob, the receiver, also randomly chooses a basis to measure each incoming photon.

After receiving a stream of photons, Alice and Bob publicly communicate which basis they used for each transmission, but not the actual measurement outcome. They discard any bits where their bases did not match. For the bits where their bases did match, they should have identical key bits. During this process, if an eavesdropper, Eve, attempts to intercept and measure the photons, she will inevitably disturb their quantum states. This disturbance will cause errors in Bob's measurements when he uses the correct basis, which can be detected by Alice and Bob during a subsequent error-checking phase.

If the error rate is below a certain threshold, Alice and Bob can proceed with classical post-processing techniques like privacy amplification to distill a shorter, highly secure secret key. If the error rate is too high, they discard the key and try again, as it indicates the presence of an eavesdropper. The security of BB84 relies on the no-cloning theorem and the fact that observing a quantum system inevitably alters it.