Quantum Cryptography — BB84 Key Distribution

This simulator runs the real BB84 protocol bit by bit: Alice picks a random bit and a random polarization basis for every photon, Bob measures each one in his own randomly chosen basis, and the two sides publicly compare only their basis choices to sift out a shared secret key. Turn on the eavesdropper to watch an intercept-resend attack push the quantum bit error rate up to roughly 25% — the same statistical fingerprint that lets real QKD hardware detect a listener on the line without ever transmitting the key itself.