A Simple Security Proof for Entanglement-Based Quantum Key Distribution

Quantum cryptography exploits the quantum mechanical properties of communication lines to enhance the security of the so-called key distribution. In this work, we explain the role played by quantum mechanics in cryptographic tasks and also investigate how secure is quantum cryptography. More importantly, we show by a simple security proof that for any state sent by the sender, the eavesdropper can only guess the output state with a probability that will allow her not to learn more than half of the classical Shannon information shared between the legitimate parties. This implies that with high probability, the shared key is secure.