Quantum cryptography utilizes the laws of physics, as opposed to mathematical assumptions, to enable the secure exchange of a secret key between two parties. It is considered more robust because mathematical assumptions can unravel with the advent of stronger computing power, whereas physics laws cannot be broken.
For two parties to exchange a secret key, they require a transmission channel on which quantum bits or qubits can be transmitted. In practice, the qubits are usually photons, the elementary particle of light, and the channel is an optical fiber for telecommunication networks or the open air for things like satellite communications.
What makes quantum cryptography powerful is the fact that mere act of reading a qubit, changes its value, which makes any attempt to intercept qubits immediately apparent.
In order for two parties to exchange the qubits that make up the secret key, they engage in an elaborate protocol (e.g. BB84) at the end of which both parties have the key.
Quantum cryptography is not used directly to transmit the secret information – it is used to distribute the random secret key used to encrypt the secret information. Once a key has successfully been transmitted, classical symmetric ciphers such as the one-time pad or AES are used to encrypt and decrypt information.
Quantum key distribution (QKD) is a technique that allows two parties to share a common secret key for cryptographic purposes using quantum bits (qubits).
BB84 is a quantum key distribution scheme developed by Charles Bennett and Gilles Brassard in 1984. It is the first quantum cryptography protocol.
Quantum cryptography can be applied in situations where it is possible to establish a transmission channel on which quantum bits can be transmitted. In most cases qubits are photons, which means the transmission channel needs to be an optical fiber or open air. Applications of quantum cryptography can therefore be found in optical networking and satellite communications.
Elliptic-curve cryptography (ECC)ת an approach to public-key cryptography based on the algebraic structure of elliptic curves over finite fields. ECC requires smaller keys compared to non-ECC cryptography to provide equivalent security.