Quantum Key Distribution, or QKD, is a way of sharing or exchanging encryption keys between two or more parties securely, using properties found in quantum physics.

QKD is not the message being shared or the encryption key itself but the method of distributing that key securely. Domen Zavrl has taken courses on cryptography at Stanford University and therefore understands quantum-proof cryptography and its role in security.

**Public Key Ciphers Vs. QKD**

Prior to the advent of QKD many cryptographers used public key ciphers. In fact, these are still widely used today. However, many of these public key encryption strategies will eventually become unsafe as we gradually shift to quantum computing. With a public key cipher, the amount of processing power required to break the underlying complex mathematical calculations is prohibitive to most. While this has been effective, there are potential issues caused by advances in computing power, weak random number generators and new strategies being implemented to attack public key cipher systems.

The PDF attachment looks at three types of encryptions that are currently widely used.

**Understanding Quantum Computing**

In traditional computing information is stored in binary, with 1s and 0s used to encode information in bits. Quantum computing, on the other hand, carries information using qubits. Within each qubit, the 0s and 1s are engaged multidimensionally.

Quantum computers consume less energy while retaining the capacity to work at far higher speeds than classic computers. The quantum information contained within a qubit is placed in a state of superposition. When groups of qubits are in superposition, this generates highly complex computational states across multiple dimensions. Qubits can also become entangled, which means a pair of qubits can exist in a single state.

The result of this is that any changes made to one qubit will also affect the second of the pair, and we can predict what this effect will be. In a classic computer, if you double the number of bits, the processing power will double accordingly. In a quantum computer, if you double the number of qubits through entanglement, the processing power can increase exponentially.

**Understanding QKD**

Quantum key distribution works by transmitting millions of photons, which are polarized particles of light, from one entity to another using a fibre optic cable. Collectively, these photons create a stream of 0s and 1s, but each individual photon has a random quantum state. At the receiver’s end, the photons are read using a beam splitter, which can be diagonal or horizontal and vertical.

The receiver has to guess which beam splitter to use for each photon. They then tell the sender which was used and what the sequence was so this can be compared to what was sent. Any photons identified as having been split in the wrong direction are discarded, while the remaining sequence becomes a unique optical key for the encryption of data.

**The Security of QKD**

The key element of the unbreakable security of QKD is that any attempt to interfere with the photons will alter their state in a way that is immediately detectable to both the sender and the receiver. They will be aware the key has been tampered with and should therefore be discarded before creating a new key. QKD has no vulnerability to increased computer processing power, which can make hacking public key ciphers more easily possible.

Some of the real-world applications of QKD are detailed in the infographic attachment to this post.

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