# How does Quantum Key Distribution (QKD) work?

## Key distribution in cryptography

The need for secure communication is as old as we can recall. The first cryptographic device used by the Spartans is dated circa 600 BC. Cryptography is the science of encoding a message, containing confidential information, so that only the recipient can read it. Over the years, various techniques have been developed to encode messages. One example is the famous ENIGMA cipher used during the Second World War and cracked  by a team of researchers leaded by Alan Turing. The only encryption method providing unconditional security is the one-time-pad(OTP) proposed by Gilbert Vernam.  Its implementation requires i) sender and receiver to share a common cryptographic key and ii) the key must be as long as the message itself.  The OTP method is, unfortunately, hard   to implement because of the difficulty in generating long keys and distribute them.

Nowadays, practical implementations of secure communication are based on public-key encryption techniques, which are based on the difficulty to solve special mathematical problems and therefore the decryption of encrypted data would take longer than the time the contained information is of value. The most famous implementation is the RSA (Rivest, Shamir, Adleman) crypto-system which uses the difficulty of decomposing a large number into its prime factors.

## What is Quantum Key Distribution (QKD) technology?

The advent of working quantum computers poses a serious threat to current encryption methods, e.g. the implementation of Shor algorithm could render RSA useless in a really short time. This has inevitably led us to look for alternative ways to encrypt data with a higher degree of security. Provable secure crypto-systems (e.g. OTP) rely on the sharing of a secret key between the sender  and receiver  to encrypt messages.

Quantum cryptography provides a way to share a secret key between two parties securely and to know that no one has intercepted it along the way. This is guaranteed by the no-cloning theorem, which formalize that arbitrary unknown quantum state cannot be perfectly copied. When it comes to quantum cryptography, it usually has to do with Alice (A) and Bob (B) whishing to exchange private information, while the eavesdropper Eve (E) is trying to steal the message without being discovered. The main goal of quantum cryptography is to provide a method to detect Eve’s activity.

In 1984 the seminal work of Bennett and Brassard (BB84) paved the way to practical implementation of Quantum Key Distribution (QKD), which has then been widely studied and improved over the years. The traditional implementation of the BB84 protocol requires Alice to use four  non-orthogonal quantum states (e.g. different polarisations of a photon) randomly selected from two bases ({H,V}, {D,A}). A sufficiently long sequence (i.e. more than twice the amount of bits the resulting key should have) of these random states is sent on a quantum channel that Bob detects using one of the two bases in a random manner. Using the public channel, Bob and Alice compare the used bases without revealing the results, identify the coincident bases (sifting) and estimate the quantum bit error rate (QBER) of the detected values compared to the values sent by Alice. Owing to the non-cloning theorem, any operation performed by Eve along the quantum channel results in an increase of the QBER.

In general, QKD is combined with conventional encryption systems, such as AES, where the generated QKD key is used by Alice and Bob to generate temporary session key with AES, to encrypt their messages over Ethernet until it expires.

## QKD protocols and uses

QRNG is a fundamental component of a QKD system. It exploits quantum physics principles to generate true unpredictable bits used for the secret keys, which are then transmitted according to the selected QKD protocol.  Unpredictability means that knowing the generation principle and the state of the device in any point in time, you are still unable to predict anything about the next produced random number.

At Quside we have developed a QRNG product family, based on our proprietary photonic integrated chip (PIC) technology to reach scalable and affordable quantum-security today. Our solutions allow using standard electronic components and manufacturing processes which assure cost-effectiveness and industrial scaling-up. Moreover, its high speed (multiple Gb/s) and available sizes make it suitable for any market.

Domenico Tulli

Co-founder & CTO

Domenico leads the technology strategy, space initiatives and EU founded projects of Quside. With more than 15 years of experience on integrated photonics, he holds an Telecom. Engineering degree from Bologna University (2006), a Ph.D in Photonics from ICFO (2012) and a MBA from the TPMBA school (2022).

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