Quantum Cryptography (or Quantum Key Distribution)

Today, we are on the edge of a quantum revolution. The advent of quantum computers in the coming years will give mankind access to unparalleled processing power with all the advantages that this brings. However, this tremendous opportunity comes with a corresponding threat. The very same power of the quantum computer will render much of today’s cybersecurity useless. The aim of this short piece is to answer the questions:

How can we address this threat and achieve quantum-safe security?
How can Quantum Key Distribution (QKD) help?

What is Quantum Cryptography (or Quantum Key Distribution)?

Quantum cryptography is a technology that relies on quantum physics to secure the distribution of symmetric encryption keys. A more accurate name for this is quantum key distribution (QKD). It works by sending photons, which are “quantum particles” of light, across optical links. Today, these links are based on optical fibers, with a corresponding distance limitation caused by loss. Work is also underway to leverage trusted quantum satellites to enable end-to-end global coverage.

The principles of quantum physics stipulate that observation of a quantum state causes perturbation. The various QKD protocols ensure that any attempt by an eavesdropper to observe the transmitted photons will indeed perturb the transmission. This perturbation will lead to transmission errors, which can be detected by the legitimate users. This verifies the security of the distributed keys.

Requirements of QKD

QKD implementation therefore requires interactions between the legitimate users. These interactions need to be authenticated. Various cryptographic means can achieve this goal.

The end result is that QKD can utilize an authenticated communication channel and transform it into a confidential communication channel. Note that in classical information theory, a provably secure (also known as Information Theoretically Secure, or ITS) authenticated channel only requires a short secret key, regardless of the amount of data. Meanwhile a provably secure confidential channel requires a secret key, which is as long as the transmitted data. QKD, which precisely provides a provably secure confidential channel with a short initial secret key, is therefore a first example of a quantum advantage, which cannot be achieved classically.

To achieve ITS, QKD should be combined with One-Time Pad (OTP) encryption. However, an OTP requires keys, which are as long as the data to be encrypted and can be used only once. This would impose strong limitations on the available bandwidth because the key distribution rate of QKD is typically 1,000 to 10,000 times lower than conventional optical communications.

Therefore, in practical implementations, QKD is often combined with conventional symmetric encryption, such as the Advanced Encryption Standard (AES), and used to frequently refresh short encryption keys. This is sufficient to provide quantum-safe security.

For more information, read ID Quantique’s white paper, entitled “Understanding Quantum Cryptography.”

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Quantum Cryptography (or Quantum Key Distribution)