What is Quantum Cryptography & What Does It Promise?

Quantum Cryptography
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The world is moving towards the quantum revolution. Quantum computers give access to unparalleled processing power. However, with all the advantages, we are also faced with unprecedented challenges which the conventional cybersecurity tools cannot help mitigate. However, in this context, the quantum cryptography techniques, or more accurately, the quantum key distribution methods can prove to be beneficial.

Quantum cryptography is different from other symmetric and asymmetric cryptography techniques as instead of mathematics; it relies more on the concepts of physics as a key aspect of its security model. This type of cryptography uses photons (particles/waves of light) and their intrinsic properties to develop an unbreakable cryptosystem. It works by sending photons across an optical link, ensuring high security from hacking.

How Does It Work

Quantum cryptography uses a series of photons to transmit data over an optical fibre channel. The key is determined by comparing the measurements of the properties of a fraction of these photons.

In quantum cryptography, the sender transmits photons through a polariser using one of the four possible polarisations and bit designations — Vertical (One bit), Horizontal (Zero bit), 45 degrees right (One bit), or 45 degrees left (Zero bit). On the receiver’s side, one of the two available beam splitters, horizontal/vertical or diagonal, is used to understand the polarisation of each received photon. This continues till the whole stream of photons is received at the other end. The receiver informs the sender about sequence-wise which beam splitter was used to receive the photon. The sender compares this information with the sequence of polarisers used to send the key. The photons read using the wrong beam splitter are discarded, and the resulting sequence of bits becomes the key.

What makes this cryptography method highly unsusceptible to hacking or eavesdropping is the state of the photon changes if it is read in transmission, and this change is detected at the endpoints. 

Quantum vs Conventional Cryptography

In classical cryptography, the original text is transformed into ciphertext which is then transmitted across a channel controlled by a data string called a key. The receiver will be able to get the original information and decipher to the original text only if he is in possession of this key. Classical Cryptography comes with two main techniques: symmetric and asymmetric cryptography. However, these cryptography techniques face a threat from quantum algorithms. For example, the famous Shor algorithm is capable of breaking asymmetric cryptography techniques such as RSA and Elliptic Curve. Another quantum algorithm known as the Grover algorithm is capable of attacking symmetric cryptography. Quantum cryptography, on the other hand, offers safe key exchanges based on the principle of quantum mechanics. 

Further, the security of conventional cryptography is threatened by weak random key generators, advances in CPU power, new attack mechanisms, and the development of quantum computers. With regard to quantum computers, in particular, such encryption is rendered useless. The data encrypted today can be intercepted and stored for decryption by quantum computers in the future. 

Quantum cryptography offers the advantages of ‘unconditional security’ and sniffing detection. These characteristics can help in solving cyberspace security problems for the future internet and applications such as the internet of things and smart cities.

Comparison With Post Quantum Cryptography

It is often seen that quantum cryptography is confused with post-quantum cryptography, majorly because of how similar they sound. However, both differ widely in principle and application. Post-quantum cryptography offers security against an attack by a quantum computer. It usually utilises public-key algorithms. Its key application areas include secret-key operations, public-key signatures, and public-key encryption to high-level operations such as secure electronic voting. Additionally, post-quantum cryptography is cheaper as compared to quantum cryptography since unlike the latter, the former can be used over many of today’s internet communication without the installation of new hardware.

Challenges Ahead

Despite the security it offers, quantum cryptology also has a few drawbacks. One of the main flaws in the range in which quantum cryptography can provide security. The reason why the length of quantum cryptology capability is short is because of interference. It means that as the distance a photon is required increase, the chances of it meeting other particles and getting influenced also increases.

To prevent eavesdropping, only one photon per time slot is transmitted. That is because, in case of multiple photon transmission, it is possible for an attacker to count the number of photons without disturbing their quantum state; additionally, this can also reveal key information. However, the flip side of having single-photon transmission leads to reduced efficiency, sometimes as low as 15%.

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Shraddha Goled
I am a technology journalist with AIM. I write stories focused on the AI landscape in India and around the world with a special interest in analysing its long term impact on individuals and societies. Reach out to me at shraddha.goled@analyticsindiamag.com.

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