Difference between Classical and Quantum Cryptography

Classical Cryptography

Classical cryptography employs a number of mathematical approaches to prevent eavesdroppers from learning the contents of encrypted transmissions. The most widely used and widely embraced among them are given below. Throughout the article, the transmitter is referred to as 'Alice,' the receiver is referred to as 'Bob,' and an eavesdropper is referred to as 'Eve.'

Quantum Cryptography

It is a kind of cryptography that uses "quantum mechanics". The creation of a quantum channel necessitates the use of polarising filters at both the transmitter and receiver ends. As a result, at the transmitter end, we can send pictures with a certain polarisation and at the receiving end, we can measure the polarisation of photons.

There are two types of polarization filters: rectilinear and diagonal. A rectilinear filter has horizontal and vertical photon orientations, whereas a diagonal filter has 45- and 135-degree photon orientations. Vertically aligned calcite crystal and two detectors such as photomultipliers can detect the two directions.

  • First, Alice creates and sends Bob a series of photons with random polarization (0, 45, 90, or 135 degrees).

  • Bob receives the photons and selects at random whether to measure their rectilinear or diagonal polarisation.

  • Following that, Bob announces the type of measurement he performed (rectilinear or diagonal), but not the measurement outcome for each photon.

  • Alice tells him straight out if he has measured each photon correctly.

  • Alice and Bob then reject any situations in which Bob made an erroneous measurement or his detectors failed to detect a photon.

Quantum Cryptography v/s Classical Cryptography

We will differentiate Quantum Cryptography and Classical Cryptography in the following four dimensions −

  • The Primary Dimension

  • Commercial Dimension

  • Application Dimensions

  • Technological Dimensions

The Primary Dimension

In principle, any traditional private channel may be simply watched invisibly, without the sender or recipient being aware of the eavesdropping.

Classical physics is the theory of macroscopic entities and phenomena such as radio waves that permits a physical characteristic of an item to be measured without interfering with other qualities. Cryptographic key-like information is contained in the computable physical characteristics of a particular item or signal. As a result, passive eavesdropping is a real possibility in classical cryptography.

Quantum theory, which is the foundation of quantum cryptography, is thought to direct all objects, although its effects are most visible in individual atoms or subatomic particles, such as microscopic systems.

  • In classical cryptography, larger keys are frequently required since processing capacity doubles every 18 months and the cost of computation decreases quickly with time [Moore's law]. Thus, a secure method based on a kbit key may become insecure in the future, necessitating constant updates.

  • Security in quantum cryptography, on the other hand, is based on the fundamental laws of quantum physics, thus the possibility of substantial changes in future needs is extremely minimal.

Commercial Dimension

There are existing commercial solutions for Quantum Cryptography; however, they are only suited for point-to-point connections. On the other hand, a crypto chip developed by Siemens and Graz Technical University enables the development of networks with many members, but at a cost of €100,000 per unit, the system is prohibitively expensive and labor-intensive.

Traditional cryptography may be implemented in software at practically no cost to the customer. Furthermore, cryptographic systems based on classical cryptography may be implemented on small hardware components such as smart cards, but in the case of quantum cryptography, shrinking to such a level requires too much research.

Application Dimensions

The digital signatures demonstrate to the receiver the validity of the digital data. A digital signature ensures that the communication was created by a recognized sender and was not altered in transit. Key generation, signing, and key verification are the three primary algorithms. However, we all know that algorithms are difficult to implement in Quantum Cryptography.

As a result, Quantum Cryptography lacks several essential capabilities like digital signatures, certified mail, and so on.

Technological Dimensions

Chinese scientists achieved the world's longest-distance quantum communication transmission, also known as "instant matter transfer technology." Researchers from the China University of Technology and Tsinghua University's Hefei National Laboratory used free-space quantum communication experiments to effectively increase the communication distance to 10 kilometers. However, traditional cryptography can be utilized to communicate across long distances of several million kilometers. According to the most recent study, Toshiba has achieved a new record bit rate for quantum key distribution, averaging 1 Mbit/s. The bit rate of classical cryptography, on the other hand, is determined by processing power largely


Quantum Cryptography is based on a combination of quantum mechanics and information theory ideas. The QC security standard is based on classical information theory principles and the Heisenberg uncertainty principle.

Experiments have shown that keys may be transferred across short distances at low bit rates. Its conjunction with traditional secret-key cryptography methods allows it to significantly increase the secrecy of data transfers. It is clear that quantum cryptography has a clear advantage over classical cryptography, even if several concerns remain unresolved.