• Cryptography Tutorial

# Quantum Cryptography

Last Updated : 21 Mar, 2023

The uncertainty principle of quantum physics builds the earliest foundations for quantum cryptography. With quantum computers of the future being expected to solve discrete logarithmic problems and the popularly known cryptography methods such as AES, RSA, DES, quantum cryptography becomes the foreseen solution. In practice, it is used to establish a shared, secret and random sequence of bits to communicate between two systems let’s say, Alice and Bob. This is known as Quantum Key Distribution. After this key is shared between Alice and Bob, further exchange of information can take place through known cryptographic strategies.

Based On Heisenberg’s Uncertainty Principle:

BB84 and variants: A single-photon pulse is passed through a polarizer. Alice can use a particular polarizer to polarize a single-photon pulse and encode binary value bits to the outcome of a particular type (vertical, horizontal, circular, etc) of a polarizer. On receiving the photon beam, Bob would guess the polarizer, and Bob can thus match the cases with Alice and know the correctness of his guesses. If Eve would have been trying to decode then polarization by Eve’s polarizer would have caused discrepancies in the match cases of Bob and Alice and thus they would know about eavesdropping. Thus in such a system if Eve tries to eavesdrop it will get to the notice of Alice and Bob.

• The B92 protocol has only two polarization states unlike the 4 in the original BB84.
• BB84 has a similar protocol SSP that uses 6 states to encode the bits.
• SARG04 is another protocol that uses attenuated lasers and provides better results than BB84 in more than one photon system.

Based On Quantum Entanglement:

E91 and Variants: There is a single source that emits a pair of entangled photons with Alice and Bob receiving each particle. Similar to the BB84 scheme Alice and Bob would exchange encoded bits and match cases for each photon transferred. But in this scenario, the outcome of the results of the match cases of Alice and Bob will be the opposite as a consequence of the Entanglement principle. Either of them will have complement bits in bit strings interpreted. One of them can then invert bits to agree upon a key. Since Bell’s Inequality should not hold for entangled particles thus this test can confirm the absence of eavesdroppers. Since practically it is not possible to have a third photon in entanglement with energy levels sufficient for nondetect ability, thus this system is fully secure.

• SARG04 and SSP protocol models can be extended to Entangled particles theory.

### Possible Attacks In Quantum Cryptography:

• Photon Number Splitting (PNS) Attack: Since it is not possible to send a single photon thus a pulse is sent. Some of the photons from a pulse can be captured by Eve and after matching of bits by Alice and Bob, Eve can use the same polarizer as done by Bob and thus get the key without being detected.

• Faked-State Attack: Eve uses a replica of Bob’s photon detector and thus captures the photons intended for Bob and further passed it to Bob. Though Eve knows about the encoded bit, Bob thinks that he received it from Alice.

### How does Quantum Cryptography work?

Quantum Cryptography works on the principle of quantum entanglement, which is a phenomenon where two particles are correlated in a way that the state of one particle affects the state of the other particle, even when they are separated by a large distance. In quantum cryptography, the two parties, Alice and Bob, use a pair of entangled particles to establish a secure communication channel.

The process involves the following steps:

1. Alice sends a stream of photons (particles of light) to Bob.
2. Bob randomly selects a subset of photons and measures their polarization (direction of oscillation).
3. Bob sends the result of his measurements to Alice through a classical communication channel.
4. Alice and Bob compare a subset of their measurements to detect any eavesdropping.
5. If no eavesdropping is detected, they use the remaining photons to encode their message.
6. The encoded message is then sent over a classical communication channel.

### Why is Quantum Cryptography secure?

The security of Quantum Cryptography relies on the fundamental laws of quantum mechanics. Any attempt to intercept or measure the photons during the transmission would disturb their state, and the disturbance would be detected by Alice and Bob, alerting them to the presence of an eavesdropper. This is known as the “no-cloning theorem,” which states that it is impossible to create an exact copy of an unknown quantum state. Therefore, the security of the communication channel is guaranteed by the laws of physics, making it impossible to hack.

### Applications of Quantum Cryptography

Quantum Cryptography has the potential to revolutionize the way we communicate by providing a secure communication channel that is immune to cyber-attacks. Some of the applications of Quantum Cryptography include:

• Financial transactions: Quantum Cryptography can provide a secure communication channel for financial transactions, making it impossible for cybercriminals to intercept and steal sensitive financial information.
• Military and government communication: Quantum Cryptography can be used by military and government agencies to securely communicate sensitive information without the fear of interception.
• Healthcare: Quantum Cryptography can be used to secure healthcare data, including patient records and medical research.
• Internet of Things (IoT): Quantum Cryptography can be used to secure the communication channels of IoT devices, which are vulnerable to cyber-attacks due to their low computing power.

### Challenges of Quantum Cryptography

While Quantum Cryptography is a promising technology, it is not without its challenges. Some of the challenges include:

1. Cost: Quantum Cryptography is an expensive technology that requires specialized equipment and infrastructure, making it difficult to implement on a large scale.
2. Distance limitations: The distance between the two parties is limited by the attenuation of the photons during transmission, which can affect the quality of the communication channel.
3. Practical implementation: The implementation of Quantum Cryptography in real-world scenarios is still in its early stages, and there is a need for more research and development to make it more practical and scalable.

Unconditional security: Quantum cryptography provides unconditional security, which means that it is impossible for an eavesdropper to intercept or copy the data being transmitted without being detected.

Key distribution: Quantum cryptography can be used for secure key distribution, which is an essential component of many encryption algorithms.

Speed: Quantum cryptography can provide secure communication at very high speeds, which is important for applications that require real-time data transfer.

Long-term security: Quantum cryptography is resistant to attacks by future quantum computers, which means that data encrypted using quantum cryptography will remain secure even in the future.

Verification of security: Quantum cryptography provides a way to verify the security of the communication by detecting any attempt to intercept or tamper with the data.

Cost: Quantum cryptography can be expensive to implement due to the need for specialized hardware and software.

Distance limitations: Quantum cryptography has distance limitations due to the nature of quantum entanglement, which means that it is currently limited to short-range communication.

Complexity: Quantum cryptography is a complex technology that requires specialized knowledge and skills to implement and maintain.

Key distribution limitations: Quantum cryptography is limited by the need for a trusted third party to distribute the cryptographic keys, which can be a potential weakness in the system.

Vulnerability to side-channel attacks: Quantum cryptography is vulnerable to side-channel attacks, such as attacks on the hardware or software used to implement the system.

Conclusion: Quantum Cryptography has the potential to revolutionize the way

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