**Quantum computing** has progressed rapidly, with companies like Google achieving quantum supremacy in 2019. As we enter 2024, viable quantum computers inch closer to becoming a reality. This has big implications for cryptography and data security. Two major encryption methods **quantum cryptography** and **RSA encryption** are impacted differently by advances in quantum tech.

## How does quantum computing affect encryption?

**Quantum computers** leverage quantum physics to perform calculations exponentially faster than classical computers. Their extra processing power threatens to break widely used encryption standards like RSA, which secures data based on the difficulty of factoring large prime numbers. **Quantum computers** can brute force this much faster. Most experts predict RSA and similar methods will be obsolete in the post-quantum era. More quantum resistant encryption is needed to keep data safe. **Quantum cryptography** provides an intriguing solution.

**What is quantum cryptography?**

Also called **quantum key distribution (QKD)**, **quantum cryptography** uses quantum physics properties like photon polarization to generate shared secret keys between parties wanting to communicate securely. It enables cryptographically secure communication by letting two remote parties create a random secret key known only to them, which they then use to encrypt and decrypt messages.

## How QKD works

QKD protocols like **BB84** exploit special properties of quantum particles to produce a shared random bit string between two users, while detecting any attempts by a malicious third party to gain knowledge of the key. This key can then be used as a one-time pad to achieve theoretically unbreakable encryption.

QKD ensures **future secrecy** if an adversary tries to intercept the key exchange, the laws of physics guarantee they will be detected with high probability. This means if a key is reported as secure today, an attacker will be unable to decipher a message encrypted with that key even given unlimited computing power in the future when powerful quantum computers exist.

## Pros of quantum cryptography

Here are some benefits driving interest in QKD:

### Unconditional security

QKD offers **unconditional** or **future-proof security** resting on the laws of quantum mechanics rather than assumptions about computational complexity. It can withstand attacks from a quantum computer.

### Theoretically unbreakable

Properly implemented QKD used to one-time pad encrypt data is **provably unbreakable** even an adversary with unlimited computational resources cannot decrypt it. This is a very attractive security guarantee.

### Quantum key renewal

Compromised keys are much less issue with QKD since fresh ones are **continuously made** as existing keys are used up, limiting any risk.

### Secure over standard fiber

QKD links have been shown to work over **existing fiber infrastructure** up to 120 km, allowing integration into metropolitan networks. No need for dedicated infrastructure.

### Point-to-point applications

Government, defense, and finance are exploring **point-to-point use cases** like intra- and inter-datacenter connections to secure critical communications. Battlefield deployment has occurred.

## Cons of quantum cryptography

However, there are also downsides constraining QKD:

### Distance limits

Quantum signals **degrade significantly** over distance as fiber loss and environmental disturbances add noise. Links are typically limited to under **300 km**, enough for metro networks but not global communication. Free space and satellite may extend range.

### Key generation rate

Initial QKD demonstration systems had key generation rates under **10 kbps**. While speeds have improved, they fall well below RSA encryption. Limits remain on how much data QKD can encrypt.

### Cost

**Specialized hardware parts** like single photon detectors and quantum random number generators are required. Overall systems are currently much **more expensive** than standardized public key encryption.

### Point-to-point

Most QKD is **point-to-point** between sender and receiver. Networking capabilities allowing key sharing between multiple users are still maturing. This constrains flexibility.

## How does RSA encryption compare?

**RSA** is the most widely used public key encryption scheme. It generates public and private keys based on the challenge of factoring large prime numbers. RSA relies on computational complexity rather than quantum mechanics for its security.

### Broad compatibility

RSA runs on any device, enjoys widespread compatibility across platforms and products, and works at global distances. It is **versatile and accessible**.

### Speed

RSA is **much faster** than current QKD demonstrations, capable of encrypting data on the scale of gigabits per second. It easily handles high bandwidth applications.

### Cost

Using standard computer hardware and software for encryption makes RSA **far cheaper** and easier to scale up than specialised QKD equipment.

### Flexibility

Keys can be freely generated and distributed in **many configurations**. RSA enables encryption between multiple parties, not just point-to-point.

## Which is better for security?

For unconditional future-proof encryption, QKD is vastly superior and offers security RSA cannot match. However, complications around key rate, cost, and distance limits mean QKD complements rather than replaces RSA encryption in the medium term. A hybrid approach maximizes strengths of each. QKD provides **unparalleled security** for the most sensitive data exchanges like government intelligence while RSA affords **convenience and accessibility** for typical business and personal applications.

## Quantum cryptography outlook

Ongoing QKD research targets issues like distance, speed, cost and flexibility through innovations like quantum repeaters, new protocols and network integration. If these challenges are overcome, QKD usage could explode in a post-quantum world as RSA falls vulnerable to attack.

## Conclusion

Quantum cryptography allows two parties to communicate with theoretically unbreakable encryption guaranteed by the laws of physics. While limits around speed, cost and distance remain, QKD offers unmatched security properties in the coming quantum computing age. It is best suited presently for securing highly sensitive communications, whereas RSA encryption retains significant general utility until faster, cheaper quantum networking matures. A balanced approach takes advantage of both technologies.

### FAQs

**Is RSA dead against quantum computers?**

No, but its security will be severely compromised by quantum algorithms like Shor’s that can easily break RSA. Hybrid encryption combining RSA with post-quantum schemes can provide smooth migration.

**How much data can QKD encrypt?**

Early QKD demonstrations were limited to speeds around 10 kbps but systems now exceed 10 Mbps over short distances. Speed/distance tradeoffs remain before QKD matches RSA throughput.

**When will quantum computers break RSA?**

Predictions range from 2030 to beyond 2040 depending on who you ask. But given the pace of progress, prudence suggests organizations start transitioning to quantum-resistant cryptography within the next 5 years.

**Does QKD require special hardware?**

Yes, single photon detectors, quantum random number generators, quantum channels and other custom equipment is needed. This increases complexity compared to ubiquitous RSA implementations in software.

**Can QKD go over the public internet?**

Not yet. QKD links have stayed isolated on dedicated quantum channels due to fragility. Research into quantum repeaters may one day enable long distance key sharing across public networks.