Quantum Cryptography Beyond Quantum Key Distribution (1510.06120v2)

Abstract: Quantum cryptography is the art and science of exploiting quantum mechanical effects in order to perform cryptographic tasks. While the most well-known example of this discipline is quantum key distribution (QKD), there exist many other applications such as quantum money, randomness generation, secure two- and multi-party computation and delegated quantum computation. Quantum cryptography also studies the limitations and challenges resulting from quantum adversaries---including the impossibility of quantum bit commitment, the difficulty of quantum rewinding and the definition of quantum security models for classical primitives. In this review article, aimed primarily at cryptographers unfamiliar with the quantum world, we survey the area of theoretical quantum cryptography, with an emphasis on the constructions and limitations beyond the realm of QKD.

An Overview of "Quantum Cryptography Beyond Quantum Key Distribution"

In "Quantum Cryptography Beyond Quantum Key Distribution," Anne Broadbent and Christian Schaffner provide an extensive survey of the landscape of theoretical quantum cryptography, emphasizing cryptographic constructions and limitations that extend beyond the more widely recognized quantum key distribution (QKD). Through this analysis, the paper brings to the forefront various cryptographic primitives enabled by quantum mechanics, such as quantum money, randomness generation, secure multi-party computation, and delegated quantum computation. It also explores the limitations posed by quantum adversaries, including the challenges of creating quantum security models for classical cryptographic primitives.

Key Concepts and Contributions

The paper underscores several key areas where quantum cryptography extends beyond QKD. It explores both the opportunities and limitations of leveraging quantum information in cryptographic protocols:

1. Quantum Cryptographic Primitives: The authors discuss the potential of quantum cryptography to perform tasks like secure multi-party computation and randomness generation, which might be infeasible using classical means alone. Notably, the paper presents quantum-secured variants of cryptographic primitives such as quantum money, bit commitment, and oblivious transfer.
2. Quantum Constructions Based on Conjugate Coding: Broadbent and Schaffner highlight how many quantum cryptographic protocols are rooted in the principle of conjugate coding, where classical information is encrypted using conjugate bases in quantum mechanics. This foundation is critical in protocols like quantum money and secure two-party computations.
3. Impossibility Results and Quantum Limitations: A significant portion of the paper is devoted to examining the constraints imposed by quantum mechanics on cryptographic protocols. For example, it addresses the impossibility of achieving information-theoretically secure quantum bit commitment or two-party quantum computation without additional assumptions.
4. Advanced Quantum Cryptographic Protocols: The authors explore more complex protocols such as device-independent quantum cryptography, which leverages quantum non-locality to enable secure cryptographic operations even when the devices used are untrusted. Additionally, they explore delegated quantum computations, wherein quantum computations are securely outsourced to more powerful quantum processors.
5. Implications and Future Directions: The survey speculates on future developments in quantum cryptography, emphasizing the need for continued exploration of quantum cryptographic schemes and the proof techniques needed to ensure their security. The potential practical applications and challenges in real-world implementations are also considered, particularly given current limitations in quantum technology.

Noteworthy Findings and Theoretical Implications

Throughout the paper, Broadbent and Schaffner meticulously dissect the theoretical underpinnings of quantum cryptography. They highlight the nuanced interplay between quantum mechanics and cryptography, providing detailed insights into the areas where quantum methods confer advantages over classical techniques. For instance, the usage of entanglement and the no-cloning theorem are pivotal quantum features that facilitate innovative approaches to information security that have no classical analogue.

Moreover, the authors address the existing gaps and limitations, provoking a deeper discussion on the boundaries of what is achievable under quantum mechanics. The paper points to the need for assumptions about adversarial capabilities and resource constraints, demonstrating that quantum security is inherently complex and fraught with subtleties.

Conclusion

"Quantum Cryptography Beyond Quantum Key Distribution" is an insightful exploration of the theoretical expansion of quantum cryptographic techniques beyond QKD. Broadbent and Schaffner provide a thorough survey that is invaluable for researchers in the field, articulating both the potential and limits of quantum cryptography. This work invites further exploration into novel quantum cryptographic protocols, their security models, and eventual practical applications, offering a roadmap for future research and development within this innovative intersection of quantum information science and cryptography.