QSig: exploring opportunities in quantum cryptography

On January 26, 2024, the University of Edinburgh will host QSig – a novel research event that will gather academics and industry experts to explore an exciting and developing field of computer science. The event, with attendees including IOG chief scientist professor Aggelos Kiayias and the Ethereum Foundation’s Justin Drake, will focus on the applications of quantum computing to cryptography, blockchain science, and financial technology. QSig is inspired by recent discoveries of novel cryptographic primitives enabled specifically by quantum technologies. Among other exciting applications, these address longstanding challenges in blockchain design and security.

Quantum computing was initially recognized as a science for studying complex quantum systems via simulation. The concept was that quantum computers would support a science of quantum simulation, akin to the wildly successful theory and practice of classical (ie non-quantum) simulation. However, an unexpected breakthrough by Peter Shor in 1994 established that quantum computation – when finally realized – will provide extraordinary leverage on purely classical computational problems arising in number theory. A well-known consequence is that scalable quantum computers will break many of our existing cryptographic workhorses, such as RSA encryption and DSA signatures.

A rich seam of research

Research over the last few decades has revealed an even richer relationship between quantum computation and cryptography than these early outcomes suggested. A flurry of recent developments in the field of quantum-enhanced protocols has suggested new and exciting ways in which financial technologies can potentially benefit from quantum technology.

In particular, we now understand that there are natural cryptographic primitives that are simply impossible to construct in the classical setting, but are spectacularly enabled by quantum computing. Among these are perfectly secure quantum key distribution schemes, encryption techniques that permit a recipient to ‘prove that they have deleted a ciphertext,’ and one-shot signature schemes with ‘self-destructing’ private keys. Essential to these remarkable new mechanisms is an unintuitive but fundamental aspect of quantum physics, which asserts that, in general, quantum states cannot be duplicated. At a more abstract level, this ‘no-cloning theorem’ articulates a striking difference between classical and quantum information. The straightforward act of copying information, which is trivial in the classical setting, is forbidden by the basic physical laws that govern quantum states.

To highlight the relevance of these new cryptographic tools in the context of blockchain science, we return to the third example listed above – ‘one-shot’ signature schemes. In principle, these quantum signature schemes comprehensively address the well-studied threat of long-range attacks in proof-of-stake blockchains with a remarkable algorithmic device. Each act of signing a message destroys the (quantum) key used to generate the (classical) signature, while simultaneously generating a fresh (quantum) key to sign the next message. Moreover, while the fresh key can be used for a future signature, it cannot be used to recover old keys. Readers will recognize this as an ideal realization of the classical notion of ‘forward security,’ which calls for periodic evolution of (classical) keys with the explicit requirement that expired keys are deleted. This ensures that old messages cannot be revised and signed even if future keys are revealed. In this sense, ‘one-shot’ signatures capitalize on quantum information theory to yield forward secure signatures that require no additional erasure assumption.

Exploration and optimization

The promise of this new genre of cryptographic tools is complicated by the unpredictable development timeline for robust, scalable quantum computing and communication. Despite impressive engineering advances, we can expect that quantum devices available in the near term will exhibit delicate tradeoffs in terms of memory size, computation length, storage times, and accuracy. These considerations demand a further dimension of exploration and optimization on the part of new cryptographic constructions. In particular, efficiency concerns, specifically those related to the computational aspects requiring quantum devices, are likely to play an expanded role in determining practical feasibility. From this perspective, one-shot signatures have another remarkable feature: while signing and key storage require a quantum computer, the signatures themselves are purely classical messages verifiable by purely classical means.

To conclude, QSig will explore the power of quantum-era cryptography, the prospects and challenges posed by medium-term quantum devices, and the direct relevance of these to blockchain science and financial technology. The goal? To contribute to future research that will ultimately resolve the remaining open questions before quantum-enhanced financial technologies become a reality.

The QSig event (sponsored by Input Output, the Cardano Foundation, and the Ethereum Foundation) takes place on January 26, 2024. For more on the event, you can also watch this video from Charles Hoskinson or this workshop, hosted by Prof. Aggelos Kiayias and Justin Drake.