CQT experts to consult on quantum security of cryptocurrency

Miklos Santha (left) and Troy Lee (right), Principal Investigators at CQT, and their collaborators are providing advice on the quantum security of cryptocurrencies.

Researchers from the Centre for Quantum Technologies (CQT) at the National University of Singapore (NUS) have been retained as consultants on the security of cryptocurrency.

The market for such digital currencies, which began with Bitcoin in 2009, is currently worth over US$150 billion.

CQT's Troy Lee and Miklos Santha will advise cryptocurrency provider Hcash on how to protect the company's digital currency against future attacks by quantum computers. Its currency Hshare had a market capitalisation of over US$300 million as of 3 November 2017.

"It's important for anyone who protects their data or money with cryptography to be prepared for quantum computers. I am very excited to be working with Hcash, which is taking quantum security seriously and building it into the design of its coin," says Troy.

The scientists will consult for Hyperchain, which provides technical services to Hcash, along with collaborators Gavin Brennen from Macquarie University, Sydney and Marco Tomamichel from the University of Technology Sydney, Australia. They offer expertise in cryptography and quantum computing.

Troy and Miklos are both Principal Investigators at CQT, NUS. Troy is also at the Nanyang Technological University in Singapore, and Miklos at the French national research organisation CNRS.

The researchers began discussions with Hcash as they worked on a detailed analysis of the threat that quantum computers will pose to Bitcoin and other cryptocurrencies. That analysis, completed 28 October, is now available as a white paper "Quantum attacks on Bitcoin, and how to protect against them" at https://arxiv.org/abs/1710.10377.

CQT Principal Investigator Divesh Aggarwal is also an author of the white paper but is not part of the consulting team.

Today's small quantum computers are not a risk, but the industry is moving fast. Companies including IBM and Google are rapidly increasing the size of their machines. Algorithms already exist that, if run on big enough quantum computers, will crack many of today's cryptography schemes.

In the white paper, the researchers estimate the speed of the quantum algorithms and project developments in quantum computing technology to put a timeline on when cryptocurrencies could become insecure – warning that the signature scheme used by Bitcoin could be broken within a decade.

The signature scheme verifies ownership of digital coins. The team also analysed the 'proof-of-work' step used to record Bitcoin transactions, concluding that quantum computers are unlikely to undermine this in the near term.

The Bitcoin signature scheme is based on 'elliptic curve' cryptography that a quantum computer could crack using Shor's algorithm. If a signature is cracked, the hacker can spend the coins, stealing them from the rightful owner.

"The main quantum bottleneck is having a quantum computer with enough qubits to run Shor's algorithm on the scheme used by Bitcoin. We estimate it would take about 500 thousand to 1 million qubits. By the most optimistic estimates, in 10 years the signature scheme of Bitcoin could be cracked in under 10 minutes by a quantum computer," explains Troy. Other cryptocurrencies that use similar security schemes will be vulnerable, too.

In the white paper, the team review alternative signature schemes proposed as quantum-safe. The researchers' work for Hcash will include recommending specific protocols to incorporate to achieve quantum security.