Google Research published updated estimates warning that future quantum computers could break cryptocurrencies sooner than previously thought, though such an event is still several years away.

In a Tuesday blog post, Google explained that most blockchain technologies and cryptocurrencies rely on the 256-bit elliptic curve discrete logarithm problem (ECDLP-256) to secure wallets and transactions.

Citing the latest whitepaper from Google Research, the blog post said that the necessary quantum computing resources to break the ECDLP-256 have decreased significantly. The team compiled two of the latest quantum circuits: one with 1,200 logical qubits and 90 million Toffoli gates, and one with 1,450 logical qubits and 70 million Toffoli gates.

"We estimate that these circuits can be executed on a superconducting qubit CRQC (cryptographically relevant quantum computers) with fewer than 500,000 physical qubits in a few minutes," the blog post said. "This is an approximately 20-fold reduction in the number of physical qubits required to solve ECDLP-256."

Google's white paper added that the reduced execution time allows quantum computers to conduct real-time attacks within bitcoin's average block time of 10 minutes. This enables "on-spend" attacks, which are quantum computing attacks that target active cryptocurrency transactions while they are still in the mempool.

"This is not merely a distant danger to dormant keys; the potential for early fast-clock CRQCs to launch on-spend attacks within bitcoin's 10-minute average block time places active transactions at immediate risk," the whitepaper said.

Targeting 2029

Google Research said the time remaining before the arrival of CRQCs still exceeds that needed to migrate blockchains to post-quantum cryptography (PQC), which is resistant to quantum attacks. However, that margin for error is "increasingly narrow," the team added.

"We urge all vulnerable cryptocurrency communities to join the migration to PQC without delay," the whitepaper said.

Earlier this month, Google announced a 2029 target for a full migration to PQC and suggested that the timeline be shared across the industry. 

Quantum computing poses a significant threat not only to cryptocurrencies but also to a variety of sectors built on current cryptographic standards, including national defense. Google said it has been working since 2016 to prepare for the post-quantum computing transition alongside other industry players taking similar approaches, such as Coinbase and the Ethereum Foundation.

The potential threat to bitcoin is cited as one of the major reasons some long-term investors have exited the crypto market. In January, Christopher Wood, global head of equity strategy at investment bank Jefferies, eliminated a 10% bitcoin allocation from his model portfolio, citing quantum computing risks.