“Hash algorithms like SHA-256 are actually believed to be very difficult for even the most ideal, biggest quantum computer we can imagine,” Beast said. “We theorize that we would need a quantum computer bigger than the moon to break 256-bit hash-based cryptography using Grover's algorithm.”
“That’s not really what we’re worried about in the next five years,” Beast said. “What we're worried about in the next five years are signatures, and that goes over with Shor's.”
Alex Pruden, chief executive of blockchain cybersecurity firm Project Eleven, described what that would mean.
“Ownership in Bitcoin is entirely conferred by your ability to sign a digital signature,” Pruden said during the panel. “With Shor’s algorithm, just knowing your public key—the thing that’s supposed to be safe to share—is enough to reverse engineer your private key. That means I own your Bitcoin simply by knowing your public key.”
Today’s machines cannot do that. However, Pruden pointed to recent technical milestones by Google, IBM, and others in quantum computing, which could portend further rapid developments ahead.
The discussion comes as the broader crypto industry increases preparations for the day when a practical quantum computer comes online.
“Basically, a third of the supply would be vulnerable to what we call a long exposure attack,” Beast said.
Isabel Foxen Duke, Beast’s co-author on BIP 360, said the problem is not purely technical.
“There are a lot of challenges with Bitcoin and quantum-hardening Bitcoin that have nothing to do with post-quantum cryptography,” she said.
Some older coins, Foxen-Duke, may never migrate to quantum-safe addresses, including those believed to belong to Bitcoin’s creator, Satoshi Nakamoto.
“There are proposals out there to freeze Satoshi’s coins and all pay-to-public-key addresses completely,” she said. “I think those are the more controversial, more complicated, and in some ways more interesting questions, because getting consensus around something like that is going to be an incredibly difficult and politically challenging problem to solve.”
However, she warned that if quantum capability arrives before consensus on migration, it would be catastrophic for the Bitcoin network.
“If 4 million Bitcoin hit the market in a matter of hours once a quantum computer arises and somebody actually takes advantage of it, that’s a potentially Bitcoin‑project‑destroying event, regardless of whether or not we have post‑quantum cryptography,” Foxen Duke said.
