Bitcoin solved the Byzantine Generals problem. Quantum just reopened it.

In 2009, Bitcoin answered an old puzzle. Generals surround a city, they can only communicate through messengers, and some of them may be traitors. Proof-of-work made lying expensive, and digital signatures made every message verifiable, because only General A could have sealed a message in A’s name. For a long time, the solution held, and the puzzle seemed closed.

It reopened because the traitor changed. The old traitor was a dishonest general who had to be outworked. The new one is a quantum computer running Shor’s algorithm, which can derive a private key from a public one. The machine is coming, and a traitor could sign in any general’s name, leaving the blockchain with no way to distinguish the forgery from the original.

The cryptographic fix exists, since quantum-resistant signatures are already known. The unsolved part is the council. Bitcoin has no commander, so the generals themselves must choose the new scheme, persuade millions of holders to move their coins, and decide what happens to the coins whose owners are gone for good. Some of the earliest coins are locked directly to public keys, with nobody left to move them. Freezing them breaks the promise that no one can touch your coins, while leaving them vulnerable allows the first quantum thief to take them. Every path requires trusting someone, and eliminating the need for that trust was the very reason the system was built.

That is the second Byzantine problem, and it is about agreement. The deadline is set by a laboratory, and the generals can only hope to finish arguing before it arrives.