Scientists say quantum computers will change the world. With their help, medical researchers could develop cures for cancer, and environmentalists could finally reduce harmful emissions into the atmosphere. However, these benefits may not extend to cryptocurrencies.
What is a quantum computer
Quantum computers are radically different from conventional computers. To understand why they are a threat to Bitcoin, we first need to understand how they work
Classical computers work with bits—zeros (0) and ones (1). Everything your laptop or smartphone does ultimately comes down to combinations of these bits. Quantum computers use qubits (quantum bits). Their key feature is that they can be both 0 and 1 at the same time (a property called superposition). Qubits can also be entangled with each other (quantum entanglement), which makes it possible to process huge amounts of data in parallel.
This means that a classical computer processes options sequentially, while a quantum computer can process many states simultaneously. For example:
2 qubits can store 4 combinations (00, 01, 10, 11) at once.
50 qubits represent over a quadrillion states (2⁵⁰)—a number so large that a conventional PC could not process it in thousands of years.
The computing power of quantum computers opens up many possibilities. In medicine, it enables accelerated modeling of molecules to create new drugs. In logistics, it enables optimization of complex routes. In finance, it enables the analysis of huge amounts of data.
Why quantum computers are dangerous for Bitcoin
Imagine you need to find one specific key in a giant keyring. A classical computer checks them one by one, but a quantum computer can "scan" all at once thanks to superposition. This makes it dangerous for cryptography: algorithms like Shor’s can break ciphers in minutes instead of billions of years.
The second threat concerns mining. Grover’s algorithm allows quantum computers to significantly speed up hash searches. Theoretically, this could lead to a 51% attack, where one user controls more than 50% of the network’s computing power.
However, breaking SHA-256 would require millions of qubits, which is currently unattainable.
Another acute problem is "old" bitcoins. According to Bitcoin Core developer Pieter Wuille, about 7 million BTC (37% of the total supply as of 2019) are stored in addresses with exposed public keys. In the future, quantum computers could compute the keys and steal all these funds.
"Attackers can already collect public keys from the blockchain and then decrypt them when powerful enough quantum computers become available. Here’s how such an attack would work: a public key is disclosed when a transaction is published. While the transaction is waiting for confirmation, a quantum adversary could run Shor’s algorithm, find the private key, and sign their own transaction for the same coins," explained the source.
Nevertheless, Mithus reassures: for now, even the most powerful quantum computer cannot crack Bitcoin encryption. The crypto community has time to prepare.
When a quantum computer will crack BitcoinToday’s quantum computers are still more like scientific experiments than blockchain hacking tools. However, this could change in the next decade.
Current quantum computers (e.g., Google Willow with 105 qubits) cannot crack ECDSA or SHA-256 yet. This requires millions of qubits with high precision.
Today’s quantum systems, like IBM’s Condor (1,121 qubits), operate in extreme conditions—at temperatures close to absolute zero. They also constantly struggle with decoherence (loss of quantum state). A real attack on Bitcoin would require millions of stable qubits (the current record is about 1,000), effective error correction, and practical implementation of algorithms. Shor’s and Grover’s algorithms are only theoretical at this stage.
Experts (Wired, WSJ) still believe a practical quantum computer will not emerge for at least a decade, but the trend is worrisome. We are likely one or two decades away from reaching a 'critical mass' of qubits capable of cracking ECDSA (a public key cryptography algorithm), unless there is a revolutionary breakthrough.
Bitcoin and Ethereum developers are already discussing the transition to quantum-resistant systems. However, this could take years. For now, Mithus recommends:
Abandon outdated address formats (P2PK), where the public key is visible in the blockchain.
Use modern standards (Bech32, P2WPKH/P2TR), where the key is disclosed only when funds are spent.
Never repeat addresses—each new payment should receive a unique address.
ConclusionsSo far, quantum computers are still largely science fiction. However, their development is only a matter of time. As Alex Mithus, "the threat is real, but not immediate." The community has at least 10 years to prepare for the mass introduction of quantum computing.
