Bitcoin Developer Wants Quantum Hackers To Risk Their BTC

Bitcoin Core developer Cory Fields proposes making any quantum theft financially risky thanks to cryptographic “mines” and a security deposit. However, this mechanism reveals a deeper problem. The network would not be able to distinguish a quantum hacker from the true owner of an old wallet.

The trap does not recognize the thief from the owner

The scenario relies on bitcoins whose public key is already visible on the blockchain. A quantum computer powerful enough could, in theory, start from this public key to find the private key allowing spending the funds.

Cory Fields proposes changing the rules after the appearance of a credible quantum threat. Anyone wishing to move bitcoins held in an old format would have to provide a guarantee equivalent to one from a wallet resistant to quantum attacks.

The envisioned operation can be summarized in several steps :

• Before “Q-Day”, the owner creates a quantum output containing a secret proof of possession of their old key ;
• After “Q-Day”, moving 1 vulnerable BTC would require providing 1 additional BTC as a guarantee ;
• The 2 BTC would be locked for a period up to thirty days ;
• The initial owner could reveal their proof during this period ;
• They would then recover their bitcoin and seize the hacker’s guarantee ;
• Without a valid dispute, the funds would be released after the deadline.

The “mine” would remain invisible until activation. An attacker could therefore not know if the wallet they target contains a prepared proof. To steal 1 BTC, they would risk losing their own bitcoin provided as a guarantee.

Fields estimates that this uncertainty could discourage attacks. The system would even encourage some owners to deliberately leave vulnerable funds to attract a hacker and recover their deposit.

The problem is that the blockchain does not know the real identity of the person signing a transaction. After a quantum breakthrough, both the true holder and the hacker could produce a valid private key. The protocol would therefore have to apply the same guarantee requirement to both.

Protecting an old bitcoin might require owning two

This symmetry creates a significant obstacle for honest holders. A person holding all their savings in an old 10 BTC wallet would have to find 10 additional BTC, already placed in a quantum-secure format, to move their funds.

The mechanism would thus protect ownership by temporarily imposing a doubling of the required capital. Large holders and platforms could probably provide this guarantee. Individuals, heirs, or owners returning after several years might remain stuck.

The preparation of the “mine” must also happen before quantum computers are capable of breaking current keys. It would therefore not serve people who have already lost their private key nor holders who died without transferring access.

This limit notably concerns old bitcoins attributed to Satoshi Nakamoto and many inactive wallets. For these funds, Fields mentions auctions where participants would pre-place hidden bids on vulnerable outputs.

When a hacker tries to move these bitcoins, bidders could recover all or part of their guarantee. The coins originally targeted could, however, be destroyed without returning to their historic owner. The goal would no longer be to return the funds, but to make theft economically pointless.

This choice raises a sensitive question: should the network allow an attacker to seize coins accessible with a valid signature, or accept their destruction to protect the system’s credibility? The answer goes beyond cryptography and directly touches property rights on Bitcoin.

A governance battle before a single line of code

The proposal is neither deployed nor formalized as an improvement ready for adoption. Fields explicitly presents it as a series of “thought experiments” and leaves the description of mechanisms necessary for activation via soft fork to later.

Several indispensable elements do not yet exist. Bitcoin would first need a standardized quantum output format. Then the time when old transactions become subject to new rules, the exact amount of the guarantee, and the duration of waiting periods must be defined.

Auctions raise other difficulties: distribution of seized guarantees, treatment of losing bids, possible remuneration of miners, and criteria authorizing destruction of vulnerable bitcoins.

Adoption would above all require sufficiently broad agreement within the ecosystem. Freezing, slowing, or conditioning the spending of some bitcoins would change the rights their owners currently enjoy. Even implemented by soft fork, such a change would be politically delicate.

Fields’ proposal thus offers less a ready-made solution than a change in method. Rather than only seeking a new quantum-resistant signature, it uses game theory to make the attack too costly.

While the United States accelerates on quantum technology, its main merit is also its weakness. It shows it is possible to trap a hacker without knowing their identity, but at the cost of constraints imposed on all holders of old bitcoins. Before facing a quantum computer, Bitcoin must therefore solve a human question: who can modify the conditions allowing an owner to spend their own funds?