Bitcoin core developer Cory Fields has proposed a system called "quantum mines," which could expose attackers attempting to steal funds from wallets vulnerable to quantum computing to the risk of losing their own bitcoins.
In a proposal posted on the Bitcoin development mailing list on November 4, Fields outlined a mechanism requiring anyone transferring vulnerable BTC to provide collateral equal to the transferred amount from a quantum-safe wallet. If the original holder has previously prepared proof of ownership, the system would allow them to reclaim their coins after the fact and claim the attacker’s collateral.
Fields, affiliated with MIT’s Digital Currency Initiative, described this concept as a potential solution to counter the future threat of quantum computing. Currently, no such mechanism is operational on the Bitcoin network.
The "quantum mine" could put the attacker's BTC at risk.
This proposal addresses a security vulnerability: Bitcoin wallets whose public keys have already appeared on the blockchain. If quantum computers become powerful enough, they could theoretically use the exposed public keys to calculate the corresponding private keys.
Some of Bitcoin's earliest transaction formats directly exposed public keys, and reuse of addresses may also render other holdings vulnerable. This concern also includes dormant wallets from Bitcoin's early years, though no publicly demonstrated quantum computer has yet been able to carry out such an attack.
Fields proposed that anyone spending these fragile coins must provide collateral equal to the amount being transferred. Following his example, if someone attempts to transfer 1 BTC, they must hold an additional 1 BTC in a quantum-safe wallet.
Both amounts will be directed to a new output with a spending restriction, which may include a 30-day waiting period. The legitimate owner of the original bitcoins can prepare a hidden ownership commitment in advance, before quantum attacks become feasible.
If the attacker later obtains the private key and attempts to transfer those bitcoins, the original owner can disclose the prior commitment during the waiting period.
Fields wrote: "However, at the same time, Alice noticed the transaction." He used this to illustrate that the legitimate owner can reclaim their funds while seizing the attacker's collateral.
Attackers do not know in advance whether the target wallet contains such commitments. Fields believes that this uncertainty may deter theft attempts, as a successful attack could ultimately cost the thief more than the value of the targeted funds.
Bitcoin developers propose an auction mechanism for lost wallets.
This "landmine" mechanism relies on the legitimate holder still controlling their private key and preparing the necessary commitments before quantum attacks become feasible.
However, many older Bitcoin wallets may no longer meet these criteria. Some holders have lost their private keys, while others have not touched their holdings for years.
To address this issue, Fields also proposed another system that allows participants to submit hidden bids against vulnerable positions before an attack occurs.
In one example, participants can place bids on a dormant wallet containing 50 BTC. If a quantum attacker later attempts to spend these coins, an auction will be initiated after the initial proof-of-ownership period ends.
Participants may disclose their pre-prepared bids and receive a portion of the attacker's collateral. According to the described mechanism, if a qualified bid prevents the attacker from retaining these coins, the vulnerable coins will be destroyed.
Fields account for different collateral allocation rules, including payments to successful bidders, miners, or other designated recipients.
He also discussed another auction design, proposed in conversation with Neha Narula, Director of the MIT Digital Currency Initiative, in which competing bidders could face financial penalties.
Neither of these auction systems guarantees that the original wallet holder will recover their funds. Their purpose is to make theft less attractive by forcing attackers to risk their own funds.
Proposed Bitcoin protection measures may affect legitimate holders.
Collateral requirements may pose challenges for individuals attempting to transfer bitcoin they legally own.
For example, if someone holds 1 BTC in a vulnerable wallet, according to Fields’ proposal, they would need an additional 1 BTC in a quantum-safe output to complete the transaction.
Therefore, if the holder leaves their entire balance in the old wallet, they may find it difficult to access these funds without additional bitcoins.
Fields acknowledged that Bitcoin cannot automatically distinguish between legitimate holders and attackers who recover the same private key through quantum computing.
“The purpose of this article is merely to present these ideas as thought experiments,” Fields wrote, noting that he did not specify the software changes required to implement these concepts.
Any implementation will first require quantum-secure transaction outputs and new consensus rules to manage vulnerable coins.
Bitcoin developers have been considering additional protective measures. As crypto.news previously reported, Coinbase urged developers in October to prepare for quantum threats, warning that migrating existing wallets could take years.
Fireblocks' research also identifies exposed public keys as a primary risk, estimating that millions of BTC may be at risk in vulnerable addresses.
Another quantum proposal by BitMEX suggests that restrictions on older wallets should only be enabled upon evidence of a successful quantum attack.
The discussions in the mailing list by Fields left several design issues unresolved, including the collateralization ratio, auction cutoff times, penalties for unsuccessful bids, and the conditions under which vulnerable tokens may be burned.