Bitcoin Developer Proposes 'Landmines' to Counter Quantum Theft

Bitcoin Core developer Cory Fields proposed hidden traps that would expose quantum thieves to financial losses before the thieves could spend stolen bitcoin. Fields, who works on Bitcoin software at MIT's Digital Currency Initiative, presented the proposal on the bitcoin-dev mailing list on Wednesday. Fields described the ideas as thought experiments, dependent on Bitcoin gaining quantum-safe wallets and on enforcement rules that Fields has not written.

Fields' example would require an attacker targeting 1 BTC to contribute another 1 BTC from a quantum-safe wallet. The proposed rules would lock the targeted bitcoin and the collateral for a waiting period, such as 30 days. Before quantum attacks become feasible, an owner could establish a separate quantum-safe "landmine" holding concealed proof of ownership of vulnerable coins. During an attempted theft's waiting period, the owner could disclose that proof, recover the vulnerable coins and collect the attacker's collateral. Fields explained that the owner would reveal the proof by spending the landmine as an input and directing the locked funds to a previously committed key. An attacker could not identify trapped holdings before putting up collateral. The mechanism would encourage owners to prepare traps before quantum computers pose a threat, potentially doubling the owner's vulnerable holdings after an attempted theft.

A powerful enough quantum computer could derive a wallet's private spending key from the public key. Some early Bitcoin payments made public keys visible immediately, potentially exposing coins linked to Bitcoin creator Satoshi Nakamoto. A dispute concerns whether Bitcoin should freeze vulnerable coins or permit spending by anyone able to supply a valid signature. A freeze could prevent theft but also deny access to owners who had not upgraded.

The proposed landmines would protect only owners who retain the relevant keys. That limitation excludes coins believed to be lost, including coins associated with Nakamoto. Fields also outlined an auction mechanism intended to protect inactive coins against quantum theft. Participants competing on the network would commit hidden bids against a vulnerable wallet in advance. A quantum theft attempt would automatically activate the highest bidder's smart contract. The contract would immediately destroy the targeted coins and transfer the attacker's locked collateral to the winning bidder as a bounty.

However, the proposed arrangements could impose additional costs on legitimate owners because the blockchain cannot distinguish an owner from a quantum thief. An owner transferring 1 BTC would also have to supply 1 BTC in collateral. An early holder with all of the holder's wealth in an old wallet would need matching funds to move those coins.