Bitcoin Developer Sketches Post-Q-Day Collateral Rule for Vulnerable Coins

Bitcoin Core developer Cory Fields outlined a “quantum landmine” concept on Nov. 4 that would make spending quantum-vulnerable Bitcoin carry a collateral requirement under hypothetical future rules. The requirement would apply only after a future “q-day,” if new validation rules were activated. Fields presented the concepts as thought experiments and left the soft-fork mechanics for later work.

In one example, an owner combines 1 BTC in a legacy output with 1 BTC in a quantum-safe output, creating a 2 BTC quantum-safe output under the assumed rules. The owner keeps a proof of knowledge of the legacy private key in a separate quantum-safe output. The landmine idea depends on having that proof ready before the hypothetical quantum threat.

A 30-day waiting period is illustrative, not a settled specification. During the wait, an owner with the proof could spend the output; after the timeout, the example would allow spending without the proof. If a quantum attacker tried to move the vulnerable 1 BTC, the attacker would also provide 1 BTC in quantum-safe collateral. An owner who had prepared the proof could reveal it during the waiting period, reclaim the funds and claim the attacker’s collateral.

The collateral requirement would also apply to legitimate holders moving vulnerable funds. In Fields’ example, that means an owner would need quantum-safe Bitcoin alongside the legacy funds being spent. Fields did not set a final collateral ratio or timeout, leaving choices such as a 1:1 ratio or other staking requirements open.

For vulnerable outputs whose private keys may be lost, Fields sketched a staked auction and a Sats Auction. A staked-auction example uses a 50 BTC vulnerable output, a 40 BTC precommitted bid and 50 BTC of collateral from an attacker. The auction designs and their outcomes remain open.

The concepts assume a quantum-safe output type and new Bitcoin validation rules; they do not themselves provide quantum-resistant signatures. BIP-361, a separate proposal for post-quantum migration and a sunset for legacy signatures, is listed as a draft. Fields wrote, “The purpose of this post is just to introduce the ideas as thought experiments.”