Perhaps the simplest possible quantum-security upgrade

Erik proposes an innovative approach to enhance Bitcoin's security against quantum threats without the need for a new signature scheme. Recognizing the specific needs of Bitcoin’s proof of utxo ownership, he suggests a chain-native authorization primitive that leverages the economic assumptions underpinning transaction finality. This solution aims to provide a quantum-resistant migration path that is immediately deployable, maintains low validation costs, and doesn't expand transaction sizes significantly.

At the core of Erik's proposal is the introduction of a minimal new introspection primitive that avoids a complete redesign of Bitcoin's Script. This involves a single opcode that reveals a challenge derived from the blockchain, specifically, the hash of a block at a selectable distance from where the UTXO was created. The distance—or offset—is determined by the locking script based on the value at risk, with larger offsets providing greater security through deeper confirmation depth. This model incorporates existing timelock opcodes to enforce a delay for accessing this blockchain-derived value, ensuring that authorization requires both a traditional signature and a hash-based proof conditioned on future blockchain data.

Erik's design adheres to a commit-challenge-response authentication framework, where the challenge comes uniquely from future blocks in the chain. This dual requirement of satisfying both a conventional signature check and a delayed chain-conditioned proof means an attacker must overcome both quantum cryptographic challenges and the economic hurdles of manipulating blockchain confirmation depths to compromise a transaction. Importantly, the proposed scheme does not necessitate significant changes to the current Bitcoin infrastructure. It retains existing address formats and keeps transaction sizes manageable while avoiding reliance on unproven cryptographic assumptions.

This quantum-resistance mechanism aligns with Bitcoin's existing economic security principles, providing a conservative yet effective method for enhancing security against quantum attacks. The proposal encourages incremental adoption and invites further development and discussion within the community, potentially through a Bitcoin Improvement Proposal (BIP).