Hybrid Post-Quantum Signatures for Bitcoin and Ethereum: A Protocol-Level Integration Strategy

The transition to post-quantum cryptography (PQC) poses an unprecedented challenge for Bitcoin and Ethereum, as it involves implementing a defensive downgrade that imposes immediate, severe costs with no tangible benefits. While quantum computers capable of breaking secp256k1 require approximately 2,100–2,400 logical qubits – with algorithmic improvements continuously reducing this threshold – current systems achieve only ~100 logical qubits. This 10–15-year threat horizon collides with the reality that convincing decentralised communities to accept 50% capacity loss and 2–3× fee increases could take 10–15 years, if achievable at all. Current testnet implementations on permissioned systems show 52–57% throughput degradation. Critically, this data comes from fundamentally different architectures than permissionless networks, which will likely experience 60–70% throughput loss due to global verification requirements, heterogeneous hardware, and compounding propagation delays. This methodological limitation – extrapolating from permissioned to permissionless systems – represents a critical infrastructure failure that introduces massive uncertainty into migration planning. Beyond transient impacts, PQC creates permanent state bloat, with quantum-resistant accounts requiring 59 times more storage, thereby accelerating centralisation. This paper presents a comprehensive framework acknowledging these harsh realities. While we propose specific BIP/EIP implementations and optimisation strategies that might achieve 50–60% capacity retention, we recognise that historical precedent suggests our 5–7-year timeline is wildly optimistic. Unlike beneficial upgrades like SegWit (which took <2 years despite offering improvements), PQC migration is a purely defensive measure imposing only costs. Blockchain communities face a stark choice: accept immediate degradation to prepare for quantum threats or risk emergency migration under crisis conditions.