A quantum computing attack against Bitcoin would primarily target the Elliptic Curve Digital Signature Algorithm (ECDSA), the cryptographic foundation used to sign and verify transactions. While most modern Bitcoin addresses (P2PKH or SegWit) hide the public key behind a hash until a spend is attempted, a quantum computer could derive a private key the moment a transaction is broadcast to the mempool. This creates a race condition where an attacker could potentially re-route a user’s funds to a different address before the original transaction is confirmed in a block.
Recent analysis from Bitcoin technical experts like Shinobi highlights that the most immediate on-chain threat involves "reused" addresses and legacy Pay-to-Public-Key (P2PK) outputs. In these instances, the public key is already known to the network, meaning a quantum attacker would not need to wait for a transaction broadcast to begin their derivation process. For the majority of users, however, the risk remains confined to the 'window of vulnerability' between the time a transaction is signed and the time it is included in a mined block.
In 2026, the discussion has shifted from 'if' to 'when,' as advancements in quantum hardware bring the industry closer to a Cryptographically Relevant Quantum Computer (CRQC). The Bitcoin developer community is currently evaluating several Bitcoin Improvement Proposals (BIPs) that introduce quantum-resistant signature schemes. These include lattice-based cryptography and Winternitz One-Time Signatures (WOTS), though these solutions face hurdles regarding transaction size and potential blockchain bloat.
Institutional investors and retail holders should watch for the development of 'quantum-safe' address types and potential soft forks that would allow for a secure migration of funds. A critical point of contention remains the fate of 'Satoshi-era' coins; if these early outputs are not moved to quantum-resistant addresses, they could become the first targets of a quantum-enabled theft, potentially impacting market sentiment and Bitcoin's perceived scarcity. Readers should monitor the NIST standards for post-quantum cryptography for clues on which algorithms might be integrated into the Bitcoin protocol.