Coinbase has published a detailed framework describing the challenges bitcoin must address before migrating to post-quantum cryptography. The exchange, which safeguards roughly 5% of bitcoin's supply, argues that any transition must protect exposed coins, fit within bitcoin's limited block space, preserve full-node validation, and accommodate both institutional custody and individual users.
The company's central argument is that bitcoin needs an adaptable architecture rather than a single cryptographic fix. Coinbase pointed to designs including P2MR under BIP-360 and P2TRv2, which could potentially allow post-quantum signature systems to be introduced or replaced without requiring an entirely new output format each time.
One potential migration path would use a 1-of-2 hybrid structure, enabling funds to be spent using either current ECDSA or Schnorr keys or a future post-quantum key. This approach could allow wallets to move early without waiting for the broader bitcoin community to decide when legacy signatures should be restricted.
Institutional Custody Complications
The requirements become more complex at institutional scale. Coinbase warned that some stateful post-quantum signature schemes, while potentially smaller and faster, introduce operational risks. Reusing a signing state can enable forgery, a challenge for both ordinary wallets and institutions running distributed signing systems across multiple servers and locations.
Coinbase therefore advocates for bitcoin to retain a stateless signing option and says post-quantum schemes should be evaluated for compatibility with multi-party computation, or MPC, which large custodians use to reduce key-theft risk. The firm also highlighted implementation requirements including constant-time verification, reliable cross-platform behavior, predictable signing, and resistance to random-number failures.
The Dormant Bitcoin Problem
Coinbase identified dormant and inaccessible coins as potentially the most difficult issue. Bitcoin contains old outputs with exposed public keys, reused addresses, inactive holdings, and funds for which private keys may have been lost. If quantum computers eventually become capable of breaking current signature systems, decisions about those coins become both technical and governance challenges.
Coinbase stated that this debate should not delay work on post-quantum signature support. There is no described immediate quantum attack on bitcoin. The significance lies in a custodian responsible for approximately one-twentieth of all BTC treating migration planning as something the industry should begin before the threat becomes urgent.
