Bitcoin’s Quantum-Resistant Upgrade Has to Work for Big Custodians, Coinbase Tells Developers

Coinbase set out what it wants from Bitcoin's shift to signatures that can resist a future quantum computer, and much of its list traces back to how a large custodian actually signs transactions.

Holding roughly 5% of all BTC in existence by its own count, the exchange described the post as "an input to an open discussion, not a set of instructions or a final Coinbase position."

Coinbase argued the work should start before anyone proves the threat is real. "We believe it is important to begin preparing well ahead of any CRQC announcement," the company wrote in the post, referring to a cryptographically relevant quantum computer, one able to derive private keys from exposed public keys. It said changes to Bitcoin's consensus rules, and the migrations that follow, can take years.

Hybrid Keys and Swappable Signatures

Coinbase said it favors an output type that lets new post-quantum signature schemes be added or replaced later without a full redesign. It pointed to BIP-360, a draft that would create a Pay-to-Merkle-Root (P2MR) output through a soft fork by dropping Taproot's key path, and to a design called P2TRv2.

The exchange also floated a 1-of-2 hybrid, in which the same coins could be spent either with a classical ECDSA or Schnorr key or with a post-quantum key. That would let active wallets add protection before the network settles what happens to older signature types.