Coinbase’s Bitcoin quantum preparation push puts SHRINCS recovery and signing counters under scrutiny, with larger fallback signatures.

Coinbase’s push to prepare Bitcoin for quantum computing addresses what happens when a wallet returns from an old backup or a signing server fails.

Brian Armstrong called on Oct. 20 for preparation and collaboration across the industry. The operational detail comes from Shashank Agrawal’s Oct. 13 Coinbase article, which asks prospective signature schemes to work with ordinary wallets, recovery procedures, and independently operating custody servers.

SHRINCS, a draft hash-based signature scheme for Bitcoin, combines smaller signatures that require a reliable record of previous signing with a larger fallback that works without that record. Under its current rules, recovery with uncertain state means taking the larger path.

That preserves a route to signing for someone who can recover the secret. The bargain is that keeping the smaller signatures requires additional operational discipline.

The current SHRINCS specification lists a 48-byte public key, stateful signatures between 548 and 4,619 bytes, and a stateless signature of 5,777 bytes.

“State” is the changing record of which one-time signing slots a key has already used. SHRINCS’ compact component (XMSS) depends on not reusing those slots. Reusing one for different messages can allow an observer to forge signatures.

A backup can recover the long-lived secret without recovering a trustworthy record of every signature created since the backup. Agrawal also explained that signatures and state updates do not necessarily result in an on-chain transaction.

When correct state is unavailable, including after restoring from a static backup, the implementation must refuse stateful signing and use only the stateless path.

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