Project Eleven's "Q-Day Receipts" Could Prove Your $BTC Is Still Yours After Quantum Forgers Crash the Party

Project Eleven on Thursday unveiled a cryptographic technique designed to let wallet owners prove control of their funds after quantum computers become powerful enough to derive private keys from public keys and forge digital signatures on Bitcoin. The proposal, outlined in an X thread by Project Eleven CEO Alex Pruden, frames the central problem as one of identity rather than defense. "How do you prove you still own a wallet after a quantum computer can forge its signatures?" Pruden wrote. "After Q-Day, once a quantum computer can derive an ECC private key from its public key, a valid signature no longer proves ownership. Both the quantum adversary and the legitimate owner are able to produce identical signatures."

The technique uses a wallet's key derivation path to let users demonstrate that they hold the parent key from which a compromised private key was generated, without disclosing that parent key itself. Because a quantum computer cannot reconstruct the seed phrase behind the derivation path, Project Eleven says the scheme can distinguish the real owner from an attacker even after the address-level private key has been broken. "So even after Q-Day, an attacker who's broken your address's private key does not hold, and can't compute, the seed phrase it was derived from. Proving you know that parent key, without revealing it, is something only the real owner can do," Pruden wrote.

According to Pruden, the work was developed in collaboration with Jim Posen, lead maintainer of the open-source Binius zero-knowledge proof system, and builds on a technique known as "signature lifting," first proposed by researchers Alon Sattath and Robert Wyborski. Project Eleven funded Posen to implement the approach using Binius, an open-source proof system designed to accelerate hash-heavy cryptographic operations.

The recovery mechanism is intended for users who miss a future migration to quantum-safe addresses, and arrives as efforts to harden Bitcoin for a post-quantum era accelerate. Coinbase's quantum advisory council is urging blockchain developers to begin preparing for a post-quantum future now, arguing in a Thursday report that the technical work of upgrading Bitcoin, Ethereum and other networks should not wait for consensus on how to handle vulnerable or abandoned coins.