Public key recovery for EC leaves in P2MR (BIP-360)

The discussion revolves around the security and usage implications of xpubs (extended public keys) in a cryptographic setting where quantum computing poses a potential threat. The primary concern is that while using xpubs to derive keys is acceptable if kept private, these must be treated with as much confidentiality as hardened derivation keys or private keys themselves. This approach essentially classifies all key derivation sources as private to enhance security against quantum attacks, which could exploit lesser-protected elements like chaincodes.

There is also a suggestion regarding the format of descriptors to increase security. Instead of including an xpub directly, it is considered more secure to use descriptors that only provide essential information for signing with a private key, without exposing the full public key or its direct derivatives. An example provided includes using a hash of the public key along with the origin identifier, which helps in aligning with practices reminiscent of how hardened public keys are managed.

Furthermore, the feasibility of transitioning the Bitcoin ecosystem towards post-quantum (PQ) security is discussed, highlighting the complexity of such a shift. It's acknowledged that a full migration to PQ-resistant structures may not be practical in a single step due to technical and user-adoption challenges. The email outlines a phased approach: initially, providing tools for users who are preemptively cautious of quantum threats; then moving to a widespread transition during an imminent threat; and finally, re-establishing a new norm post-quantum threat materialization with advanced mathematical solutions. This tiered strategy aims to balance security needs with practical usability and acceptance among users.

Finally, there is a realistic perspective on the adoption challenges, emphasizing that significant tooling and infrastructure development is necessary even for limited-scope transitions. The implication is that while proactive measures are essential, they need to be implemented thoughtfully to ensure they do not overly disrupt current practices or deter users due to complexity or reduced functionality.