Bird of Prey 2: non-malleable Schnorr + PQ signatures

The discussion surrounding the importance of Signature Upgrade Functionality (SUF) in the context of post-Segwit Bitcoin transactions highlights several technical nuances. Post-segwit, the dependence on signatures for transaction ID integrity has been minimized, addressing previous concerns about malleability in cryptographic schemes like ECDSA or SQIsign. The conversation shifts towards the consideration of hybrid cryptographic schemes, where mutations in signatures would necessitate multiple signings of the same message with an identical hybrid key. This scenario mirrors the current state in Bitcoin's naive multisig scripts, where signers might change their signature for the same message without introducing new vulnerabilities. Furthermore, the use of deterministic signing algorithms presents a solution for honest signers to sidestep these malleability issues altogether.

Another significant aspect of this discussion involves the potential advantages of adopting a hybrid scheme, particularly regarding space efficiency. Boris Nagaev proposes an intriguing method that could potentially integrate BIP340 within a hash-based scheme, adding only 48 bytes of overhead, as detailed in his contribution to the Bitcoin development group (read more here). This proposal suggests that hybridizing cryptographic methods could lead not only to enhanced security but also to better optimization of space, which is a critical factor in blockchain technology. Such innovations prompt further examination and discussion among experts to explore these hybrid schemes' feasibility and practical implications in real-world applications.