Allowing Duplicate Keys in BIP 390 musig() Expressions

The allowance of duplicate participant public keys under BIP 327 is highlighted as a noteworthy feature, despite the potential complexities it introduces into the signing procedures. The rationale behind this decision appears to stem from a desire to maintain flexibility and simplicity in transaction processing, although it raises concerns about the security implications of such an approach.

Ava Chow's correspondence sheds light on the specific issue of deterministic nonce generation when duplicate public keys are present. The standard practice of deriving nonces from a combination of the message, the set of public keys, and each participant's private key could lead to the creation of identical nonces if public keys are duplicated. This situation, while not immediately threatening in terms of private key exposure, introduces risks associated with unexpected outcomes during the signing process. The discussion reflects an awareness of these risks and a cautious approach to managing them, emphasizing the need for careful consideration of security assumptions in scenarios involving duplicate keys.

Furthermore, the dialogue touches upon the challenges associated with implementing musig() descriptor expressions, particularly the restriction against repeated participant public keys. The practical difficulties of enforcing this restriction are brought into question, especially in light of the fact that MuSig2, a protocol enhancement, permits such duplications to facilitate easier implementation. Ava proposes amending the BIP to eliminate this restriction, seeking feedback from the developer community on the potential impact of this change. The conversation underscores the dynamic nature of Bitcoin protocol development, where proposals for modifications are subject to community scrutiny and debate in pursuit of optimal security and functionality.

This discourse among Bitcoin developers illustrates the intricate balance between innovation and security in the evolution of cryptocurrency protocols. The willingness to reconsider established norms, such as the unique public key requirement, in response to practical implementation challenges reflects a broader theme of adaptability within the field. As the discussion continues, the outcome will likely influence future practices related to public key management and nonce generation within the Bitcoin network.