Make pathological transactions with more than 2500 legacy signature operations non-standard

In a detailed exploration of challenges surrounding Bitcoin's Coinjoin transactions and sigop limits, the intricacies of transaction validation, particularly in relation to Bitcoin Improvement Proposals (BIPs) and their implications on network security and software development practices, are discussed. This examination highlights a scenario involving a Coinjoin transaction among ten pseudonymous peers, focusing on how the transaction assembly could inadvertently run afoul of Bitcoin's policy rules due to the inclusion of a legacy input by one participant. This legacy input, designed to be space-efficient while minimally contributing to the transaction's value, becomes problematic under the MAX_TX_LEGACY_SIGOPS rule, which limits the number of signature operations for legacy inputs. The issue is compounded by the lack of awareness or understanding of this policy rule among participants or coordinators, especially in a decentralized setting without centralized error messaging protocols.

The difficulty in identifying and resolving such policy violations in collaborative transactions underscores the broader challenge of ensuring compatibility and compliance with evolving network policies. Moreover, the discussion touches upon the potential for DoS attacks exploiting these policy constraints, suggesting that the reduction in sigop limits could inadvertently lower the cost for adversaries to launch such attacks. Despite this, the responsibility for addressing these issues often falls outside the purview of full-node operators, placing greater emphasis on developers of downstream software to adapt and respond to policy changes.

Additionally, the conversation delves into the specifics of BIP141, clarifying its stipulations regarding segwit inputs and the exacting requirements for scriptSig validity. This specificity is crucial for preventing misinterpretation and unintended application of rules to segwit spends, which could lead to broader validation issues. The dialogue advocates for clearer references and less ambiguity in BIP documentation, arguing that explicit connections between proposals and a precise technical definition of terms like "legacy" and "segwit" inputs can significantly enhance clarity and implementation fidelity.

Finally, personal experimentation with transaction scripting is cited, demonstrating the potential for creating standard-compliant transactions that heavily utilize sigops within the bounds of MAX_STANDARD_TX_WEIGHT. This example serves to illustrate both the flexibility and the potential vulnerabilities within Bitcoin's current policy framework, highlighting the ongoing need for careful consideration of policy impacts on transaction creation and validation practices.