OP_CC: A simple introspection opcode to enable cheaper consolidations

This opcode functions by evaluating inputs spending the same scriptPubKey (SPK) differently: it returns false for the first transaction input that spends a particular SPK and true for every subsequent input that does the same. The practical implication of this mechanism is the potential for significant savings in transaction size, as demonstrated through a straightforward scenario where multiple Unspent Transaction Outputs (UTXOs) associated with the same SPK are spent in a single transaction. Specifically, the first UTXO requires a traditional key-path spend, including a signature, while the subsequent UTXOs can be unlocked via script-path spends without needing additional signatures, thereby conserving 58 witness bytes per input compared to conventional methods.

Despite the efficiency gains promised by OP_CC, concerns have been raised regarding its encouragement of address reuse—a practice generally discouraged due to privacy implications. However, an alternative perspective suggests that the current prevalence of address reuse may indicate a level of acceptability of its privacy trade-offs among users, especially considering OP_CC's optional nature. Additionally, the introduction of Cross-Input Signature Aggregation (CISA) offers a parallel path towards achieving similar efficiencies through Schnorr signatures, boasting not only space savings but also broader benefits. The CISA proposal is further supported by ongoing research and development efforts, as evidenced by the linked information at CISAResearch.org.

Another related concept, OP_CIV by Tadge Dryja, shares some functional similarities with OP_CC but is distinguished by its flexibility and potential post-quantum considerations. Despite these alternatives, OP_CC is highlighted for its simplicity and ease of implementation for wallets, presenting a viable option for enhancing economic bandwidth without necessitating block size increases.

The discussion also touches upon OP_CHECKCONTRACTVERIFY (CCV) as a mechanism capable of facilitating signatureless consolidation transactions through a specific taptree construction. This approach would allow all value from consolidated inputs to flow to a designated output without requiring signatures, thereby enabling external fee payments while maintaining the option for standard key spends to any destination. This concept underscores the ongoing exploration of innovative solutions aimed at optimizing transaction efficiency and flexibility within the Bitcoin ecosystem.