OP_CIV - Post-Quantum Signature Aggregation

The proposed method, OP_CIV or OP_CHECKINPUTVERIFY, is designed to be compatible with any signature type, including those based on post-quantum cryptography. This technique allows a transaction input to demonstrate its connection to another input within the same transaction by referencing it and using its signature, thus avoiding the need for an additional one. This process not only ensures the integrity of transactions through a script that checks for correct parameters but also enhances privacy protection against potential analytical attacks aimed at identifying the ownership of multiple unspent transaction outputs (UTXOs).

A key feature of OP_CIV is its potential to significantly reduce transaction sizes by requiring only one signature per transaction in many cases. However, challenges emerge, particularly for wallets generating numerous addresses without direct linkage between UTXOs, necessitating multiple signatures. Moreover, deterministic wallets face complexities in key recovery due to the increased possibilities when addresses point to all existing UTXOs within a wallet. Proposed solutions include limiting the number of OP_CIV scripts or incorporating scripts for spent TXOs to facilitate key recovery. Additionally, concerns regarding replay attacks and address reuse are mitigated by OP_CIV's design, which references outpoints rather than addresses or keys, and by utilizing SIGHASH_ALL for signing inputs.

The practical applications of OP_CIV extend beyond simply reducing transaction sizes. Its integration into Bitcoin could enable new types of smart contracts and secure linkages between transaction inputs, marking a significant step towards blockchain technology's adaptation to post-quantum cryptographic standards. This idea was initially presented by Tadge during TABConf, with the aim of gathering community feedback and further refining the approach.

In related discussions, the debate on using "addresses" versus "outpoints" for Bitcoin transactions has highlighted privacy and ease of implementation as key considerations. Prioritizing outpoints over addresses could discourage address reuse and enhance privacy and security, despite potential complexities in implementation. Such approaches underscore the evolving methodologies in Bitcoin transactions, balancing technical hurdles with strategic benefits.

Furthermore, the dialogue among developers has introduced innovative ideas for streamlining transactions through stateless address generation and CISA. By assigning every address a unique index and generating shared keys, this method promises simplified backups and a reduction in transaction size, crucial for incorporating post-quantum cryptographic methods effectively. Simultaneously, these discussions acknowledge the privacy implications and trade-offs involved, emphasizing a nuanced understanding of Bitcoin's transactional dynamics in the context of advancing cryptographic practices.