Full-Chain Membership Proofs

Full-Chain Membership Proofs (FCMPs) are a proposed privacy enhancement for Monero that aims to replace the existing ring signature mechanism. FCMPs prove that the output being spent is one of any output on the entire Monero blockchain, rather than just a subset, significantly expanding the anonymity set from around 16 to over 100 million outputs. Two FCMP proposals exist - one integrated with the upcoming Seraphis upgrade, and another called FCMP++ that can be deployed more quickly without requiring Seraphis. FCMP++ also introduces additional features like transaction chaining, outgoing view keys, and forward secrecy to further improve Monero's privacy protections