| summary | Monero's transaction proving system | |||
|---|---|---|---|---|
| title | FCMP++ | |||
| terms |
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FCMP++ (Full-Chain Membership Proofs + Spend Authorization + Linkability) is the system Monero uses to prove that @transaction inputs are valid and unspent without revealing which outputs are actually being spent.
Each FCMP++ input proves it does spend one output from the whole set of eligible outputs on the @blockchain. The proof itself does not leak any information about which specific output is real.
From a chain observer’s perspective, it is not possible to determine the real input.
Compared to @CLSAG:
- The anonymity set is the entire set of Monero transactions in history, not a ring of 16.
- Double-spend detection still works using linkable tags that cannot be tied back to a concrete on-chain output.
- Spend authorization is cleanly separated: the proof shows the spender controls the key to some valid output in the global set, but never which one.
FCMP++ also enables new functionality:
- Transaction chaining - safely spending outputs from still-unconfirmed transactions, useful for advanced protocols and potential layer-2 designs.
- Outgoing view keys - optional keys that let a chosen auditor or watch-only device see both incoming and outgoing transfers, without granting any spend rights.
- Stronger long-term privacy - the design ensures that transactions do not expose information that future analysis can use to recover the real inputs.