Ring signature
A signature that proves one member of a group signed, without revealing which one; Monero uses it to hide spent outputs.
A ring signature lets someone sign on behalf of a group so that a verifier knows one member signed but cannot tell which one. Monero uses ring signatures to hide which coin is actually being spent in a transaction.
When you send XMR, your wallet picks a set of other outputs from the blockchain as decoys and signs with the whole set, or ring. Observers see all ring members as possible sources of the payment. Monero uses a fixed ring size set by the protocol, currently 16, so every transaction looks alike.
Ring signatures work together with stealth addresses, which hide the recipient, and RingCT, which hides amounts. A key image prevents the same output from being spent twice without revealing which output it was. Researchers continue to study decoy selection, and Monero's developers have been working on upgrades that go beyond ring signatures.
