A ring signature allows for secure signing of messages without compromising the anonymity of the signer. This works by hiding the identity of the signer within a group of users that each have keys that can be used to make the signature, then using cryptography to make it computationally infeasible for anybody to determine which set of keys was used to produce the signature in any given transaction.
Ring signatures offer privacy improvements over a predecessor protocol called group signatures. Group signatures are centrally managed/coordinated and require setup procedures, while ring signatures groups don't need to be managed and don't have initial setup procedures.
The cryptocurrency Monero implements a linkable ring signature scheme to ensure that transactions on its blockchain are private. Used along with stealth addresses, this ring signature scheme makes all transaction outputs in Monero untraceable while ensuring that the network can't decipher which outputs are spent or unspet.
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