Ring Signature

Initially designed in 2001, ring signatures was proposed as a method to "leak a secret". For example, a ring signature could be used to provide an anonymous signature from "a high-ranking White House official", without revealing which official signed the message. Ring signatures are similar to group signatures but differ in two key ways: first, there is no way to revoke the anonymity of an individual signature, and second, any group of users can be used as a group without additional setup. This signature implementation was then incorporated in cryptocurrency design which gave birth to Monero and other privacy coins.