Group signature
A group signature scheme is a method for allowing a member of a group to anonymously sign a message on behalf of the group. The concept was first introduced by David Chaum and Eugene van Heyst in 1991. For example, a group signature scheme could be used by an employee of a large company where it is sufficient for a verifier to know a message was signed by an employee, but not which particular employee signed it. Another application is for keycard access to restricted areas where it is inappropriate to track individual employee's movements, but necessary  to secure areas to only employees in the group. Essential to a group signature scheme is a "group manager", who is in charge of adding group members and has the ability to reveal the original signer in the event of disputes. In some systems the responsibilities of adding members and revoking signature anonymity are separated and given to a membership manager and revocation manager respectively. Many schemes have been proposed, however all should follow these basic requirements: The "ACJT 2000", "BBS04", and "BS04" (in CCS) group signature schemes are some of the state of the art. (Note: this might be  an incomplete list.) Boneh, Boyen and Shacham published in 2004 ("BBS04", Crypto04) is a novel group signature scheme based on bilinear maps. Signatures in this scheme are approximately the size of a standard RSA signature (around 200 bytes). The security of the scheme is proven in the random oracle model and relies on the Strong Diffie Hellman assumption (SDH) and a new assumption in bilinear groups called the Decision linear assumption (DLin). A more formal definition that is geared towards provable security was given by Bellare, Micciancio and Warinschi.