In standard signature schemes, the sender computes a signature on his message using his secret key. The receiver can check, using the signature and the sender's public key, that the message was indeed composed by the sender and that it was not modified in transit. To achieve this, the public key must uniquely identify the sender. However, in many scenarios this strict identification is not necessary or even desirable. Whenever an application only needs assurance that the sender belongs to a certain group of possible senders, anonymous group signatures can be used.
Anonymous group signatures allow each member of a group to sign messages without disclosing their identity. For this, each group member gets their own private key that is associated to the group's public key. In contrast to standard digital signature schemes, the message receiver can only check whether some group member signed the message, but not which specific member did it. The actual signer can only be determined by a special entity in the system (the group manager).
Besides anonymity, other properties of group signatures play an important role in some applications. For example, it may be useful to restrict the number of messages that each group member can sign. Furthermore, techniques for revoking group membership are important. In particular, these and other extensions of group signatures can be used to construct anonymous reputation systems.
Reputation systems are an important tool to allow customers and providers of goods and services to gather useful information about past transactions. In order to receive trustworthy, reliable, and honest ratings, a reputation system should guarantee the customer anonymity and at the same ensure that no customer can submit more than one rating. Of course the ratings should be publicly verifiable by third parties.
Some of the required properties for reputation systems are already known for group signatures. Some, however, are not. For example, a reputation system does not consist of a single group, but rather there is a group for each rateable product (or service). For the security of reputation systems these different groups cannot be analyzed in isolation.
The goal of this research area is the extension of group signature schemes and the construction of anonymous reputation system on the basis of group signatures.