In addition to being secure Digital Signatures, they need to allow for the efficient implementation of two protocols:
The second protocol allows the signature owner to prove that he has a signature on a number of messages, without revealing the Digital Signature and only a (possibly) empty subset of the messages.
The combination of these two protocols allows for the implementation of digital credential and ecash protocols.
Signatures with efficient protocols may have United States and other country Patents Signatures with efficient protocols is a form of a Group Signatures
Signatures with efficient protocolss have been enhanced since the original.