It is impossible to unobservably violate the protocol, or to modify the object after it has been comitted. Typically the key of the signature defines the meaning of the signed object, rather than the contents of the object signed, so that Bob doesn't end up signing a blank check. Used in digital bearer instruments, where Bob is the clearing agent, and Chaumian credentials, where Bob is the credential issuer. When an the object is transferred, the trasferee can request the clearing agent to verify that the key has never before been cleared, and issue a new key. The clearing agent prevents multiple clearing of particular objects, but can be prevented from linking particular objects one or both of the clearing nyms who transferred that object.
Digital cash is a popular form of digital bearer instrument.
A backup service which comes online upon failure of the current service. Usually triggered by a dead-man switch. Data necessary to the operation of smart property. For example, a complex, proprietary firing sequence needed to operate a computerized engine, a CAD file needed to manufacture a specialized part, etc. To avoid theft of service, ZKIP is required to open an encrypted channel to the device.