Value exchange systems enabling security and unobservability

The main problem arising in value exchange over a network, e.g. in the exchange of digital money for other valuable information, is the lack of simultaneity of the exchange, yielding a temporary advantage for one party, who could then stop communication. The situation is even worse when this party is anonymous. This normally is the case when digital payment systems enabling unobservability are used. But third parties can be used to overcome this problem. We compare two rather different approaches using third parties. The first tries to provide security by third parties identifying perpetrators in case of detected fraud, whereas the second uses a third party as trustee who takes an active part in the value exchange and can be completely controlled by each absolutely anonymous party.