Future industrial networks will consist of a mixture of old and new components, due to the very long life-cycles of industrial machines on the one hand and the need to change in the face of trends like Industry 4.0 or the industrial Internet of things on the other. These networks will be very heterogeneous and will serve legacy as well as new use cases in parallel. This will result in an increased demand for network security and precisely within this domain, this thesis tries to answer one specific question: how to make it possible for legacy industrial machines to run securely in those future heterogeneous industrial networks. The need for such a solution arises from the fact, that legacy machines are very outdated and hence vulnerable systems, when assessing them from an IT security standpoint. For various reasons, they cannot be easily replaced or upgraded and with the opening up of industrial networks to the Internet, they become prime attack targets. The only way to provide security for them, is by protecting their network traffic. The concept of encryption gateways forms the basis of our solution. These are special network devices, that are put between the legacy machine and the network. The gateways encrypt data traffic from the machine before it is put on the network and decrypt traffic coming from the network accordingly. This results in a separation of the machine from the network by virtue of only decrypting and passing through traffic from other authenticated gateways. In effect, they protect communication data in transit and shield the legacy machines from potential attackers within the rest of the network, while at the same time retaining their functionality. Additionally, through the specific placement of gateways inside the network, fine-grained security policies become possible. This approach can reduce the attack surface of the industrial network as a whole considerably. As a concept, this idea is straight forward and not new. Yet, the devil is in the details and no solution specifically tailored to the needs of the industrial environment and its legacy components existed prior to this work. Therefore, we present in this thesis concrete building blocks in the direction of a generally applicable encryption gateway solution that allows to securely integrate legacy industrial machinery and respects industrial requirements. This not only entails works in the direction of network security, but also includes works in the direction of guaranteeing the availability of the communication links that are protected by the gateways, works to simplify the usability of the gateways as well as the management of industrial data flows by the gateways.