This Service Set defines Services used to open a communication channel that ensures the confidentiality and Integrity of all Messages exchanged with the Server. The base concepts for OPC UA security are defined in OPC 10000-2.
The SecureChannel Services are unlike other Services because they are not implemented directly by the OPC UA Application. Instead, they are provided by the Communication Stack on which the OPC UA Application is built. For example, an OPC UA Server may be built on a stack that allows applications to establish a SecureChannel using HTTPS. In these cases, the OPC UA Application shall verify that the Message it received was in the context of an HTTPS connection. OPC 10000-6 describes how the SecureChannel Services are implemented.
A SecureChannel is a long-running logical connection between a single Client and a single Server. This channel maintains a set of keys known only to the Client and Server, which are used to authenticate and encrypt Messages sent across the network. The SecureChannel Services allow the Client and Server to securely negotiate the keys to use.
Logical connections may be initiated by the Client or by the Server as described in OPC 10000-6. After the connection is initiated, the SecureChannel is opened and closed by the Client using the SecureChannel Services.
An EndpointDescription tells a Client how to establish a SecureChannel with a given Endpoint. A Client may obtain the EndpointDescription from a Discovery Server, via some non-UA defined directory server or from its own configuration.
The exact algorithms used to authenticate and encrypt Messages are described in the SecurityPolicy field of the EndpointDescription. A Client shall use these algorithms when it creates a SecureChannel.
It should be noted that some SecurityPolicies defined in OPC 10000-7 will turn off authentication and encryption resulting in a SecureChannel that provides no security.
When a Client and Server are communicating via a SecureChannel, they shall verify that all incoming Messages have been signed and encrypted according to the requirements specified in the EndpointDescription. An OPC UA Application shall not process any Message that does not conform to these requirements.
The relationship between the SecureChannel and the OPC UA Application depends on the implementation technology. OPC 10000-6 defines any requirements that depend on the technology used.
The correlation between the OPC UA Application Session and the SecureChannel is illustrated in Figure 13. The Communication Stack is used by the OPC UA Applications to exchange Messages. In the first step, the SecureChannel Services are used to establish a SecureChannel between the two Communication Stacks which allows the secure exchange of Messages. In the second step, the OPC UA Applications use the Session Service Set to establish an OPC UA Application Session.
Figure 13 – SecureChannel and Session Services
Once a Client has established a Session it may wish to access the Session from a different SecureChannel. The Client can do this by validating the new SecureChannel with the ActivateSession Service described in 5.6.3.
If a Server acts as a Client to other Servers, which is commonly referred to as Server chaining, then the Server shall be able to maintain user level security. By this we mean that the user identity should be passed to the underlying Server or it should be mapped to an appropriate user identity in the underlying server. It is unacceptable to ignore user level security. This is required to ensure that security is maintained and that a user does not obtain information that they should not have access to. Whenever possible a Server should impersonate the original Client by passing the original Client’s user identity to the underlying Server when it calls the ActivateSession Service. If impersonation is not an option then the Server shall map the original Client’s user identity onto a new user identity which the underlying Server does recognize.