All OPC UA Applications shall establish a SecureChannel before creating a Session. This SecureChannel requires that both applications have access to Certificates that can be used to encrypt and sign Messages exchange. The Application Instance Certificates installed by following the process described in 6.1.2 may be used for this purpose.
Figure 21 above assumes Client and Server have online access to a CertificateA uthority (CA). If online access is not available and if the administrator has installed the CA public key on the local machine, then the Client and Server shall still validate the application Certificates using that key. The figure shows only one CA, however, there is no requirement that the Client and Server Certificates be issued by the same authority. A self-signed Application Instance Certificate does not need to be verified with a CA. Any Certificate shall be rejected if it is not in a trust list provided by the administrator.
Both the Client and Server shall have a list of Certificates that they have been configured to trust (sometimes called the Certificate Trust List or CTL). These trusted Certificates may be Certificates for Certificate Authorities or they may be OPC UA Application Instance Certificates. OPC UA Applications shall be configured to reject connections with applications that do not have a trusted Certificate.
Certificates can be compromised, which means they should no longer be trusted. Administrators can revoke a Certificate by removing it from the trust list for all applications or the CA can add the Certificate to the Certificate Revocation List (CRL) for the Issuer Certificate. Administrators may save a local copy of the CRL for each Issuer Certificate when online access is not available.
A Client does not need to call GetEndpoints each time it connects to the Server. This information should change rarely and the Client can cache it locally. If the Server rejects the OpenSecureChannel request the Client should call GetEndpoints and make sure the Server configuration has not changed.
There are two security risks which a Client shall be aware of when using the GetEndpoints Service. The first could come from a rogue Discovery Server that tries to direct the Client to a rogue Server. For this reason the Client shall verify that the ServerCertificate in the EndpointDescription is a trusted Certificate before it calls CreateSession.
The second security risk comes from a third party that alters the contents of the EndpointDescriptions as they are transferred over the network back to the Client. The Client protects itself against this by comparing the list of EndpointDescriptions returned from the GetEndpoints Service with list returned in the CreateSession response.
The exact mechanisms for using the security token to sign and encrypt Messages exchanged over the SecureChannel are described in OPC 10000-6. The process for renewing tokens is also described in detail in OPC 10000-6.
In many cases, the Certificates used to establish the SecureChannel will be the Application Instance Certificates. However, some Communication Stacks might not support Certificates that are specific to a single application. Instead, they expect all communication to be secured with a Certificate specific to a user or the entire machine. For this reason, OPC UA Applications will need to exchange their Application Instance Certificates when creating a Session.