OPC UA security is concerned with the authentication of Clients and Servers, the authentication of users, the integrity and confidentiality of their communications, and the verifiability of claims of functionality. It does not specify the circumstances under which various security mechanisms are required. That specification is crucial, but it is made by the designers of the system at a given site and may be specified by other standards.

Rather, OPC UA provides a security model, described in Part 2, in which security measures can be selected and configured to meet the security needs of a given installation. This model includes security mechanisms and parameters. In some cases, the mechanism for exchanging security parameters is defined, but the way that applications use these parameters is not. This framework also defines a minimum set of security Profiles that all OPC UA Applications support, even though they may not be used in all installations. Security Profiles are defined in Part 7.

Application level security relies on a secure communication channel that is active for the duration of the application Session and ensures the integrity of all Messages that are exchanged. This means users need to be authenticated only once, when the application Session is established. The mechanisms for discovering Servers and establishing secure communication channels and application Sessions are described in Part 4 and Part 6. Additional information about the Discovery process is described in Part 12.

When a Session is established, the Client and Server applications negotiate a secure communications channel. Digital (X.509) Certificates are utilized to identify the Client and Server. The Server further authenticates the user and authorizes subsequent requests to access Objects in the Server.

OPC UA includes support for security audit trails with traceability between Client and Server audit logs. If a security-related problem is detected at the Server, the associated Client audit log entry can be located and examined. OPC UA also provides the capability for Servers to generate Event Notifications that report auditable Events to Clients capable of processing and logging them. OPC UA defines security audit parameters that can be included in audit log entries and in audit Event Notifications. Part 5 defines the data types for these parameters. Not all Servers and Clients provide all of the auditing features. Profiles, found in Part 7, indicate which features are supported.

OPC UA security complements the security infrastructure provided by most web service capable platforms.

Transport level security can be used to encrypt and sign Messages. Encryption and signatures protect against disclosure of information and protect the integrity of Messages. Encryption capabilities are provided by the underlying communications technology used to exchange Messages between OPC UA Applications. Part 7 defines the encryption and signature algorithms to be used for a given Profile.