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.

The set of Objects and related information that the Server makes available to Clients is referred to as its AddressSpace. The OPC UA AddressSpace represents its contents as a set of Nodes connected by References.

Primitive characteristics of Nodes are described by OPC-defined Attributes. Attributes are the only elements of a Server that have data values. Data types that define attribute values may be simple or complex.

Nodes in the AddressSpace are typed according to their use and their meaning. NodeClasses define the metadata for the OPC UA AddressSpace. Part 3 defines the OPC UA NodeClasses.

The Base NodeClass defines Attributes common to all Nodes, allowing identification, classification and naming. Each NodeClass inherits these Attributes and may additionally define its own Attributes.

To promote interoperability of Clients and Servers, the OPC UA AddressSpace is structured hierarchically with the top levels the same for all Servers. Although Nodes in the AddressSpace are typically accessible via the hierarchy, they may have References to each other, allowing the AddressSpace to represent an interrelated network of Nodes. The model of the AddressSpace is defined in Part 3.

Servers may subset the AddressSpace into Views to simplify Client access. Subclause describes AddressSpace Views in more detail.

The OPC UA Object Model provides a consistent, integrated set of NodeClasses for representing Objects in the AddressSpace. This model represents Objects in terms of their Variables, Events and Methods, and their relationships with other Objects. Part 3 describes this model.

The OPC UA object model allows Servers to provide type definitions for Objects and their components. Type definitions may be subclassed. They also may be common or they may be system-specific. ObjectTypes may be defined by standards organizations, vendors or end-users.

This model allows data, Alarms and Events, and their history to be integrated into a single Server. For example, Servers are able to represent a temperature transmitter as an Object that is composed of a temperature value, a set of alarm parameters, and a corresponding set of alarm limits.

The interface between Clients and Servers is defined as a set of Services. These Services are organized into logical groupings called Service Sets. Service Sets are discussed in Clause 6.4 and specified in Part 4.

OPC UA Services provide two capabilities to Clients. They allow Clients to issue requests to Servers and receive responses from them. They also allow Clients to subscribe to Servers for Notifications. Notifications are used by the Server to report occurrences such as Alarms, data value changes, Events, and Program execution results.

OPC UA Messages may be encoded as XML text or in binary format for efficiency purposes. They may be transferred using multiple underlying transports, for example TCP or web services over HTTP. Servers may provide different encodings and transports as defined by Part 6.