When using a Brokerin the PubSubmodel, the same shared symmetric key concepts as defined in 4.5.3.2can be used to provide Confidentialityand Integrity. Furthermore, communication to the Brokercan be secured according the rules defined for the Broker. These rules are not defined in the OPC UA specification but are defined by the Middleware. In many cases the Middlewarerequires the authorization of both the Publishersand the Subscribersbefore they can interact with the Broker. The Brokerinteractions can provide security mechanisms to meet Confidentiality, Integrityand application or user Authenticationas security objectives. If the published message is not secured using the shared symmetric key concepts, the message content is visible to the Brokerwhich creates some risk of man-in-the-middle attacks. The use of the shared symmetric keys eliminates this risk.