Protocol for classes implementing listeners for certificate challenges. The SAP Mobile Platform library should be able to connect to a relay server via HTTPS (or HTTP) using the iPhone SDKs native connectivity APIs. If HTTPS is used, and a connection attempt occurs, some negotiation takes place. Depending on what SSL certificate is installed on the server, a different sequence can occur. A certificate is delivered from the server to the device. If the certificate is trusted by the system or application keychain, then the connection is established. Otherwise, a MessagingClientLibrary delegate is called, if there is a listener registered to handle it, to determine whether the certificate should be trusted.