The authentication protocol that is used by Microsoft Office Communications Server 2007 and its clients has been updated to require a signature in all messages. This includes the first message that carries the challenge response.
The version number of the protocol that is advertised in the challenge response has increased to version 4. Challenge responses that use version 4 of the protocol must contain a signature. The signature is a digest of the identifying headers in the message. The signature is calculated by using the session key.
Client applications that are built by using the Unified Communications Client API SDK must use this update to register and to work with Communications Server 2007. Communications Server 2007 requires the first challenge response message to contain a signature.