Session Directories can make encrypted session announcements using private key encryption, and carry the encryption keys to be used for each of the conference media streams in the session. While this does not solve the key distribution problem, it does allow a single conference to be announced more than once to more than one key-group, where each group holds a different session directory key, so that the two groups can be brought together into a single conference without having to know each other's keys.
This is discussed in detail in chapter ten.