private broadcasts
ordinarily, kast attaches your receiver key to each broadcast that you send. This kind of key inclusion allows anyone to subscribe to your channel as soon as they hear about your broadcast.
in some circumstances, however, you may want to broadcast files to a selected group of people, and not everyone on the network. In this case, you should mark your broadcast as private.
private broadcasts do not include receiver keys, so the only way someone can subscribe to your channel securely is by obtaining the receiver key directly from you (through email, your web page, etc.). Making a broadcast private can be a way of forcing subscribers to come to your web page or otherwise contact you for subscription information.
keep in mind that marking a broadcast as "private" does not make your broadcast content invisible to unknown people in the network. The content still travels through the public network, and anyone can get it. However, without your receiver key, they cannot get it in a reliable and secure way, since they cannot establish a secure subscription to your channel. Thus, by listening in on a private channel, outsiders can receiver content, but they have no guarantee that the content has not been tampered with.
kast is not designed for distribution of secret information. If you have content that you do not want outsiders to access, do not broadcast it with kast.