title: Encrypted and authorized multicast
body: <p>I have a server (will be noted as 'A') that transmits multicast to some end stations.
  I would like only those that are privileged to see the information, to actually see it.</p>
  
  <p>There is a third party server ('B') that manages unicast communications with both 'A' and the end stations and SSL with both of them.</p>
  
  <p>My goal is to implement a secured multicast:
  Two necessary givens:</p>
  
  <ol>
  <li>It has to be multicast.</li>
  <li>I want the security in the Application level (not the Network or Transport)</li>
  </ol>
  
  <hr>
  
  <p>Questions:</p>
  
  <ol>
  <li><p>Are there such implementations? I've tried searching for "secured multicast" etc. and have come across only IP Multicast security. I want an applicative implementation. If so, Could you recommend some and how to use them?</p></li>
  <li><p>I thought about generating a special symmetric key - during the communication of 'A' and 'B' and during the conversation of 'B' and end station = to pass it (over SSL) to the end stations.</p>
  
  <p>a. Is that a good idea?</p>
  
  <p>b. A problem I see is that revealing the key becomes easier because of the amount of stations. I thought about replacing the key every hour. I shall inform the stations the key has changed by sending multicast messages: (sequence_number, encrypted_message) - this way every time the key changes then sequence_number++.</p></li>
  </ol>
  
  <p>What do you think of the implementation? Have better ideas?</p>