Applications postbox tutorial

[dbosk]

This add the slides for the tutorial. We should fix:

• Shorten their development time to 30 minutes.
• Provide solution for the basic case.
• Make sure we can start working on the anonymous case.
• Add a solution for the anonymous case.
• 20 minutes suffice!
• Find literature to refresh memory of this topic. Particularly sender-receiver anonymity (Add material on sender--receiver anonymity #65).
• Add link to DIGG's requirements (Swedish only).