"Compare the follwing schemes for obtaining Bob's public key, in terms of bandwith and computation efficiency, security, flexibility, and any other criteria you can think of: downloading Bob's key from the node located at a particular IP adress (via an unauthenticated interaction), looking up Bob's key in a directory via an authenticated interaction, having an authenticated conversation to the directory, having the directory sign the information you request, storing and retrieving certificates from the directory, having no directory but having each principal responsible for keping its own certificate, and sending it to someone who needs to talk to it?"