3. (24 points) Compare the following schemes for obtaining Bob's public key, in terms of bandwidth and computation efficiency, security, flexibility, and any other criteria you can think of: a. (4 points) downloading Bob's key from the node located at a particular IP address (via an unauthenticated interaction) b. (4 points) looking up Bob's key in a directory via an unauthenticated interaction c. (4 points) having an authenticated conversation tothe directory d. (4 points) having the directory sign the information you request e. (4 points) storing and retrieving certificates from the directory f. (2 points) having no directory but having each principal responsible for keeping it own certificate and sending it to someone who needs to talk to it