Computer Science Assignment - Managing Encryption Keys

Answer should be original, and similarity (plagiarism) not more than 10% this is very very very important otherwise I will be in trouble.

Answer should be a clear explanation and should be clear understanding (300 words enough).

Answer should be references on Harvard style (Not use Wikipedia) should be Academic Articles or books.

When use citation from any reference please keep it between "citation" (Author,Year),please citation should be not more than 5%

As reference you can use the following book:

Security Engineering: A Guide to Building Dependable Distributed Systems (2nd Edition)

Author: Anderson, R.

ISBN: ISBN-10: 0470068523 ISBN-13: 978-0470068526

Publisher: John Wiley & Sons

Assignment is:

1) why are timestamps used in the Kerberos protocol?
2) Assume that Alice shares a secret s with her company’s server computer. When Alice is on a trip, she tries to store an important message in the CEO’s account directory. This message needn’t be encrypted since confidentiality is not important here, but the CEO needs to be guaranteed that the message really is from someone who knows the secret s (e.g. Alice) when the CEO opens his/her computer the next day. A naïve protocol to achieve this may look like this:
Alice-->Server: “This is Alice, I have an important message to forward to the CEO”
Server-->Alice: “OK, please encrypt R” (where R is a random nonce)
Alice-->Server: “Es(R)” (that is, R is encrypted with the secret s)
Server-->Alice: “Please send the message”
Alice-->Server: “The CEO needs to attend a meeting in Liverpool on May 1, 2006”
The server stores this message in the CEO’s directory
Is this protocol secure? If not, how do you feel it could be modified to make it so?