1. 1 Intro To Encryption Exercise 12

2. 2 Problem What may be the problem with a central KDC?

3. 3 Solution Performace may decrease when clients increase. Especially when multicast clients are present Security may be a very big issue (single point of failure, master keys recovery).

4. 4 Problem How should you improve performance? Is replication a good performance improvement? How does it affects security?

5. 5 Solution A naïve approach may be to replicate the KDC This may solve some performance issues when done properly This may decrease security because of multiple exact copies of the same information.

6. 6 Problem Suggest a solution for sharing the load of computation and communication among servers. The solution must maintain the security and not degrade it.

7. 7 Solution Use secret sharing with threshold. Is this solution sufficient? NO!!!!  We must define (how?) the amount of servers and the threshold needed

8. 8 Problem Revise the previous solution to accommodate better definitions for security

9. 9 Solution After conducting surveys and cryptanalysis we have come to the conclusion that in a given time 20% of our servers are (may be) corrupt. Thus we shall decide on the scheme (n,80%n) for secret sharing.

10. 10 Problem Is this solution sufficient for secret consistency?

11. 11 Solution NO!!!! We don’t know which 20% of our server are corrupt. Which 80% should we trust? How do we know that some servers don’t sent bogus secrets?

12. 12 Problem Modify the solution to provide secret verification.

13. 13 Solution Use verifiable secret sharing by distributing public verification values. Where do we publish the values? Who signs the values to ensure they are authentic?  May be other sets of servers that act as a CA  May be a single server which is “super secured” (later)

14. 14 Problem How do we refresh the servers’ secrets?

15. 15 Solution A simple solution may be a single server which acts as the refresher. I.e. calculates the new secret and spreads it. The server may remain offline while not needed, thus preventing corruption.

16. 16 Problem How can we check when a server is being tempered?

17. 17 Solution Limit number of failed logins Lock accounts on several wrong logins  Lock for a certain period  Lock indefinitely Use a WORM logging mechanism.  Most attackers will not want to be detected Use a cookie mechanism/Reverse DNS/IDENT  Ensure that the client is indeed using his IP address and not using a bogus one.