1. Authentication Applications The Kerberos Protocol Standard
Rabie A. Ramadan
Lecture 7

2. Outline I. Introduction II. Introduction to Kerberos v4
III. Details of Kerberos v4
IV. Kerberos v5
V. Realms and Inter-Realm Authentication

3. Introduction
Note: some of the slides and figures are taken from Dr. Jean-Anne presnetation

4. Introduction Open distributed environment
Workstation
Introduction
Open distributed environment
Users at workstations wish to access servers distributed throughout the network
Servers must restrict access to authorized users
Servers must authenticate request for service

5. Introduction
Workstation (your computer) can not be trusted to authenticate its user correctly to network services:
Three threats exist:
User pretends to be another user.
User alters the network address of a workstation.
User eavesdrops on exchanges and use a replay attack.

6. Solutions AS knows all the passwords of all users
Server (V)
Authentication
Server (AS)
Client (C)
AS knows all the passwords of all users
AS shares unique secret Keys with each server

7. A Simple Authentication Dialogue
Server (V)
Authentication
Server (AS)
Client (C)
Security holes?
Password sent as plain ASCII
No time limits for tickets
Man-in-the middle can steal the ticket and fake IDC ( simple, because it is sent as clear text).

8. More Secure Dialogue Idea: Introducing a Ticket Granting Server (TGS)
Kc’ : A key that is derived from the user password

9. Problems with the Previous Protocol
1. Lifetime associated with the ticket-granting ticket:
If too short → the user is repeatedly asked for the password
If too long → a greater opportunity to replay exists.
The threat is that an opponent will steal the ticket and use it before it expires.
2. There may be a requirement for servers to authenticate themselves to users.
The false server would then be in a position to act as a real server and capture any information from the user and deny the true service to the user.
An opponent could eavesdrop on the network and capture a copy of the ticket-granting ticket and then wait for the legitimate user to log out. Then the opponent could forge the legitimate user's network address and send the message of step (3) to the TGS. This would give the opponent unlimited access to the resources and files available to the legitimate user.
Therefore, a network service (the TGS or an application service) must be able to prove that the person using a ticket is the same person to whom that ticket was issued.

10. What is Kerberos? Network authentication protocol
Developed at MIT in the mid 1980s
Relies on conventional encryption, making no use of public-key encryption.
Available as open source or in supported commercial software
Two versions: version 4 (passing slowly away) and 5 coexist.

11. Kerberos 4 Overview
Stallings Fig Discuss in relation to Table 14.1 which details message exchanges.

12. Version 4: Authentication Dialogue
Authenticatorc

13. First Step : C to AS

14. Second step: AS to C

15. Third step: C to TGS

16. Fourth step: TGS to C

17. Fifth step: C to V

18. Sixth step: V to C

19. Kerberos Realms A Kerberos environment consists of:
a Kerberos server
a number of clients, all registered with server
application servers, sharing keys with server
This is termed a realm
typically a single administrative domain

20. Request for Service in Another Realm V.4
Users on one realm may need access to servers in other realms
Please consult the book
for more details

21. Difference Between Version 4 and 5
Encryption system dependence (v.4 DES)
Message byte ordering (v.4 arbitrary; v.5 defined by ASN1 Standard)
Ticket lifetime (v.4 21h max; v.5 arbitrary)
Authentication forwarding to other hosts (v.4 no; v.5 yes),
A client accesses a server. The server can not act on another server on behalf of the client)
Inter-realm authentication: v.4 (v5. simpler)

22. Kerberos Version 5 Developed in mid 1990’s
Provides improvements over v4
addresses environmental shortcomings
encryption algoithms, network protocol, byte order, ticket lifetime, authentication forwarding, interrealm authentication
Specified as Internet standard RFC 1510

23. New Fields in V5 Realm: Indicates realm of user
Options: Used to request that certain flags be set in the returned ticket
Times: Used by the client to request the following time settings in the ticket:
from: the desired start time for the requested ticket
till: the requested expiration time for the requested ticket
rtime: requested renew-till time
Nonce: A random value to be repeated in message (2) to assure that the response is fresh and has not been replayed by an opponent

24. New Fields in V5
Subkey: The client's choice for an encryption key to be used to protect this specific application session. If this field is omitted, the session key from the ticket (Kc,v) is used.
Sequence number: An optional field that specifies the starting sequence number to be used by the server for messages sent to the client during this session. Messages may be sequence numbered to detect replays.

25. Kerberos Limitations
Every network service must be individually modified for use with Kerberos
Doesn’t work well in time sharing environment
Requires a secure Kerberos Server
Requires a continuously available Kerberos Server
Stores all passwords encrypted with a single key
Assumes workstations are secure
Scalability